Cephalopods in the Marine Ecosystems of the Paleozoic - Springer Link

ISSN 0031-0301, Paleontological Journal, 2008, Vol. 42, No. 11, pp. 1167–1284. © Pleiades Publishing, Ltd., 2008.

Cephalopods in the Marine Ecosystems of the Paleozoic I. S. Barskov, M. S. Boiko, V. A. Konovalova, T. B. Leonova, and S. V. Nikolaeva Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia e-mail: [email protected], [email protected] Received February 17, 2008

DOI: 10.1134/S0031030108110014

CONTENTS INTRODUCTION CHAPTER 1. FUNCTIONAL INTERPRETATION OF THE BODY PLAN IN TAXA OF THE ORDER RANK 1.1. State of the Modern System of Cephalopods 1.2. Functional Interpretation of Morphological Characters of the Cephalopod Shell 1.3. Improving and Regulating of Buoyancy 1.4. Orientation and Support of Orientated Position 1.5. Body Plans and Major Evolutionary Trends in Cephalopod Orders CHAPTER 2. LIFE-FORMS OF CEPHALOPODS 2.1. Concept of Life-Forms 2.2. Life-Forms of Cephalopods 2.3. Ecological (Adaptive) Significance of Constructive Differences of the Outer Shell in Fossil Cephalopods, Criteria and Methods of Their Assignment to Various Life-Forms 2.4. Life-Forms of Cephalopods with a Curved Shell 2.5. Life-Forms of Cephalopods with a Straight Shell 2.6. Life-Forms of Cephalopods with a Planispiral Shell 2.7. Life-Forms of Cephalopods with Planispiral Shell in which the Whorls Were Not in Contact 2.8. Life-Forms of Cephalopods with a Conispiral Shell 2.9. Life-Forms of Cephalopods with a Heteromorphic Shell CHAPTER 3. ECOLOGICAL SPECIALIZATION AND ECOGENESIS OF PALEOZOIC CEPHALOPODS 3.1. Ecological Structure of the Modern Cephalopod Taxocoenosis 3.2. Ecological Structure of Paleozoic Cephalopods 3.2.1. Order Ellesmerocerida 3.2.2. Order Endocerida 3.2.3. Order Actinocerida 3.2.4. Order Orthocerida 3.2.5. Order Pseudorthocerida 3.2.6. Order Tarphycerida 3.2.7. Order Lituitida 3.2.8. Order Barrandeocerida 3.2.9. Order Discosorida 3.2.10. Order Oncocerida 3.2.11. Order Ascocerida 3.2.12. Order Nautilida 3.2.13. Order Anarcestida 3.2.14. Order Tornoceratida 3.2.15. Order Clymeniida 3.2.16. Order Praeglyphioceratida 3.2.17. Order Goniatitida 3.2.18. Order Prolecanitida 3.2.19. Order Ceratitida 3.3. Morphological Diversity of Life-forms and Ecogenesis of Cephalopod Taxocoenosis in the Paleozoic CHAPTER 4. ECOLOGICAL STRUCTURE OF PALEOZOIC AMMONOID COMMUNITIES IN THE URALIAN PALEOBASIN 4.1. General Background 4.2. Ecological Structure of the Paleozoic Ammonoid Communities in the Urals 4.3.1. Early Devonian 4.3.2. Middle Devonian 4.3.3. Late Devonian 4.3.4. Mississippian (Early Carboniferous) 4.3.5. Pennsylvanian (Middle and Late Carboniferous) 4.3.6. Early Permian CONCLUSIONS REFERENCES 1167

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BARSKOV et al. The authors dedicate this paper to the memory of A.A. Shevyrev

INTRODUCTION Cephalopods are the largest and most diverse group of the Paleozoic marine biota. By now about one and a half thousand valid genera and over four thousand species have been described. In the last century hundreds of papers have been written on Paleozoic cephalopods, including fundamental studies on all major groups and geochronological intervals. The historical development of higher taxa, their morphological and taxonomic diversity, have been relatively fully studied, their diversity dynamics have been tracked, and their biostratigraphic significance, which was the basis for the stratigraphy of the Late Paleozoic, has been demonstrated. The ontogenetic studies of Paleozoic ammonoids became the basis of the onto-phylogenetic method in paleontology and allowed the recognition of the major evolutionary patterns, which became important across many fields of biology. All this knowledge was to a large extent summarized half a century ago in Osnovy paleontologii (Fundamentals of Paleontology, Ed. by V.E. Ruzhencev, 1962) and Treatise on Invertebrate Paleontology. Part K, Part L, Ed. by R.C. Moore (1957, 1964). In recent decades new data were accumulated in all the above fields, and some new methods and approaches in Paleozoic cephalopod studies were proposed and successfully tried (this includes microstructural studies and various sorts of mathematical methods). All these numerous and diverse studies show that in the Paleozoic cephalopods represented one of the major elements of the ecological structure in marine ecosystems and played a significant role in the evolution of the entire biosphere. At the same time, our knowledge of the ecology and lifestyle of fossil cephalopods are far from sufficient, and this field is the least explored in cephalopod studies. Until now both professional and semipopular literature widely promotes an image of all cephalopods as active pelagic predators. This belief is based on a quite unjustified transferal of the lifestyle and behavior of the best known extant cephalopods to the whole of this morphologically and taxonomically diverse group, including numerous fossil cephalopods; even among the modern cephalopods, active pelagic predators constitute less than half of the taxocoenosis. The lifestyle of fossil cephalopods with an outer shell have only been seriously studied in about ten or so papers. These papers are generally those that address genera and species with unusual morphology (see Shevyrev, 2005). In Treatise (1964) only the data on the modern Nautilus are cited. In Osnovy paleontologii there is a small section with general views on the lifestyle of some cephalopods (Shimansky, 1962). Very

few papers discuss the general problems of adaptive specialization of fossil cephalopods, ecogenesis of groups, which would be important for understanding their role in past ecosystems. Louis Dollo (1922) was the first to attempt to propose a general scheme of adaptive specialization of fossil cephalopods. Barskov (1976, 1988, 1989, etc.) substantiated the recognition of life-forms based on shell geometry and hydrostatics. The main task facing the present authors was to apply functional and ecological approaches to the study of the entire community of Paleozoic cephalopods. The first chapter discusses the general state of the macrosystem of Paleozoic cephalopods. The considerable variety of proposed systematic schemes and the lack of unanimity in understanding of the rank and composition of the higher taxa of cephalopods probably result from the morphological approach being used in isolation. We attempt to substantiate the differences in the body plans and ranks of the major groups of Paleozoic cephalopods based on the functional morphological method. Fundamental differences in the body plans allowing the recognition of orders are related to different mechanisms of manipulating buoyancy and orientation within the framework of the archetype of this class. In the second chapter, the functional morphological approach is used to substantiate adaptive types, i.e., life-forms of cephalopods with different morphology. They are based on the evaluation of cephalopod potential to inhabit different adaptive zones of the sea, which is largely dependent on shell architecture and geometry, including mechanisms for maintaining orientation, and other features important for an animal with a shell operating as a buoyancy device. In the third chapter, the life-forms are recognized in each of 23 cephalopod orders based on the criteria proposed in the second chapter, and the ecogenesis of these orders is discussed throughout their evolutionary history. The ecological structure of the entire cephalopod taxocoenosis is discussed and its changes and the changes in the taxonomic composition if each life-form throughout the Paleozoic in relation to changes in abiotic factors are considered. The fourth chapter discusses in detail the evolution of the ecological structure of ammonoid communities in the Uralian Paleobasin from the Early Devonian to the Early Permian inclusive. Changes in the ecological structure and taxonomic composition are discussed in the context of their potential connection with the geological history of the basin.

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CEPHALOPODS IN THE MARINE ECOSYSTEMS OF THE PALEOZOIC

CHAPTER 1. FUNCTIONAL INTERPRETATION OF THE BODY PLAN IN TAXA OF THE ORDER RANK 1.1. State of the Modern System of Cephalopods Although a sensible system of cephalopods at the megataxon level has been intensively sought since the mid-20th century, no agreement has been achieved on the number of orders and subclasses, i.e., on the body plans within the archetype of the class (Flower and Kümmel, 1950; Flower, 1964; Shimansky and Zhuravleva, 1961, Donovan, 1964; Teichert, 1967, 1988; Zeiss, 1969; Zhuravleva, 1972; Salvini-Plawen, 1980; House, 1981; Drushchits and Shimansky, 1982; Starobogatov, 1983; Leonova, 2002; Shevyrev, 2005, 2006a, 2006b; etc.). Approaches to classification have been very varied, which is reflected in the subdivision of the class into between two and eight subclasses. There is more consistency in the understanding of the orders, although the number of recognized orders varies from 15 to 30. The latest review of the macrosystem was undertaken by Shevyrev (2005, 2006a, 2006b). Unfortunately, this system, like those proposed earlier, is nothing more than another shuffling of orders, some of which is widely accepted, and some is only accepted by a few authors. However, even the widely accepted orders have been placed in all subclasses in the systems proposed by various authors. In our opinion, the main reason for the lack of success in reconstructing the macrosystem of cephalopods was that when the orders were established and united in the subclasses, the comparative taxonomic analysis was usually based on the morphological characters of the shell and/or its parts with an estimation of their functional or ecological roles. The disparity and taxonomic value of characters are understood differently by different authors. This makes an agreement on the number of orders of fossil cephalopods and on their combinations in yet higher taxa, virtually impossible. In our opinion, functional interpretation of morphological characters of the cephalopod shell is the criterion on which the evaluation of disparities can be based. In recent cephalopod taxa, the description of the morphological state of the soft body anatomy, which has experimentally testable physiological and functional significance, may characterize their body plan, and consequently, their affinity to different higher rank taxa. In fossil cephalopods (which constitute three-quarters of the entire taxonomic diversity of cephalopods), a simple description of morphological characters is insufficient to form an opinion on fundamental differences in the body plan, and cannot lead to anything but to another shuffling the possible interpretations. At present, to make progress in understanding the disparity of the body plan, the evaluation of disparity of morphological characters of the cephalopod shell and its parts has to be supplemented by interpretation of the functional significance of this disparity and of the degree of its functional realization. PALEONTOLOGICAL JOURNAL

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Therefore, the orders recognized in this study will be discussed based on the explanation of fundamental differences in the shell morphology from the functional interpretation of a particular body plan specificity in the members of an order at the time when this order evolved, and on the major trend in the evolution of this specificity (referred to as “the main cluster of development” by Ruzhencev (1960)). 1.2. Functional Interpretation of Morphological Characters of the Cephalopod Shell Cephalopoda are the only class of mollusks whose origin can be traced from fossil material. Confirmed cephalopod remains are presently known only from the end of the Cambrian. The Early Cambrian genera Volbortella and Salterella, and the Middle Cambrian Vologdinella and Olenecoceras, previously assigned to cephalopods, are now excluded from this class of mollusks. Hence, in contrast to all other molluscan classes, whose fossil remains are known as early as the Lower Cambrian, the first confirmed cephalopods are known only from the topmost Cambrian (genera Plectronoceras, Paleoceras, and Ectenolites). Therefore, there is more chance of finding taxa which may be cephalopod ancestors. It is currently widely accepted that cephalopods evolved from monoplacophorans with a relatively high-coned shell. The origin of cephalopods was discussed in most detail by Kobayashi (1987) and Dzik (1981). The origin of the cephalopodan archetype began with the appearance of the septa and siphuncle in the apical part of the shell, the features that allowed the development of the gaseous-fluid float and colonization of the pelagic zone, then a new adaptive zone, at that time inaccessible for other groups of mollusks. Continuous septa are present in the apical parts of the shell of many groups of gastropods, and also in the fossil groups Hyolitha and Tentaculita, which are sometimes placed together in the molluscan class Coniconchia, and in the Cambrian monoplacophorans Helcionella and Knightoconus) (Yochelson et al., 1973; Kobayashi, 1987). According to Kobayashi (1987), Helcionella, with its taller high-coned shell is a more appropriate candidate for the role of a morphological ancestor of cephalopods than Knightoconus, with its low-coned shell. Thus, the development of septa is not a feature unique to cephalopods, whereas the appearance of the siphuncle is a fundamentally new character, as of a part of the body that remains in the chambers and is capable of controlling the buoyancy of the animal. According to Dzik (1981), the development of septa happens at the larval stage and begins with the retention of a bubble between the posterior mantle and the shell filled with liquid of a density less than that of the molluscan body or of seawater. Functionally, this is necessary to facilitate the existence of the larva in the pelagic zone. At the next stage, the fluid-filled sac is separated by a solid carbonate or organic septum. Rhythmical alternation of these two processes led to the development of septa in

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septate monoplacophorans and tentaculites. In Dzik’s interpretation, the siphuncle of cephalopods, responsible for calibration of buoyancy, is not homologous to any structures in other mollusks and may have originated from the part of the larval shell that was involved in soft body attachment to the shell. Kobayashi (1987) suggested that in the earliest Cambrian cephalopods, septa in the apical region of the shell were imperforate, whereas the siphuncle appeared later, at the adult stages. Starobogatov (1974) suggested that the appearance of the imperforate septa is the first step toward the decollation of the apical parts of a high-conical shell. Functionally, this is related to the necessity of maintaining compactness and stability of the shell through the lowering of the center of gravity. Decollation is widely developed in other molluscan groups. The development of septa and siphuncles is a result of incomplete and unfinished decollation, i.e., the septa are formed on the convex, anatomically anterior side, and lateral sides of the endogastric shell, whereas the posterior half of the body sac, which extended up to the apex, remains free of septa. This resulted in the functional reorientation of the body and shell of the animal. The anterior side became functionally the dorsal side, whereas the posterior side, where the siphuncle is located became the ventral side. The previously entire mantle epithelium secreting the shell was subdivided into three independent sections: shell walls, septa, and posterior siphuncular section. Later, another zone of secretion formed, that was responsible for the development of endosiphuncular deposits. According to this interpretation, the cephalopod siphuncle is homologous to the apical part of the body sac. This interpretation is, in our opinion, more realistic than Dzik’s (1981) hypothesis of the origin of the siphuncle from the larval muscle cord. The siphuncle of the earliest cephalopods is wide. Its diameter in many Ellesmerocerids, Endocerida, and Actinocerida is over a third and even more than half the diameter of the soft body in the body chamber. The structure of the soft tissue of the siphuncle with a welldeveloped system of blood vessels is impossible to explain using Dzik’s hypothesis. The gas-fluid float in the shell presented at least two major problems: necessity to develop and regulate the buoyancy and orientate and stabilize the body in space in a position comfortable for life. 1.3. Improving and Regulating of Buoyancy Different ways of regulating buoyancy are reflected in the shape and structure of the connecting rings of the siphuncle. Because the epithelium of the siphuncular zone derived from the external epithelium of the mantle, three layers homologous to the three successive layers in the shell wall and septa (spherulite-prismatic, nacreous, and semi-prismatic) are secreted when the siphuncular sheath (connecting rings) are formed. The

earliest cephalopods of the order Ellesmerocerida had thick three-layered, apparently strongly mineralized connecting rings, the ends of which directly correspond to three layers of the shell walls. This structure suggests low permeability of the siphuncular cover and, hence, low ability to regulate buoyancy. Demineralization facilitates the intensification of this process, which is shown by their varying degrees of thinning, mineralization, and differentiation in the longitudinal and transverse directions. In modern Nautilus, the main part of a connective ring is a demineralized homologue of the nacreous layer, with highly porous organic membranes (Gregoire, 1968, Mutvei, 1980). Denton and GilpinBrown (1961) showed that the change in buoyancy in Nautilus occurs by osmotic pumping of the fluid from the chamber of the phragmocone through the specialized epithelial cells, with cytoplasm containing high concentrations of salts. A similar mechanism is used for changing buoyancy in Sepia and Spirula (Denton and Gilpin-Brown, 1961, 1971; Denton et al., 1961). Ward and Martin (1978) showed that for Nautilus, living at depths of 200 m and deeper, the simple osmotic mechanism is insufficient to regulate buoyancy, and the presence of other mechanisms, e.g., a partial osmotic pump, is possible. Strictly speaking, the osmotic mechanism of buoyancy regulation works in one direction only, i.e., to increase buoyancy by pumping liquid from the chambers. The decrease in buoyancy, i.e., filling the chambers with liquid is not supported by osmotic mechanisms. To explain this phenomenon, Barskov (1999) suggested capillary transfer of the liquid in the reverse direction. It is most likely that the regulation of buoyancy is controlled by both processes, while these processes are restricted to different zones of the siphuncular cover, as these are derived from different layers of originally trifoliate connecting rings of ancient Ellesmerocerida. It is possible that the osmotic regulation of buoyancy is controlled by those zones of the siphuncular cover which are homologous to the nacreous layer, whereas capillary transport is controlled by the variously mineralized homologues to the spherulite and/or semiprismatic layers. In addition, the capillary regulation of buoyancy may be also controlled by the zones of septa and septal necks contacting the connecting rings. Judging from the existing data on the structure of the connecting rings in various groups of fossil cephalopods (Flower, 1957; Hewitt, 1982; Mutvei, 1972, 1997; Druschits et al., 1976), there are several possible types of the structure of the siphuncular cover suggesting different ways of controlling buoyancy. (1) Trifoliate, strongly calcified connecting rings (typically developed in Ellesmerocerida) (Fig. 1.1). (2) Connecting rings with a transverse differentiation of layers (Discosorida) (Fig. 1.2). (3) Connecting rings in which the main part consists of homologues of the nacreous layer (Nautilida), whereas the homologues of other layers are locally restricted.


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c

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Fig. 1.3. Connecting rings of Endocerida, genus Emmonsoceras (1) and Aktinocerida, genus Discoactinoceras (2): (c) connecting ring, (s) septa, (co) contact layer (after Teichert, 1964, p. 182, p. 212).

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Fig. 1.1. Multilayered connecting rings of Ellesmerocerida: (1) Ellesmeroceras, (2) Paracyclostomiceras, (3) Bathmoceras: (3a) dorsal part of the siphuncle, (3b) ventral part of the siphuncle.

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v g Fig. 1.4. Connecting rings of actinosiphonate Oncocerida: (1) Pectinoceras, (2) Conostichoceras, (3) Jovellania.

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Fig. 1.2. Structure of the connecting rings in Ruedemannoceras boycii (Discosorida): (a1, a2) first and second amorphous layers; (b1, b2) inner and outer layers of bullete; (c) conchiolin zone; (g) granular zone; (v) vinclulum (after Teichert, 1964, p. 322). PALEONTOLOGICAL JOURNAL

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(4) Connecting rings in which the main functional part consists of homologues of semiprismatic layer (Endocerida, Spirula) (Fig. 1.3). (5) Thick connecting rings with numerous, variously built, radial endosiphuncular deposits (actinosiphonate oncocerids) (Fig. 1.4). (6) Modified connecting rings of ammonoids. Apart from modification in the structure of the siphuncular cover, the intensification of the exchange between the siphuncle and chambers was also facilitated by the development inside the siphuncle of taxa with a straight shell of longitudinal and transverse 2008

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organic membranes, separating the deposits (Endocerida, Intejocerida, Actinocerida). 1.4. Orientation and Support of Orientated Position This is a second major problem that cephalopods had to face after having acquired a gas-fluid float. The problem of stabilizing the shell in an oriented position comfortable for life was solved in a number of different ways: (1) Using the weight of the wide ventral siphuncle. (2) Through the development of endosiphuncular deposits. (3) Through the development of deposits inside the chambers (cameral deposits). These strategies are mostly effective for forms with a straight shell, since they can provide a horizontal position, suitable for active swimming. (4) Coiling in a flat spiral. This is one of the best ways to solve the problem of stability of the animal in the water, because it results in the close approximation of the centers of gravity and buoyancy, and the animal is constantly in a state of unconditional balance, allowing any position in relation to the bottom or the surface of water with the minimal energy loss, only using the arms (tentacles), or the funnel. (5) Decollation of the posterior end of the shell leading to the approximation of the centers of gravity and buoyancy. (6) In the shell in the state of unconditional balance the center of gravity in the phragmocone will be always above and on the same vertical line with the center of gravity of the animal in the body chamber. In the shell morphology this is reflected by a narrow or almost completely closed aperture. Comments on the Origin of the Endosiphuncular and Cameral Deposits Endosiphuncular deposits and the connecting rings were formed by the siphuncular epithelium. It is not clear whether the epithelium secreting the endosiphuncular deposits is a separate epithelial zone like one that secretes septa and unattached regions of the connecting rings, or the endosiphuncular deposits are formed in the final secretory phase by the same zone of the epithelium that forms the connecting rings. In the former case it can be expected that the endosiphuncular deposits will contain layers of all three secretory phases: spherulite, nacreous, and semiprismatic. The existing data on the microstructure of these deposits showing homogenous fine-prismatic structure of the endosiphuncular deposits and the absence of the repeated layers of various structures suggest that the endosiphuncular deposits are formed in the last, semiprismatic secretory phase by the same epithelial region as the connecting rings. Evidently, the endosiphuncular deposits were not massive,

but porous, capable of retaining a large amount of liquid to increase the weight of the mollusk. Despite the recurrent discussion of the problem of the cameral deposits, there are still alternative views on their origin: (1) The deposits are formed by a mantle zone, which remains in the chambers after these are formed and continues functioning by secreting carbonate deposits in the apical part of the shell; (2) No mantle is present in the chambers, whereas the deposits are secreted by the cameral extrapallial liquid (a substance between the mantle and carbonate layers at the anterior edge of the shell during its accretionary growth. Recently, those accepting the former point of view brought in new arguments supporting the existence of the cameral mantle. Klebaba (1999a; 1999b) suggested that the connecting rings are gradually resorbed in the apical zones of the shell in orthocerids, beginning from the dorsal side; the mantle grows over the cameral walls and septa and secretes cameral deposits. Zhuravleva and Doguzhaeva (1999) discovered structures in chambers of some pseudorthocerids and actinocerids, which they interpreted as remains of soft tissue and blood vessels, and discovered pores in the connecting rings, enclosing these vessels. In our view, all these data are not confirmed, whereas the structures found are artifacts that appeared due the incompleteness or poor preservation of the material studied. Additionally, the hypothesis of the existence of live soft tissue inside the isolated chambers of the siphuncle is quite simply against common sense. Nevertheless, cameral deposits certainly did develop in live mollusks. Their development was only possible to explain by suggesting that in the live mollusk the cameral deposits were not strictly speaking “deposits,” as it was a series of porous organic membranes, which served as reservoirs for the cameral liquid which entered the chambers through the porous zones of the connecting rings. It is possible that the chambers contained no unbound fluid at all, which, being highly inert, could have presented difficulties to active swimming. 1.5. Body Plans and Major Evolutionary Trends in Cephalopod Orders (Functional morphological features of the body plans of Paleozoic cephalopods and major trends in the changes in the geological history.) Order Ellesmerocerida Flower, 1950 (Late Cambrian–Late Ordovician). The first cephalopods and known from the Upper Cambrian beds of northeastern China (Manzhou). At the end of the Cambrian, this region was situated slightly north of the equator, and, judging from paleotectonic reconstructions, occupied a specific borderline position between the Panthalassa Ocean and the just born Paleotethys Ocean, and was the center of origin and primary diversification of cephalopods. These earliest cephalopod taxa belong to the order Ellesmerocerida. According to Schindewolf (1933), the earliest ellesmerocerids were crawling


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benthic organisms with a flat foot as in gastropods. Their imperfect float only allowed these mollusks to rise over the substrate for a short time, possibly to aid escape from predatory arthropods, such as trilobites, many of which were larger and were widely distributed at the time. Their body plan included such general features as short chambers, a thick siphuncular cover, originally consisting of three layers, similar to the shell wall and septa. The presence of diaphragms (imperforate septa in the apical part of the siphonal region) in many genera support the hypothesis that the phragmocone, as a separate part of the shell, and the siphuncular cover evolved as a result of decollation. All this suggests low buoyancy and poor buoyancy control. The genus Plectronoceras is basal in Cephalopoda. As early as the end of the Cambrian, the ellesmerocerids were represented by almost all morphological types, including endogastric Plectronoceras, smooth straight Paleoceras, straight or curved annulated Walcottoceras and Tamdoceras. The early Ordovician expansion of ellesmerocerids, when they constituted the major part of the cephalopod taxocoenosis, was not accompanied by the appearance of any new morphological types or by the dominance of one shell shape. The diversity of the shell shape and siphuncular segments in the Early Ordovician Ellesmerocerida included all morphotypes that ever existed in this group, excluding the coiled shells. Should they be found in the younger rocks, they, based on morphology alone, could have been assigned to different orders. In the Middle and Late Ordovician ellesmerocerids were represented by approximately ten genera with both the orthoceraconic (Cochlioceras, Bathmoceras, Bactroceras) and cyroceraconic shells (Cyrtocerina, Shideleroceras) (Fig. 1.5). Morphological features of main lineages of ellesmerocerids, which are assigned to separate families (Baltoceratidae, Bassleroceratidae, and Protocycloceratidae), may be interpreted from the point of view of perfection of the floating function of the phragmocone and buoyancy control as trends toward the development of new body plans. Baltoceratidae have a straight shell and ventral siphuncle, which is considerably narrower than in other ellesmerocerids. The evolution of the family shows a morphogenetic trend toward the development of endosiphuncular deposits facilitating horizontal stability. At least in two genera (Cryptendoceras and Rhabdiferoceras) the ventral side of the connecting ring possesses longitudinal rods extending along the entire siphuncle and widening apically. This morphology of the connecting rings is virtually one step away from the development of the endosiphuncular deposits found in the order Pseudorthocerida. Morphologically it may be interpreted in the following way: the rods became separated from the connecting rings and were formed independently. PALEONTOLOGICAL JOURNAL

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2a

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Fig. 1.5. Ellesmerocerida with orthoceraconic (1) and cyrtoceraconic (2) shell. (1) Bactroceras: (1a) ventral side, (1b) longitudinal section; (2) Cyrtocerina: (2a) lateral view, (2b) longitudinal section of the siphuncle (ectosiphuncle).

The family Bassleroceratidae includes secondarily exogastric shells. This shell shape does not suggest a more efficient propulsive mechanism and active swimming compared to the originally endogastric shell. This trend developed, leading to the narrowing of the siphuncle and thinning of connecting rings and giving rise to the order Oncocerida. Bassleroceratidae are different from Oncocerida in the presence of very short phragmocone chambers and relatively wide siphuncle with thick connecting rings (characteristic features of ellesmerocerid morphology). The increase in the exogastric curvature up to contacting whorls and thus development of a spirally coiled shell with respective hydrostatic and hydrodynamic properties led to the appearance of the order Tarphycerida. Genera of the family Protocycloceratidae have all the characters observed in the order Endocerida: straight shell, wide marginal siphuncle, structure of the siphuncular cover. However, members of this family do not have endosiphuncular deposits, and all of them have an annulated shell. The functional significance of the annulation is in the increase of the internal volume of the chambers, with their length unchanged, and in facilitating the attachment of the soft body in the shell in its hypostomal position. Ellesmerocerids gave rise to 12 orders. At the first stage of the separation of the orders, the shell shape of the ancestral forms was of primary significance,

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although in later evolution the external shell morphology could become considerably different. The discussion below begins with orders, which originally had a straight or slightly curved exogastrically longiconic shell. The presence of a straight shell is adaptively useful only when it is accompanied by a mechanism allowing the horizontal orientation of the shell. Order Yanhecerida Chen et Qi, 1979 (Late Cambrian or Early Ordovician). Endemic Chinese taxa with a straight shell with all characteristic features of Ellesmerocerida, including low chambers of the phragmocone and a wide siphuncle with tubular segments. The presence of siphonal deposits is a fundamental functional difference allowing their placement in a separate order. The structure of these deposits is still unknown. However, the very presence of these deposits in the shell as a mechanism of orientation and stability of the shell certainly suggests a different body plan from that of Ellesmerocerida, the active swimming ability, and possible presence of an organ allowing this function. In Ellesmerocerida these mechanisms were absent. The origin and further evolution of Yanhecerida remain unknown. Order Protactinocerida Chen et Qi, 1979 (Late Cambrian or Early Ordovician of China). Like in Yanhecerida, the appearance of the straight shell was accompanied by the siphonal deposits, which served the same function but were based on a different morphology of the siphuncle. In Protactinocerida, the siphuncular segments expand into the phragmocone chambers, and the siphuncle is moniliform. The different morphology of the siphuncle in these orders suggest different structures, which may be used as evidence of their taxonomic separation. The morphology and structure of the endosiphuncular in Protactinocerida are also unknown. The outline of the siphuncular segments are similar to Actinocerida, which appeared later, and as is seen from their name, were their supposed ancestors. However, a considerable stratigraphic gap between these orders and the unknown structure of their endosiphuncular deposits do not allow a positive link. If the apomorphy that separated cephalopods from other mollusks, which for the first time allowed macroscopic shelled organisms to take off from the bottom, be compared to the invention of the hot air balloon, then Yanhecerida and Proactinocerida became the first zeppelins capable of active directional movement. Order Endocerida Teichert, 1933 (Early–Late Ordovician). High phragmocone chambers, wide siphuncle, and diversity in the structure of the siphuncular cover, all this suggests a more efficient means of buoyancy control. During the Ordovician, Endocerida were the largest bottom-dwelling pelagic animals, active predators at the top of the trophic pyramid. Perhaps, from that time, the roles changed and the bottomdwelling cephalopods instead of being the prey of arthropods became their predators.

At the same time the mechanism of stability control arose, which worked by filling the siphuncle with continuous endosiphuncular deposits, coating the connecting rings, as in Yanchecerida. The continuous deposition inside the siphuncle interrupted the communications between the siphuncle and chambers and eventually prohibited buoyancy control in the zones of the phragmocone where they were present. Thus this body plan was seriously internally controversial. At the end of the Ordovician or at the very beginning of the Silurian Endocerida became extinct. Order Intejocerida Balashov, 1960 (Early Ordovician). The attempts to find a compromise between the need in the increasingly heavy apical end of the phragmocone by development of endosiphuncular deposits and retention of communications between the siphuncle and the chambers through a series of longitudinal organic membranes, which cut through the massive deposits and worked as channels connecting the siphuncular epithelium with the chambers (important for buoyancy control) were unsuccessful. This morphology was characteristic of the order Intejocerida, which branched off the Endocerida in the Early Ordovician. This approach was not efficient, and Intejocerida became extinct as early as the Early Ordovician. Nevertheless, as shown below, this method of intensification of the exchange between the siphuncle and the chambers was used on several occasions. Order Orthocerida Kuhn, 1940 (Early Ordovician–Triassic). Representatives of this order had a straight or weakly curved exogastric shell. In addition, orthocerids had two more fundamental features that separated them from all previously evolved orders: appearance of the mechanism controlling orientation and stability of the shell (cameral deposits) and changes in early ontogeny. The former was first tried in the coiled and secondarily straight tarphycerids and lituitids, and the latter evolved for the first time in orthocerids, but was later inherited by the entire evolutionary lineage leading to ammonoids and coleoids. Morphologically, this can be observed as a formation of a small spherical chamber (protoconch). The presence of the protoconch suggests an incompletely formed hydrostatic apparatus of one chamber only at the first postembryonal stage. This means that in contrast to Ellesmerocerida, a group with no protoconch and with a phragmocone of several chambers at the first postembryonal stage, orthocerids were capable of earlier access to the pelagic zone (before they reached the phragmocone stage). This, on the one hand, promoted a wider distribution, and, on the other hand, opened opportunities of evolutionary changes at early ontogenetic stages. The realization of the wide distribution opportunities is supported by the known fact that in the second half of the Ordovician and in the Silurian, orthocerids were the most widespread cephalopod group and existed for about 300 million years, until the end of the Triassic, and possibly longer. The possibilities of the evolutionary changes at the early stages were to a full


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extent used by bacritids (descendants of orthocerids, and further, by ammonoids and coleoids, which dominated in the Jurassic and Cretaceous). For a more detailed description of major features of the evolution of early cephalopods see Barskov (1989). Order Pseudorthocerida Barskov, 1968 (Early Ordovician–Triassic). The treatment of this taxon as an order appears to have been a simple increase in the rank of the superfamily Pseudorthoceratacea (previously in the order Orthocerida) (Barskov, 1968). There were two major features of pseudorthocerids that supported the idea of a higher rank for this taxon: (1) Absence of a protoconch; (2) Presence of endosiphuncular deposits, different from pendant deposits, found in Actinocerids, from the endocones of endocerids, polyptychocones of early discosorids and from endosiphuncular deposits, which are present in some later genera of orthocerids (family Geisonoceratidae). In “typical” Late Paleozoic pseudorthocerids the endosiphuncular deposits are initially formed in the septal foramen (as in Actinocerida and Orthocerida), but later spread only adorally, forming the lining of the connecting rings of the next preceding segment of the siphuncle, where they become thicker, but do not merge with the deposits of this segment. Thus, the deposits are formed in such a way that the communication between the siphuncle and chambers is maintained in almost all chambers of the phragmocone. A detailed study of the Early Paleozoic representatives showed that the development of the endosiphuncular deposits in this cephalopod group toward the stage typical of the Late Paleozoic taxa was very gradual. Originally these deposits developed only adapically, like endocones (opistoneckal deposits) later both adorally and adapically (bilocal deposits). Only in the Silurian they acquired a typical shape spreading only adorally from their place of origin in the septal foramen (Barskov, 1972, 1989). Thus, the establishment of the typical body plan of pseudorthocerids was a very gradual process. Order Actinocerida Teichert, 1933 (Early Ordovician–Middle Carboniferous). Morphologically, Actinocerida continue the evolutionary trend that began with the order Protactinocerida. Characteristic features of this body plan include siphonal segments expanding into chambers, up to the development of rounded and nummuloid and development of endosiphuncular deposits. The expansion of the siphuncular segments suggests an increase in the exchange between the siphuncle and the chambers by increasing the surface area of the exchange, and contributing towards horizontal stability comfortable for active swimming. The conflict between the need for regulation of stability and its unavoidable limitation by the development of deposits in the apical regions led to the development in Actinocerida of so-called pendant deposits. They did not line the connecting rings, like endocones in endocerids, but were formed discretely in each septal foramen and spread within the siphuncle pressing the connective tissues of the siphuncle and leaving longitudinal and PALEONTOLOGICAL JOURNAL

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Fig. 1.6. Reconstructed siphuncle in Actinocerida (after Teichert, 1964, p. 196).

transverse spaces for blood vessels and deposit-free peripheral space near connecting rings (perispatium) in each segment, where the exchange between the siphuncle and phragmocone chambers took place. Usually, the structure of the siphuncle in Actinocerida is interpreted as follows. Longitudinal canals run along the entire length of the siphuncle (they most certainly were part of the blood system), arterial (in Nautilus arteries with peristaltic epithelium) and venous (in Nautilus a system of lacunas in the connective tissue lacking epithelium). Each segment also possessed radial canals providing the connection with the perispatium, possessing epithelium responsible for emission of gas into the chambers of phragmocone and for liquid extraction and removal to maintain buoyancy. The reconstructions of the siphuncle in Actinocerida led to interpretation of radial canals as tubular structures, similar to longitudinal canals (Fig. 1.6). This led to the conclusion that, because the both arterial and venous branches of the blood systems are tubular, the venous branch had blood vessels similar to those in the arterial branch. Thus, the fundamental organization in Actinocerida was more developed than even in the modern Nautilus, which does not have true vessels (i.e., with their own walls) in the venous branch of the blood circulatory system. This reconstructed endosiphuncular system and its interpretation are the basis for the separation of Actinocerida as a high-rank taxon, originally as a superorder (Shimansky and Zhuravleva, 1961) and later as a subclass (Teichert, 1964). However, the interpretation of the intrasiphuncular system of Actinocerida as of being originally highly

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phragmocone than the cylindrical segments of Intejocerida and certainly provided more possibilities for maintaining buoyancy. Nevertheless, it is hardly possible to suggest a fundamentally different level of organization (and especially a higher level) compared to other ancient cephalopods with an outer shell. At the same time, this does not exclude the taxonomic treatment of Actinocerida at a rank higher than order. Clear separation of Actinocerida from other contemporary groups is emphasized by the fact that only Actinocerida (taxa with a straight shell) had septa with lobes and saddles (family Ellinoceratidae, Fig. 1.8), which is a character found only in coiled post-Paleozoic Nautilida (Triassic Clydonautilidae, Paleogene Aturiidae), and ammonoids.

(b)

(a) Fig. 1.7. Earliest Actinocerida, genus Polydesmia: (a) longitudinal section, (b) cross section.

Fig. 1.8. Genus Ellinoceras (Actinocerida) with fluted septa.

organized is not correct. The organization was acquired by some representatives of the order in the course of evolution and was not the same in all members. The fossil material on the earliest Actinocerida, e.g., the genus Polydesmia (Fig. 1.7) suggests that these representatives did not have a well-developed system of tubular radial canals, but had only longitudinal canals, which were connected to the perispatium by a series of longitudinal and transverse membranes. In this respect, the system of communications between the siphuncle and the chambers in the presence of the endosiphuncular deposits was completely analogous (homologous?) to the system that was developed inside the siphuncle of Intejocerida. Supposedly, the transverse membranes (“radial canals”), separating in each segment the pendant endosiphuncular deposits and connecting longitudinal arterial vessels from the perispatium, belonged to the arterial branch, whereas longitudinal radial membranes cutting through deposits represented the venous branch of the blood circulation system of the siphuncle. The expanded siphuncular segments of Actinocerida had a larger area of contact with the chambers of the

The presence of a straight or weakly curved shell and mechanisms of stability control in the form of cameral or endosiphuncular deposits, which are characteristic of the above orders, suggest that they had a mechanism enabling active swimming, i.e., a hyponome. We have no (and perhaps will never have) knowledge of its morphology in extinct early cephalopods. However, with regard to those groups which have phylogenetic descendants in the modern fauna (Nautilus and coleoids), at least two variants of the morphology of the hyponome may be suggested. In the lineage leading to nautilids, the hyponome had a primitive morphology and consisted of two lobes, which could open into an almost flat crawling foot. It is logical to suggest that the hyponome in Nautilus is not a simplification of a previously complexly built hyponome and that some members of the nautilid-related orders Oncocerida and Discosorida could have a similar or even more primitive hyponome. It also possible to suggest that some members of the orders Orthocerida, Pseudorthocerida, and Bactritida, which were ancestral to the modern coleoids, which have a tubular hyponome better adapted to active swimming, had a more advanced mechanism for active swimming. Order Ascocerida Kuhn, 1940 (Middle Ordovician–Silurian). The construction of the shell in the order Ascocerida from the functional point of view suggests that the shell architecture was not used for control, as the shape of the phragmocone and the entire shell are fundamentally different from those in other orders. At early growth stages, Ascocerida had a curved, exogastric orthocerid-like shell, but, in contrast to Orthocerida, did not have cameral deposits to control stability. At a certain growth stage, a large posterior part of the phragmocone was completely decollated, and the shell became more compact, thus shortening the distance between the centers of gravity and buoyancy. Later in ontogeny, the shell became egg-shaped, with a passive float on the dorsal side, since in some taxa the siphuncle did not extend to the early chambers. Order Bactritida Shimansky, 1951 (Early Ordovician? Devonian–Triassic). The treatment of Bactritida as a separate order in an intermediate position between orthocerids and ammonoids, and their taxonomic rela-


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tionship to coleoids are generally agreed. However, the morphological traits of Bactritida, shared with the order Orthocerida, from which bactritids evolved, are less frequently discussed. Characters that are different from orthocerids include: marginal position of the siphuncle, which is in contact with the shell wall (typical orthocerids have a central or almost central siphuncle), and the absence of the cameral and endosiphuncular deposits in Ascocerida. Shared characters include a straight or weakly curved shell and the presence of a protoconch. Many Late Silurian orthocerids had eccentric siphuncle closely approximating to the ventral side of the shell, but not touching it. This morphology makes the relevant genera more similar to Bactritida than to Orthocerida. Hence, it is necessary to determine fundamental functional consequences of the marginal siphuncle, and of the absence (as in some other straight nonammonoid cephalopods) of cameral deposits. These functional differences are so highly specific that it is quite impossible to assign Bactritida to Orthocerida, or even to Ammonoidea. The absence of the cameral and endosiphuncular deposits in Bactritida with a straight shell suggests that they did not have a mechanism for orientation of stability control for the horizontal position, which was typically present in all earlier cephalopod groups with a straight shell (orthocerids, pseudorthocerids, actinocerids, and endocerids). Hence, bactritids either could not orientate the shell and body horizontally, or they had a different mechanism to control stability. In the first case it would be necessary to assume that these animals’ live orientation in water was hypostomic, i.e., with the apical end facing up, or at an angle. The shell of bactritids is longiconic, i.e., the animal had high buoyancy and therefore bactritids apparently inhabited the upper pelagic zone. The pelagic affinity of bactritids is supported by their small subspherical protoconch, suggesting a large number of small-sized eggs and, hence, direct or larval development. Hatchlings were pelagic. The near-wall position of the siphuncle has important functional consequences. The siphuncle in orthocerids was enclosed in a hard tube composed of connecting rings and was central or subcentral. Apart from its main function, this siphuncular tube certainly served a structure connecting and enforcing the unattached part of the septum. The transition of the siphuncle to the ventral wall and its contact with the shell wall made the free parts of the septa unconnected. As a natural consequence, the septum became curved to form a wide omnilateral lobe, morphological structure characteristic of bactritids that was inherited by ammonoids and initiated the process of increasing sutural complexity. The attachment of the siphuncle to the shell wall naturally led the suture to break at this spot to form a neck lobe, which was also inherited by ammonoids. In addition, the ventral position of the siphuncle changed the means of buoyancy and stability control and shell orientation. It is known that Nautilus controls its buoyancy by filling the chambers with liquid, and its osmotic PALEONTOLOGICAL JOURNAL

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removal through the siphuncle. Reyment (1973) experimentally showed that the amount of liquid in the phragmocone chambers sufficient for neutral buoyancy is minimal in the involute Nautilus-like shells, and increased in more evolute taxa. The presence of liquid in the chambers imposes significant constraints to active swimming due to inertia of this liquid. While slowing down and stopping the liquid would move inside the chamber. Cameral deposits are one of the ways to reduce this effect. It is quite evident that in the live animal these were not massive, heavy deposits, as are found in fossils, but a system of organic or weakly calcified membranes capable of absorbing and retaining significant amounts of liquid. The porous skeletons of extant sepiids are a functional analogue of such a structure. Liquid tied within the micropores does not have its own inertia and does not impede active movement. In straight shells with a non-marginal siphuncle (Orthocerida, Pseudorthocerida), in which the cameral deposits were present on the ventral side, the siphuncle retained its function. In bactritids, which had a siphuncle in contact with the ventral wall, the development of the cameral would have precluded the siphuncle from functioning normally. Evidently, in the horizontally orientated bactritid shell, in which the chambers are filled with liquid, the siphuncle would have also become dysfunctional. A straight longiconic shell has very high buoyancy, which increases as the shell grows. An animal lacking a mechanism for increasing the weight of the shell would have to have lived very near the water surface, possibly even with the apical end sticking out of the water, or to float on the surface in a non-orientated position. Clearly, this situation is highly improbable. Therefore, a mechanism for increasing the shell weight and for buoyancy control in bactritids must have existed, but was apparently different from those discussed above. A marginal siphuncle is different in that the functional epithelium of the siphuncle responsible for filling the chambers with liquid and its removal is directly connected with the organic membranes covering the shell wall and septa (in taxa with a non-marginal siphuncle this connection is performed through connecting rings, which in primitive forms are quite complexly built). Soaking of liquid by the organic lining of the shell walls and the septa may decrease buoyancy, whereas the direct contact of the porous organic membranes with the siphunclular epithelium makes this control easier. The curvature of septa resulting in development of lobes and saddles increase their surface and potential for buoyancy control. The increase in weight of the phragmocone resulting from the liquid tied in the membranes and the minimal quantity of free liquid in the phragmocone chambers, which, due to its inertia, could have hampered active movement, promoted the propulsive swimming. It is possible that this means of buoy-

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Fig. 1.9. Late Devonian species of the order Discosorida with a straight and weakly curved shell: (1) Vertorhizoceras rapidum, (2) Flowerites austririphaeus, (3) Vertorhizoceras ivanovi, (4) Kadaroceras inausum.

ancy control was inherited by ammonoids from the ancestral bactritids and is the most probable reason for the septa (and sutures) of ammonoids and nautiloids being so different (Barskov, 1999). In this respect, bactritids are closer to ammonoids than to orthocerids, and can justifiably be assigned to the same superorder as ammonoids (as in most western literature), or be recognized as a separate taxon of the same taxonomic rank as ammonoids (as in the Russian cephalopod literature; see Zhuravleva and Shimansky, 1961; Shimansky, 1979; Shevyrev, 2005). Order Discosorida Flower, 1950 (Middle Ordovician–Late Devonian). The body plan of the most diverse Paleozoic orders, Discosorida and Oncocerida, was formed based on the curved shell shape. Evidently, the problems of maintaining stability and buoyancy control in a curved shell are completely different from those in a straight and coiled shell and are different in exogastric and endogastric shells. Discosorids origi-

nally had an endogastric shell with elements of the siphuncle convexly extending inside the chambers, whereas oncocerids had an exogastric shell, with elements of the siphuncle originally almost cylindrical. As the curved shell continued to grow, its center of buoyancy was displaced more and more apically, and the animal with an endogastric shell in the absence of additional mechanisms of stability control should inevitably have turned upside down, with the funnel above the head. The inconvenience of this position is self-evident. To avoid that, early discosorids elaborated the mechanism of maintaining stability of the shell in “the normal” position by accumulating deposits inside the siphuncle (polyptychocones), which formed the lining of the connecting rings in the apical regions of the phragmocone. In this case, the shell was balanced like a scale, i.e., here were two centers of gravity: in the body chamber and in the apical zone of the phragmocone. The center of buoyancy was somewhere in


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between. This organization imposed serious limitations on the possibility of active swimming. The shell could only function as a passive float. A normal position of these animals is hypostomic, and many such forms had a narrow aperture. Two trends may be recognized in the evolution of Discosorids: maintenance of structures for passive floating and an inactive mode of life, which suggest a weak propulsive mechanism. The shell became widely conical (the apical angle increased). The second trend lead to the acquisition of the weakly curved, almost straight shell, more adapted for active swimming. The latter approach was realized at the end of the order’s existence in the Late Devonian and resulted in the appearance of a number of genera with various kinds of straight shell (Fig. 1.9). Order Oncocerida Flower, 1950 (Middle Ordovician–Early Carboniferous). The original shell shape is an exogastric cyrtoceracone. The continuing growth with an absence of additional mechanisms of stability control, an animal with such a shell shape had a live position promoting better possibilities for swimming than endogastric shells. However, even with this organization and shell shape, the possibilities of improvement of swimming and, hence, the development of the propulsive mechanism were limited. As early as the very beginning of their evolution in the Middle Ordovician, several major lineages were separated within oncocerids. These lineages had different morphology suggesting that they approached the problems of stability control and of active swimming differently (or rejected the active swimming). There are four initial morphological varieties (Sweet, 1964a; Zhuravleva, 1994): (1) Graciloceratidae–Oncoceratidae—exogastric cyrtoceracones with a narrow deposit-free siphuncle. (2) Tripteroceratidae—orthoceracones with a narrow deposit-free siphuncle and a characteristically subtriangular shell cross-section. (3) Valcouroceratidae—exogastric cyrtoceracones with a wider siphuncle and actinosiphonate structures within it. (4) Diestoceratidae—endogastric shell with an actinosiphonate siphuncle. The above shows that there are two groups of oncocerids with a varying siphuncle structure: the two former families have a relatively narrow siphuncle, with no endosiphuncular deposits, whereas the latter two possess actinosiphonate deposits. These structures represent radial outgrowths of connecting rings from the siphuncular wall to its middle. There are at least five types of actinosiphonate structures, different in the number, thickness, and direction of these outgrowths (Sweet, 1964a). From the formal morphological position, the differences between the actinosiphonate and nonactinosiphonate groups may suggest significant differences in the body plan, which supports their separation at the order level, as was suggested by Teichert (1933, 1939), who proposed the order Cyrtoceroidea for actinosiphonate taxa and the order Gomphoceroidea PALEONTOLOGICAL JOURNAL

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for nonactinosiphonate taxa. However, as was already suggested by Hyatt (1900), who separated cyrtoconic cephalopod shells from orthoconic and coiled shells, which were later recognized as the order Oncocerida, actinosiphonate structures develop independently, and are not a high ranked character. Similar structures are also observed in later representatives of another order (Discosorida). Three morphogenetic trends are observed in almost all of these original branches: straightening of the shell, coiling of the shell, and the development of widely conical rapidly expanding shells with narrowing and closed aperture. In the branch beginning with the Ordovician cyrtoceraconic Valcouroceratidae with an actinosiphonate siphuncle, the tendency to straightening of the shell is clearly tracked in the lineage Jovellaniidae (Silurian)–Tripleuroceratidae (Early Devonian–Carboniferous?)–Aktjubochilidae (Late Devonian). The tendency towards coiling the shell in this lineage lead to the separation of the family Naedyceratidae with a gyroceraconic and a low-trochoid shell and the Devonian genera Notoceras, Kotelnikoceras, and Lorieroceras from the family Notoceratidae. Another initial branch related to the persistent family Oncoceratidae, in the Devonian, gave rise to the family Ptenoceratidae, which included ornamented taxa with a gyroceraconic, trochoceraconic, and natuliliconic shell, which are assigned by some authors to the order Nautilida. In the Devonian this lineage also exhibited a third tendency, i.e., development of widely conical shells with a closed multilobed aperture (Trimeroceratidae). Somewhat earlier, in the Silurian, the morphologically similar forms (family Hemiphragmoceratidae) appeared in the other actinosuphuncular branch from the exogastric rather than endogastric taxa, related to the initial family Diestoceratidae. In the course of subsequent evolution the majority of the genera became adapted to a passive floating way of life by developing a short-conical shell functioning as a passive float, similar to that of discosorids. The live orientation of the animal was also hypostomic. In the adults of some taxa, the aperture was almost completely closed (Fig. 1.10), leaving only small openings for the funnel, arms, and eyes. Another change also related to the permanently hypostomic orientation was polymerization of the attachment muscles at the base of the body chamber. At the same time, taxa with a straight orthoceraconic shell appeared many times among oncocerids, suggesting that active swimming was becoming more important. The acquisition of the coiled planispiral shell in cephalopods solved two problems: it retained the compactness of the shell during the long period of growth, and maintained the more or less stable position of the centers of buoyancy and gravity, which facilitated maintenance of the orientated position in the water. In the evolution of cephalopods, taxa with a coiled shell

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Fig. 1.10. Oncocerida with an almost closed aperture: (1) genus Octamerella, (2) genus Inversoceras; (a) apertural and (b) lateral views.

appeared at least five or six times, at various taxonomic levels. The appearance of a few taxa with a coiled shell in several lineages in the order Oncocerida did not result from the initial body plan, in contrast to the groups discussed below, in which the coiled shell was a basis for their subsequent evolution. Later these groups could also include forms with other types of organizations, including orthoconic and cyrtococnic, but the initial morphology of those was, in contrast to Oncocerida, a coiled shell. Paleozoic taxa with the originally coiled shell, are assigned either to one order Tarphycerida (Balashov, 1962), or to two orders Tarphycerida and Barrandeocerida (Moore, 1964), or Tarphycerida and Lituitida (Starobogatov, 1983, Shevyrev, 2006a). Are the functional features of members of the three orders sufficiently different for their morphological implications to be recognized as separate body plans and for the taxonomic substantiation of the order rank?

The structure of the siphunclular cover (thick, multilayered connecting rings) and its relatively large diameter at least in early representatives of in early Tarphycerida and Lituitida, was inherited from primitive Ellesmerocerida, suggesting an incomplete exchange function of the siphuncle. In this respect these groups were at an “Ellesmerocerid” stage of buoyancy control. This imperfection of the exchange function of the siphuncle became an impediment offsetting the advantages of the coiled shell. In the process of evolution, both orders experienced reversed morphological evolution and returned to the morphology of their ancestors: to the curved or even straight shell (genus Rhynchorthoceras), which is particularly distinctly observed in the morphological changes throughout shell ontogeny. At early stages the shell is coiled, later it becomes progressively uncoiled and eventually almost straight for a most of its length. This is especially clearly observed in Lituitida (Fig. 1.11). In Tarphycerida the shell uncoils at the end of the last volution. In all tarphycerids, preservation permitting, the apertural part does not overlap the last whorl. This morphology suggests that these ammonoids, because of the way their shells were built, could not maintain a position when the axis of the jet would be on the same horizontal axis as the center of gravity of the animal. This excluded the possibility of jet propulsion in their movement. The coiled part of the shell is solely a compact float. Thus, members of Tarphycerida and Lituitida, if they did not have additional mechanisms for stability control, could have only been passively floaters that were hypostmously orientated (aperture facing down). The presence in many such taxa of a narrowed or closed hypostome confirm this very clearly (Fig. 1.12). However, at least some of them developed cameral deposits which provided the animal with an additional stability and orientated position suitable for propulsion (Fig. 1.13). This mechanism of orientation is the most efficient for taxa with a straight shell and its appearance in coiled cephalopods is a unique feature, which is only found in Tarphycerida and Lituitida, and is unknown in later taxa with a coiled shell (Natulida, Ammonoida, some families of Oncocerida) and even in the order Barrandeocerida, direct descendants of Tarphycerida.

Fig. 1.11. Heteromorphic shell of the genus Ancistroceras (Lituitida). PALEONTOLOGICAL JOURNAL

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1b Fig. 1.12. Genus Ophioceras (Tarphycerida) with a strongly narrowed aperture.

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Fig. 1.14. Barrandeocerida with a trochoid shell: (1) Sphyradoceras, (2) Piesmoceras; (a) apertural and (b) lateral views.

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Fig. 1.13. Cameral deposits of Lituitida. (1) Lituites and Tarphycerida: (1a) longitudinal section and (1b) lateral view. (2) Centrotarphyceras, (3) Aphetoceras, (4, 5) Curtoceras, (6) Campbelloceras.

Representatives of the order Barrandeocerida Flower, 1950 (Middle Ordovician–Late Devonian), which had a coiled shell (like their ancestral group Tarphycerida) and Lituitida display the following fundamental features of their organization. The connecting rings are thin, and have a diameter and structure similar PALEONTOLOGICAL JOURNAL

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to those in the later taxa, suggesting a more advanced exchange function of the siphuncle. Originally, the shell of Barrandeocerida had a larger expansion rate. This is essential because in this case the body chamber, given all other variables unchanged, has a larger volume and is isometric, rather than vermiculate in shape. This suggest a larger mantle cavity, allowing a large mass of propulsive muscles. These features suggest that Barrandeocerida was a more advanced group. The adaptive radiation of this order at the end of the Ordovician and during the Silurian and Devonian led to the appearance of taxa with a heteromoph shell morphologically similar to that in the earlier genera of Tarphycerida and Lituitida. In addition, these were the first cephalopods with a trochoid, rather planispiral shell (Fig. 1.14). A trend toward coiling into a compact flat spiral to a full extent was realized in the order Nautilida Agassiz,

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1847 (Early Devonian–Recent), the major families of which directly or indirectly evolved from Oncocerida (Kümmel, 1964, p. K412). However, in contrast to all other orders, the principle characters of the nautilid body plan (compact planispiral shell, thin subcentral siphuncle) were not acquired instantaneously. Their acquisition was a very long process, which was eventually completed in the Late Paleozoic (Shimansky, 1979). The assignment of the Devonian genera with an uncoiled shell to Nautilida is not unequivocally based, whereas the origin of the Late Paleozoic true Nautilida is unknown (Kümmel, 1964, p. K412). The earliest members had a cyrtoconic, gyroconic, loosely trochoid, and almost straight shells, which are not characteristic of typical Nautilida. The siphuncle in most nautilids was ventral. All these parameters position these taxa closer to their contemporary Oncocerida. Essentially, the only character that unites all these genera and distinguishes them from Oncocerida is the presence of specific ornamentation in the form of wing-like lateral protrusions. Ammonoids Functionally, the origin of ammonoids is related to a change in the mechanism of maintaining stability and orientation and in the change in buoyancy control. The hypothesis of their origin from the order Bactritida (a small group of subclass rank or within the subclass Ammonoidea) is currently widely accepted. The key features of the ammonoid body plan can be described in three major morphological parameters: coiled shell, complex suture, and the presence of a protoconch. As shown above, coiling of the shell has an obvious function, i.e., retention of a compact shell throughout the long growth period and facilitating stability control by approximating the centers of gravity and buoyancy and the position of indifferent balance. The tightly coiled ammonoid shell was formed very quickly (within a single zone in the Middle Emsian). As mentioned above, the same process in Nautilida continued through the most part of the Devonian and in the Carboniferous. The development of complexly fluted septa (complex sutures) is the main cluster in ammonoid evolution. Many explanations (including mathematical) of the functional significance of the complex suture have been proposed, but the seemingly most convincing are those connecting the complex septa and, hence, sutures by enforcing the shell to resist hydrostatic pressure. It is likely that a complex septum and part of the septum next to the shell wall, which is observed as a sutural outline reinforced the shell. However this could only have a functional sense if the chambers of the phragmocone were empty or were filled with gas at a pressure lower than the pressure of the surrounding water. If the chambers, as in the modern Nautilus, contained gas and fluid, the development of such complex septa would be superfluous. The shell of Nau-

tilus with its simple septa and suture was shown experimentally to be able to resist up to 50 hPa, which equates to a depth of 500 m. Ammonoids were unlikely to have inhabited great depths. The increased complexity of septa cannot be convincingly explained by the necessity of shell reinforcement only and therefore the evolutionary trend to increased complexity displayed by most ammonoids cannot be explained from this point of view either. Barskov (1999) proposed a functional explanation of the appearance of fluting in ammonoid as a measure of buoyancy control (major problem for cephalopods with an outer shell), which was in various ways solved in the above mentioned groups, that makes it possible to understand why this particular character became important in the evolution of ammonoids (main cluster in Ruzhencev’s terminology). The posteriorad curvatures of the septa (lobes in the suture) played the role of reservoirs for liquid that controlled the shell buoyancy. Ammonoid orders are distinguished based on the differences in the incipient curvatures of the septum at the first, postembryonic planktonic stage, mainly in the number of lobes in the primary suture and pathways of their subsequent differentiation, in the shape of spherical or spindle-like protoconch and the body chamber of a single low whorl. The number of the primary lobes and mode of their subsequent subdivision apparently had functional significance only at these first stages of postembryonic development. The type of suture is determined based on the primary suture and several subsequent septa. The hatching larva formed the second septum (shown on the shell surface as a primary suture). The number and arrangement of the lobes in the primary suture mainly defined the body plan of a mollusk. The original type of the suture was formed within the five first septa. The earliest ammonoids had a two-lobed suture inherited from bactrites (VO—ventral and omnilateral lobes). At the following stage the dorsal lobe appears (VO : D). Further on the omnilateral lobe is replaced by the umbilical lobe, and the sutural formula becomes VU : D. The two following elements to appear are the external lateral lobe L (initial formula VLU : D) and internal lateral lobe I (VU : ID, VLU : ID, and later VL : ID), which appear in different orders almost simultaneously. All subsequent modifications of the suture develop on the basis of these five main lobes, which determine the separation of the subclass into orders. According to this classification (Bogoslovskaya et al., 1990; Leonova, 2002; Shevyrev, 2006b; etc.), Anarcestida Miller et Furnish, 1954 (Devonian) is the earliest ammonoid order. Anarcestida gave rise to other orders of Ammonoidea. The first Anarcestida appeared in the Early Devonian (Emsian) and rapidly reached considerable diversity (37 genera are known from the Emsian only). In general, the diversity of Anarcestida was very high and included 114 genera, from the Emsian to the Famennian, inclusive. The order included


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Table 1. Modern system of the subclass Ammonoidea Order/suborder

Primary suture

Original type of suture

Anarcestida Agoniatitina Auguritina

VO VO : D

VU : D VO VO : D (V2V1V2)O : D

Anarcestina

VU : D

VU : D

Gephuroceratina Timanoceratina Tornoceratida Praeglyphioceratida Clymeniida

VU : D VU : D VU : D

Prolecanitida Prolecanitina Medlicottiina Goniatitida Ceratitida

VU : D VU : D VU : ID VU : D VU : D VL : ID (V1V1)LU1 : ID

(V2V1V2)U : D (V1V1)U : D VLU : D (V2V1V2)LU : ID VU VLU VU U VLU : ID VU : D VLU : ID 1 VLU U … : ID (V1V1)LU : ID (V1V1)L : ID

VU

five suborders: Agoaniatitina (Emsian–Givetian), Auguritina (Emsian), Anarcestina (Emsian–Famennian), Gephuroceratina (Frasnian), and Timanoceratina (Frasnian). The initial morphotype in Anarcestida was a discoid loosely coiled shell, sometimes with an umbilical perforation (suborder Agoniatitina). The suture inherited from bactritids evolved from a twolobed primary suture (ventral and omnilateral lobes– VO—Agoniatitina) or from the three-lobed primary suture (ventral, omnilateral, and dorsal lobes VO : D— Auguritina or ventral, umbilical, and dorsal (VU : D)— Anarcestina, Gephuroceratina, Timanoceratina). Thus, the history of the evolution of the earliest order displays the transition from bactritids to ammonoids. The sutural outline was very changeable throughout ontogeny. The adult whorls of Anarcestida show almost the entire variation of the septal margin (sutural) outline that can be found in Paleozoic ammonoids in general, with the exception of complexly dissected “Mesozoic”type sutures, which appear for the first time at the end of the Paleozoic. For instance, a wide tripartite lobe appeared for the first time in the Early Devonian, in the suborder Auguritina. Later, in the Late Devonian, this character was repeated in the member of the suborder Gephuroceratina. In some Gephuroceratina the entire suture reached a very high level of complexity because of the appearance of auxiliary ventral or umbilical lobes (up to 54 lobes). In the suborder Timanoceratina the ventral lobe was bipartite. In total, the body plan in the earliest ammonoid order was characterized by a sutural formula VU : D. PALEONTOLOGICAL JOURNAL

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Sutural ontogeny VO : ID, 3–6 lobes 4 to 8 lobes, subdivision of the ventral 4 to 8 lobes, subdivision of the umbilical Up to 54 lobes Total of 6 lobes Total of 12 lobes Total of 8 lobes Total less than 12

Position of the siphon Marginal, ventral " " " " " Unstable Marginal, ventral Marginal, dorsal

Formation of umbilical lobes Marginal, ventral 8 to 22 " 14 to 50 " " Usually owing to the forma- On the adult whorls, tion of umbilical lobes marginal ventral

The shape of the shell in Anarcestida evolved very rapidly, from an advolute shell with loosely coiled whorls and evolute shell with an umbilical perforation in the Emsian to discoconic and oxyconic involute shell in the Givetian, from which time almost all known morphotypes of the ammonoid shell are known to have existed. Order Tornoceratida Wedekind, 1918 (Middle Devonian–Late Permian). The order displays a new body plan, i.e., a four-lobed primary: ventral, external lateral, umbilical, and dorsal lobes (VLU : D). Tornoceratids were fundamentally different from other groups in having a non-marginal, often unstable position of the siphuncle. The ventral lobe was simple (undivided), which is related to the non-marginal position of the siphuncle. In some tornoceratids this is observed only in late ontogeny. In the process of the evolution of ontogeny the sutural complexity increased to 6– 12 lobes in Tornoceratina and to 8 in Pseudohaloritina. The shell is variable in shape, especially in the Late Devonian and Early Carboniferous, from strongly compressed to inflated, from involute to evolute, with an isometric, narrow high or slit-like whorl cross section (suborder Tornoceratina). In late Tornoceratida, recognized as a suborder Pseudohaloritina, the shell is always more or less involute, with an almost isometric whorl cross section. In members of this group, the ornamentation is very coarse, consisting of various spines, nodes, complexly bent ribs, generally uncommon in Paleozoic ammonoids. Apparently, the non-marginal position of the siphuncle had certain advantages because it persisted in some taxa throughout several

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(a) V2V1 V2

(b)

L

U

U1 D1 D1

(c)

Fig. 1.15. Sutural ontogeny in Prolecanitida: (a) Becanites africanus C1t (Korn et al., 2003, p. 1129); (b) Synartinskia principalis P1s (Leonova and Voronov, 1989, p. 115); (c) Epicanites loeblichi C1 (Spinosa et al., 1975, p. 259).

Paleozoic epochs. Boiko (2005) suggested that the subcentral position of the siphuncle could prolong the planktonic stage in the development of the young mollusk thereby facilitating wider distribution in the basin. This phenomenon is also observed at the early ontogenetic stages of many Mesozoic ceratites and ammonites. Tornoceratida in the Devonian and Carboniferous included 76 genera, in the Permian 14–16 genera, mainly of the suborder Pseudohaloritina. Order Clymeniida Hyatt, 1884 (Late Devonian, Famennian)—the only ammonoid order with a dorsal position of the siphuncle. This order existed only in the second half of the Famennian but reached an unprecedented taxonomic and morphological diversity (70 genera). The suture developed along two pathways: VU VLU : D or VU U:D; the ventral lobe was often replaced by the ventral saddle. In some groups, the ventral saddle possesses an incipient ventral lobe, simple or bipartite. The dorsal lobe is simple, bipartite,

sometimes absent. The shell is variable in shape, mainly discoconic, from involute to evolute. Boiko (2005) suggested that the dorsal position of the siphuncle precludes fast liquid removal from the phragmocone chambers, i.e., the dorsal siphuncle does not allow efficient buoyancy control. In most clymeniids the shells were compressed and evolute. These organisms could only be planktonic and, given their dorsal siphuncle, could hardly be adapted to vertical migrations. Order Praeglyphioceratida (Famennian–Tournaisian). The main feature of organization is the presence of a broad tripartite ventral lobe. Apart from that there are also external lateral, umbilical, internal lateral, and dorsal lobes. The shell is involute and discoconic. This is a very small order (not more than 10 genera). Apparently, ammonoids of this group once again tried to solve the problem of buoyancy control by developing auxil-


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iary reservoirs on the ventral lobe. In general, this trend did not achieve further progress. Order Prolecanitida (Tournaisian–Scythian). Taxa assigned to this order (54 genera), throughout their history, occupy a particular, distinct morphometric space and form a compact group with a well-defined trend toward increased degree of whorl overlap and sutural complexity (Saunders and Work, 1997). For many years it was thought that the initial sutural type for all prolecanitids is trilobate, including a ventral lobe, umbilical lobe, and a dorsal lobe. Spinosa et al. (1975) showed that in the Early Carboniferous prolecanitid Epicanites the development of the lobe U is followed by the lobe L (VU : D VLU : ID) (Fig. 1.15). Our study of a specimen of Epicanites, kindly donated by our American colleagues, supported their conclusion. Korn et al. (2003) studied the ontogeny of the Tournaisian prolecanitid Becanites africanus and also concluded that the suture of the early prolecanitids and goniatitids developed in a similar way: primary suture VU : D third suture VLU : ID (Fig. 1.15). In younger members of the order (suborder Medlicottiina) the primary suture has an internal lateral lobe (fourth), as ontogenetic studies of the Permian prolecanitid genus Synartinskia have shown (Leonova and Voronov, 1989), the development follows the pathway VU : ID VLUU1U2..: ID (Fig. 1.15). Apparently, it is time to reconsider the well-established view on prolecanitids as on a group following the U-type ontogeny. The main cluster of evolution of the most diverse suborder Medlicottiina was the progressively increasing complexity of the top of the external saddle by the development of many adventive lobes on the ventrolateral shoulder. The number of the external and internal inner lobes also increases in phylogeny. Order Goniatitida (Tournaisian–Changhsingian) is the most taxonomically and morphologically diverse group of Paleozoic ammonoids. The total number of genera in this order is 330. The initial type the suture VLU : ID. The primary suture is VU : D. Members of the suborder Goniatitina, which included most Carboniferous and some Permian families, typically had “goniatitic” eight-lobed sutures composed of ventral, dorsal, and paired external and internal lateral and umbilical lobes. However, in some groups, e.g., in the Permian suborder Cyclolobina, the number of lobes reached 60, whereas the level of their dissection was comparable to that of Mesozoic ammonoids. Goniatites represented all size classes known in Paleozoic ammonoids, from dwarves of less than 1 cm in diameter to giants, with a diameter of more than half a meter. Like prolecanitids, goniatitids had a marginal ventral siphuncle. The shell shape varied from serpenticonic evolute (Rhymmoceras, Svetlanoceras) to oxyconic, completely evolute (Girtyoceras, Kazakhoceras) or spheroconic (Proshumardites, Neocrimites). The whorl section varied from high and isometric to low and slitlike. Naturally, this diversity of the morphological PALEONTOLOGICAL JOURNAL

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structures determined a wide range of adaptations. Goniatitids occupied all ecological niches in the Late Paleozoic basins and, unlike other Paleozoic orders (dominated by one or two life forms), their representation in major types of life forms was reasonably uniform, and throughout their evolutionary history, goniatitids inhabited the entire water column including bottom layers, and near the surface. Morphometrically, they never overlapped the morphospace of prolecanitids. Order Ceratitida (Roadian–Rhaetian). The first ceratitids appeared at the very end of the Paleozoic, at the Lower–Middle Permian boundary. Their initial sutural formula is VL : ID, although in the most primitive taxa the primary suture could be trilobite, and in advanced taxa it could be five-lobed. Despite the fact that ceratitids are related to prolecanitids in their origin, their sutural plans are different in the shape of the ventral lobes: in prolecanitids, the ventral lobe is tripartite, whereas in ceratitids it is bipartite. The earliest ceratitids had an evolute, medium-sized shell with a primitive suture. The position of the siphuncle changed throughout ontogeny, and in many taxa the siphuncle was a subcentral at early stages. In adult ceratitids the siphuncle is always marginal, ventral. Taxa of higher rank (suborders) (total number 10, and two in the Permian) are distinguished by the major types of the sutural ontogeny (Shevyrev, 2006b). In the systematics of ceratitids of the family- and genus-level, the suture does not have a decisive importance, as in the systematics of gonitatitids or prolecanitids. Shell ornamentation is considerably more widely used for Late Permian groups. The most characteristic morphological feature of the Permian ceratitites is a complete absence of taxa with a subspheroconic and spheroconic shells, widespread in goniatitids. Almost the entire diversity of Permian ceratitids was represented by more or less evolute taxa with a low and moderately wide whorl cross section, with the exception of Araxoceratidae, the shell of which had an unusual shape with a flattened or keeled venter and strongly extended umbilical shoulders (Fig. 1.16); some of these taxa we assign to the benthopelagic life form. Ceratitids lived in the water column and judging from the shell morphology were mainly inactive (planktonic life forms) or more active (nektobenthic life forms). We attempted to show that the functional explanation of differences in the body plans of the above groups may be used as a basis for recognition of separate orders. These groups, except bactritids and ammonoids, control buoyancy similar to how it is done by the modern Nautilus, i.e., by filling the phragmocone chambers with liquid. We could not find any fundamental functional differences that could be used as criteria supporting their assignment to different subclasses. The original shell shape could be used as the most likely functional character for such an assignment: orthoconic or cyrtoconic, which indeed determine fundamental differences in the orientation and stability control, but not in buoyancy control. Had this

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Fig. 1.16. Late Permian ceratitid Araxoceras with a rotoconic shell.

interpretation been accepted, it would have resulted in the acceptance of the system proposed by Zhuravleva (1972): two subclasses Nautiloda and Orthoceroda, with ammonoids and coleoids assigned to Orthoceroda. This decision seems unacceptable because it does not take into account fundamental differences in buoyancy control in ammonoids. Separation of Endocerida and Actinocerida, and also Ellesmerocerida (Shevyrev, 2005a) as subclasses does not have functional basis, more important than in other orders. It is possible that further studies will allow the recognition of fundamental differences in these groups (apart from just morphological), which will increase their taxonomic ranks. However, at present we do not have such data. Therefore in this work we subdivide groups of Paleozoic cephalopods, without giving them a formal taxonomic rank: nonammonoid and ammonoid. This subdivision is quite well accepted in the descriptive work on post-Silurian cephalopods. Ammonoids are taxonomically the more homogeneous group, and it is unlikely that they can be regarded as a separate subclass. ORDERS OF PALEOZOIC CEPHALOPODS Nonammonoids 1. Protactinocerida Chen et Qi, 1979–Cm 2.Yanhecerida Chen et Qi, 1979–Cm 3. Ellesmerocerida Flower, 1950 Cm–O3 4. Intejocerida Balashov, 1960 O1 5. Endocerida Teichert, 1933 O1–O3 6. Actinocerida Teichert, 1933 O1–C2

7. Orthocerida Kuhn, 1940 O1–T 8. Pseudorthocerida Barskov, 1968 O1–T 9. Tarphycerida Flower, 1950 O1–D3 10. Lituitida Starobogatov, 1983 O1–D2 11. Oncocerida Flower, 1950 O2–C1 12. Ascocerida Kuhn, 1940 O2–S 13. Discosorida Flower, 1950 O2–D3 14. Barrandeocerida Flower, 1950 O2–D3 15. Nautilida Agassiz, 1847 D1–R 16. Bactritida Shimansky, 1951 O1? D–T Ammonoidea 17. Anarcestida Miller et Furnish, 1954 D 18. Tornoceratida Wedekind, 1918 D–P3 19. Goniatitida Hyatt, 1884 C1–P3 20. Praeglyphioceratida Ruzhencev, 1957 D3–C1 21. Clymeniida Hyatt, 1884 D3 22. Prolecanitida Miller et Furnish, 1954 C1–T1 23. Ceratitida Hyatt, 1884 P2–T CHAPTER 2. LIFE-FORMS OF CEPHALOPODS 2.1. Concept of Life-Forms Alexander von Humboldt (1806) was the first to introduce the concept of what we now call a “lifeform.” He recognized 19 major forms of plants, which shared “physiognomic” appearance. Although the idea of “major forms” was widely used in botany as early as the 19th century, the first formal definition of life-forms for plants was formulated only a hundred years later by Warming (1908, p. 27), who wrote that a life-form is a


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form in which the vegetative body of an individual plant is in harmony with the environment throughout its entire life, from seed to death. There are several systems of life-forms used in botany, with a system proposed by Raunkiaer (1905) being the most popular. In this system the similarity of the external appearance of the plants united in one life-form is connected with the position of new buds over the ground. In zoology the idea of life-forms appeared later and was formulated for the first time by Friederichs (1930) who wrote: “one and the same life-form includes those living beings (species, generations, or stages of development) which live in similar environment and have a similar lifestyle … a tadpole belongs to the same lifeform as most fishes, whereas a frog belongs to another (cit. from Kashkarov, 1933, p. 123). Attempts to explain morphological similarities as a response to similar requirements of the environment resulted in the establishment of general systems of lifeforms as hierarchical structures parallel to taxonomic systems. Three such systems have been proposed (Gams, 1918; Friederichs, 1930; Aleev and Burdak, 1984). Aleev (1986) used the term “ecomorph,” rather than the “life-form.” He argued the necessity of introducing a new field of “ecomorphology” as one of the fundamental disciplines of general biology. Despite the apparent usefulness of building general systems of lifeforms (ecomorphs) for discussion of general problems of ontogeny, phylogeny and relationships between the organism and environment, they have not been used to solve problems of structure, initial stages or further evolution of taxa. According to Gams (1918) the characterization of a life-form (although he did not formally define this term) should be based on “epharmonic” (adaptive, ecological) characters. His paper initiated a widespread understanding of a life-form as a group of organisms, which, despite being taxonomically distant, have a similar shape which resulted from adaptation to the environment. However, until now, a widely accepted definition of, or unified criteria for recognition of, life-forms have not been available in either botany or zoology. There are at least five contradicting approaches to their recognition and understanding (Aleev, 1986). For instance, there is an understanding that each taxon, each species, represents a separate life-form (Severtsov, 1937). In contrast, Remane (1943) believed that each species cannot represent only one life-form but is a member of different systems of life-forms. Kuhnelt (1970) suggested using seven parallel systems of life-forms. From our point of view, the differences in approach and methods of recognition and understanding of life-forms reflect the differences in goals and tasks of different research. It is also certain that in different groups, the understanding and criteria of recognition of life-forms may be different. Below we cite several definitions of life-forms taken from the internet: (1) A life-form is a uniform biologiPALEONTOLOGICAL JOURNAL

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cal form characterized by common shape that is related to the development of this form and its inner structure of organs, the form appeared in certain ecological environment and reflecting the adaptations to this environment (http://www.cladonia.ru/dict.html#); (2) a lifeform is an ecological type of animals that includes representatives of different orders inhabiting similar environments and elaborating a similar lifestyle and a similar body shape (http://www.glossary.ru/); (3) a life-form of a species is an external form reflecting a mode of interactions with the environment (http://shkola.lv/index.php); (4) a life-forms is group of individuals (of different species or within one species) with similar ecological and morphological adaptations to living in a similar environment (http://enciclopaedia.ru/); (5) a life-form is a general ecological characterization of a species, genus, and any larger systematic category (http://www.aquaworlds.com/aquaculture/). The Paleontologicheskii slovar’ (Paleontological…, 1965, p. 117) defines it in the following way: “a group of organisms recognized based on common features of their appearance, reflecting adaptations to a specific environment.” Despite the differences, all the above definitions show a mutual interdependence of three groups of similarities on which the recognition of life-forms is based: (1) similar morphological appearance (shape); (2) similar lifestyle and consequently similar physiological characters; (3) living in a similar environment as the reason for the two above group of similarities. Differences in definitions depend on which group of these similarities is given priority. In the original definitions of a life-form both in botany and zoology, priority is given to similarity in the lifestyle and biotope, and to a lesser extent, to morphological similarity. Each taxon has its own specific historically evolved system of environmental adaptations. Organisms inhabiting the same biotope and with a similar lifestyle are not necessarily morphologically similar. For instance, pelagic microplanktonic forms have different morphology (compare foraminifers, radiolarians, ostracodes, and diatoms). The same applies to meso- and macroplanktonic organisms. For instance, the morphology of pelagic pteropods that have a similar lifestyle may be quite diverse. Among modern cephalopods, planktonic octopuses, squids, and spirulids are morphologically different. Also, similar morphology does not necessarily indicate the same biotope, or a similar lifestyle. For instance, note the similarity of the shell in the pelagic Jantina and benthic Natica and many other examples. Certainly, in all cases, morphological characters have certain ecological adaptive significance. In the ecological sense, a life-form is a group of organisms recognized based on their affinity to certain ecotopes, and in general to a particular space in the ecosphere. Life-forms are recognized among the presently living organisms primarily based on direct observations of organisms inhabiting certain environments, of their lifestyle and liaising similarities in morphological expression of adaptations with this ecotope and lifestyle. For

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fossil taxa, the determination of their affinity to a certain adaptive zone and interpretation of their lifestyle are based on functional interpretation of morphology of skeletal remains and also on their affinity to certain facies and on taphonomic observations. Recognition of life-forms within taxa of any rank is important and useful because firstly, it allows characterization of the ecological structure of a taxon and tracking of historical changes in the geological past, the understanding of the ecological background of the phylogeny and the taxon’s ecogenesis. Secondly, the evaluation of the taxonomic and morphological diversity in a region or basin allows identification of the morphological and taxonomic “richness,” proportions and relative size of adaptive zones in this region or basin. Thirdly, tracking chronological changes in the ecological structure of taxonomic groups allows evaluation of changes in the spatial distribution and the size, structure, and proportions of ecotopes in the system of the Earth’s ecosphere in the past. Of numerous general definitions of the life-form, the definition proposed by Krivolutskii (1971) is the most appropriate in this analysis of fossil cephalopods. The life-form is identified as an adaptive type formed “among representatives of a single taxonomic group (although of high rank), when characters of considerable convergent similarity appear in different branches of this group when these representatives inhabit a similar environment.” When talking about marine habitats “a similar ecological environment” is understood as the same adaptive zone. The term “adaptive zone” identifies a certain space (pelagic or benthic zone) with its physical parameters in combination with a certain lifestyle in this space. Hence, the first level of the hierarchy of life-forms is their division into benthic (epifaunal, infaunal, psammon) and pelagic (nekton, plankton) groups. Further division may be based on the trophic type (suspension feeders, detrital feeders, scavengers, predators of various kind, etc.), on degree and type of mobility and any other parameters, depending on the taxonomic group and targets of study. In this paper, while studying lifeforms of fossil cephalopods, we are naturally confined to the two first hierarchic levels of life-forms. Taxonomic genus is chosen as a major unit of lifeforms, when analyzing ecological specialization of cephalopod orders and its changes in the Paleozoic epochs, whereas the changes in the ecological structure of cephalopod communities inhabiting the Uralian Paleobasin are studied at the species level. 2.2. Life-Forms of Cephalopods Dollo (1912, 1922), a founder of ethological paleontology, was the first to recognize ecological groups of fossil cephalopods corresponding to the term “lifeforms” (Fig. 2.1). Dollo’s conclusions on ecology and lifestyle of fossil cephalopod groups are now only

interesting from a historical point of view, but his principle of ecological interpretation, in which genera are assigned to plankton, nekton, or benthos based on morphological analysis retains its full importance. Dollo’s ideas correspond more to reality than a widespread misconception that is perpetuated in text-books and even some scientific papers (e.g., Kröger, 2005) that all ammonoids were active nektonic predators. The latter is not only incorrect and cannot be assumed taking into account the design of the outer shell of cephalopods functioning as a gas-liquid float, but simply contradicts common sense. The first systematization of extant cephalopods into adaptive zones was proposed by Voos in 1967; he recognized epi, meso-, bathy-, and abyssopelagic lifeforms (cit. after Nesis, 1973). These groups were based on the habitats (pelagic or benthic zones) and depth, i.e., immediately observed occurrence in certain adaptive zones. Other ecological and morphological features were not analyzed. This approach is evidently not suitable for fossil forms, for which affinity to a specific adaptive zone can be mainly based on the morphological and functional analysis. Nesis (1973) proposed a detailed classification of life-forms of extant cephalopods. Nesis’s classification is based on the dichotomous hierarchical principle, with a taxonomic genus as a classification unit. At the uppermost hierarchical level all cephalopods are divided into inhabitants of benthic zone, shelf, and pelagic zone. The lowermost level contains 23 generic ecological groups. Apart from sharing the same biotope, genera belonging to the same ecological group share similarities in the degree of mobility and motion mechanism, and feeding strategy. Morphological characters are included in the characterization of groups, but are not used as criteria of classification, although this is possible. Nesis’s 23 ecological groups are not equally significant life-forms, and in the later publications Nesis reduced their number to five (Nesis, 1975, 1976) (benthic, benthopelagic, nektobenthic, nektonic, and planktonic). General characteristics of five life-forms of cephalopods (after Nesis, 1976): Benthic life-form. These animals are bottom-dwelling, mainly crawling, rarely using their hyponome. Size from small to large (the latter is rare). Solitary. Scavengers. Eggs are large, development is direct, without pelagic stages. This group mostly includes octopuses. Benthopelagic life-form. Bottom-dwelling animals, do not crawl on the bottom, but slowly move over the bottom using their hyponome for very short distances. Good at maneuvering. Scavengers and stealth predators. Development is direct, without a pelagic stage. Deep-water octopuses. The only extant representative with an outer shell is Nautilus. Nektobenthic life-form. There are two types of this life-form, the type represented by Sepia and the type represented by neritic squids.


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CEPHALOPODS IN THE MARINE ECOSYSTEMS OF THE PALEOZOIC Ectocochlia

Endocochlia

Baculites secondary planktonic

Opisthoteuthis tertiary benthic

Ammonites secondary benthic and primary nektonic

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Pyrrhoteuthis secondary nektonic

Rhabdoceras secondary planktonic Ceratites benthic and primary planktonic

Octopus secondary benthic Bactrites secondary planktonic

Doratopsid secondary planktonic

Goniatites secondary benthic and primary planktonic

Spirula secondary planktonic

Lituite secondary planktonics

Decapodes primary nektonic

Nautiluses secondary benthic Orthoceras primary planktonic Archimollusk primary benthic

Fig. 2.1. Major stages in ecogenesis of cephalopods (after Dollo, 1912, 1922).

Sepia type. These are intermediate forms between benthic and pelagic. They are closely connected with the sea floor where they spend nights, ambush, and lay eggs. Small and medium-sized. Feed on moving prey. Capable of darting forward. Ambush predators. Often live in group. Eggs large, laid on the bottom. Neritic squid type. Forms transitional to nektonic cephalopods. Live on the shelf. Connected with the bottom only to lay eggs. Fast, active predators, hunting in groups. Eggs are small. The development has a pelagic stage. Nektonic life-form. Permanent inhabitants of the pelagic zone, not connected with the bottom. Swim quickly using the hyponome. Active group predators. Eggs are small, in large numbers. The development includes a pelagic “larva” stage, which is very different from the adult animal. In modern fauna, this group is represented by oceanic squids. Planktonic life-form. These are strictly pelagic animals small, medium-sized, or sometimes large. Feed on plankton. Sometimes live in schools. Incapable of moving for long periods using the hyponome. Eggs are usually pelagic. Development is very prolonged. Larvae PALEONTOLOGICAL JOURNAL

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are often completely different in appearance from the adults. The above groups, excluding the nektonic form (actively swimming organisms capable of chasing their prey), which cannot be reliably substantiated for ectocochliate cephalopods with a gaseous-fluid buoyancy device, will be used in the discussion below. One and the same life-form may be represented by animals with different shell morphology (straight, curved, planispiral with various combinations of parameters, or heteromorphic), but with functional characteristics indicating a certain life-form. 2.3. Ecological (Adaptive) Significance of Constructive Differences of the Outer Shell in Fossil Cephalopods, Criteria and Methods of Their Assignment to Various Life-Forms The characterization of life-forms of extant endocochliate cephalopods is based on direct observations of their habitats, breeding, and feeding strategies. Similarities of their morphology, including their body shape, morphology of their hyponome, head, and mantle appendages are recorded but are not recognized as definitive criteria. However, according to the above def-

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inition of the “life-form,” it is the convergent morphological similarity that is definitive for the recognition of life-forms and assignment of cephalopod genera to one of these. The recognition of life-forms among fossil cephalopods is impossible by direct observation of their life and distribution in the adaptive zones of the sea. In addition, parameters of the adaptive zones of the sea could have been different in the geological past. Therefore the characteristics given below for life-forms of fossil cephalopods are mainly based on functional analysis of constructive parameters of the shell, mainly of those elements of the shell that have adaptive significance and are supplemented by some soft body features, which may be derived from the shell. The similarities to the morphology of extant cephalopods may be of little help, because in the modern fauna, shelled cephalopods which are morphologically similar to fossil cephalopods and have a similar hydrostatic mechanism are represented by approximately ten genera of sepiids and two genera of nautilids. Note that three major morphological types of modern shelled cephalopods (a few genera of Sepiida; Spirula; and Nautilus and Allonautilus belong to different life-forms: nektobenthic, planktonic, and benthopelagic, respectively. Of these, only the nautilids have an external shell. The recognition of life-forms when dealing with fossils is based on the shell and siphuncle morphology, which determine the main hydrostatic and hydrodynamic qualities of the animal allowing their assignment to a particular life-form. This is supplemented by some general biological assumptions, knowledge of the type of individual development and taphonomy, through which a possible environment may be suggested. The main hydrodynamic and hydrostatic characteristics, from which the constructive features of the outer shell may be inferred, are listed below. (1) Degree of buoyancy. The degree of buoyancy, i.e., the size of the gas-filled phragmocone in comparison to the size (weight) of the soft body may be estimated quantitatively based on the shell expansion rate. In straight and curved cephalopods, the degree of buoyancy depends on the function of the angle of the conical shell, whereas in the coiled cephalopods this parameter depends on the whorl expansion rate. The smaller expansion angle and lesser whorl expansion rate suggest greater buoyancy, which in most cases has to be compensated for by chambers filled with fluid. (2) Ways to support orientation in the water are often expressed in the shell shape, in the presence or absence of specialized mechanisms of orientation (cameral and endosiphuncular deposits, in the specific arrangement of septa, and in the ornamentation). For instance, the presence in straight shelled taxa of a ventral siphuncle with deposits suggest that they lived in a horizontal position (horizontal longitudinal axis) and, hence, were relatively actively swimming. Sinusoid septa positioned on the dorsal side of the shell, as in some ascocerids and modern sepiids, also suggest a horizontal

position of the body. A tightly coiled shell, in which the general center of gravity coincides with the center of buoyancy, is in indifferent balance with the water and does not need additional mechanisms of orientation. Extensive ornamentation (long spines, collars, or longitudinal ribs) is interpreted as a means of support of the shell orientation. (3) Active swimming is possible because of the development of the propulsive mechanism, which includes a variously sized and shaped mantle cavity and a hyponome. Swimming ability can be evaluated based on the proportions of the body chamber and on the degree of the hyponomic sinus on the shell. For instance, it is evident that shells with a very long, or very short, or strongly flattened body chamber could not have had a mantle cavity that would allow a powerful propulsive thrust and could not therefore be efficient swimmers or predators. (4) Streamlining of the shell may be one of the additional characteristics supplemented to the ability to swim and depends both on the shell shape and ornamentation. Different life-forms have hydrostatic and hydrodynamic qualities variously developed and combined, resulting from different shell morphology. Several fundamentally different types of shell geometry may be recognized among ectocochliate cephalopods, resulting in different shell hydrostatics and hydrodynamics. Major morphological types of cephalopod shell: (1) Straight (orthoceraconic); (2) Curved (cyrtoceraconic); (2a) Shells endogastrically curved (ventral side, on which the hyponome is positioned, is concave, while the dorsal side is convex); (2b) Shells exogastrically curved (ventral side is convex, while the dorsal side is concave); (3) Coiled in one plane (planispiral) with non-contacting whorls (gyroceraconic), with contacting whorls (nautilicones), among which, depending on the whorl expansion rate, degree of whorl overlap and the crosssectional shape, special names are used (see below); (4) Spirally coiled (trochoid), with possibly nontouching whorls, or in contact with varying degrees of overlap; (5) Heteromorphic shells, changing their shape in ontogeny; for instance, with a trochoid coiling replaced by planispiral coiling, tightly coiled whorls replaced by loosely coiled whorls, a planispiral shell replaced by cyrtospiral or orthoceraconic. Depending on geometric parameters, within virtually each of the above shell types, the hydrostatic and hydrodynamic properties may be different, and therefore, the animals could belong to different life-forms. Below an attempt is made to assign cephalopods with a straight and curved shell, with a coiled planispiral, trochoid, or heteromorphic shell to various life-forms.


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(c)

(a)

(b)

(d)

1

2

Fig. 2.2. Hypothetical orientation of exogastric (a, b) and endogastric (c, d) shells throughout growth; (a, c) juvenile shells; (b, d) adult shells (1) position of the center of buoyancy; (2) position of the center of gravity.

2.4. Life-Forms of Cephalopods with a Curved Shell Two types of shell may be recognized according to the way they are curved. In exogastric shells the ventral side of the shell is convex, and the dorsal side is concave. In endogastric shells, the ventral side of the shell is concave, and the dorsal is convex. The endogastric shell is a more primitive morphological type of the external cephalopod shell. Endogastric and exogastric shells have different potentials for ecological adaptations. In the absence of specialized mechanisms for orientation and stability control, in a floating endogastric shell, as it grows, the center of buoyancy shifts apically. In such cases, the animal was orientated with its concave ventral (hyponomic) side facing up, and the dorsal side and PALEONTOLOGICAL JOURNAL

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head facing down (Fig. 2.2). This situation facilitated scavenging on the bottom using head tentacles, but did not help active swimming. Thus, such shell construction allowed only slow passive floating above the bottom. This allows the placement of ammonoids with such shells within the benthopelagic life-forms. The construction of the exogastric shells allows broader opportunities for ecological adaptations. As the shell grew, the shell orientation changed to raise the dorsal side of the aperture upwards, whereas the ventral (hyponomic) side became horizontal (Fig. 2.2.). This orientation is fundamentally different from the one described previously, because it facilitates the use of the hyponome for swimming. An animal with an exogastric shell is capable of more active swimming over

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Table 2. Morphological criteria for recognition of life-forms of ectocochliate cephalopods with a curved shell Life-form

Apical angle

Relative size of phragmocone and living chamber

Benthic

More than 30° Phragmocone is equal to or less than living chamber Benthopelagic More than 15° Phragmocone is equal to or more than living chamber Nektobenthic Less than 15° Phragmocone is more than living chamber Planktonic More than 30° Phragmocone is more than living chamber

the bottom, which allows the placement of animals with such shells within the nektobenthic life-forms. The above can be applied to relatively narrow-conical curved shells with an apical angle not exceeding 15°. In widely conical shells the direction of curvature is not essential, since the center of buoyancy would be shifted insignificantly throughout growth, and in any case animals would only be able to have the aperture and hypostome facing down. The shell construction of such forms does not allow active swimming, and irrespective of the direction of curvature, taxa with such shells may be placed within the benthic life-forms (if the apical angle does not exceed 30°), or within the benthopelagic life-forms, or the planktonic life-form (see below). The morphological traits of curved shells, which allow the recognition of life-forms, are discussed below (Table 2). Benthic life-form. The apical angle exceeds 30°. The phragmocone volume is equal to or less than the volume of the body chamber. The aperture is widely open. The hyponomic sinus may be absent. The endosiphuncular and cameral deposits are absent. Benthopelagic life-form. The apical angle is between 15° and 30°. The volume of the phragmocone can slightly exceed the body chamber volume. The aperture is narrow, partly oblique, or closed. The size of the phragmocone may slightly exceed that of the body chamber, the aperture is narrowed, often oblique, or closed. The hyponomic sinus is present. The endogastric strongly curved shells may have endosiphuncular or cameral deposits in the apical parts of the shell. Nektobenthic life-form. The apical angle is less than 15°. The volume of the phragmocone exceeds the volume of the body chamber. The aperture is open, the hyponomic sinus is present. Mainly exogastric taxa. The endosiphuncular and cameral deposits (facilitating stability) may be present. Planktonic life-form. The main morphological character is the presence of very narrow, or closed aperture, suggesting permanent orientation with the aperture facing downwards. Mostly small in size. Widely conical shells, with the apical angle of over 30° may be barrel-

Aperture shape

Hyponomic sinus

Cameral and endosiphuncular deposits

Widely open

May be absent

Absent

Narrowed, oblique, closed Open

Present

May be present

Present

May be present

Closed

Present

Absent

shaped. Mollusks with longiconic (narrowly conical) shells, which had higher buoyancy, apparently, occupied the upper horizons of the pelagic zone, while those with breviconic (widely conical) shells inhabited deeper layers. Major types of cephalopod life-forms with a curved shell are shown in Fig. 2.3. 2.5. Life-Forms of Cephalopods with a Straight Shell The presence of a straight, relatively longiconic shell is adaptively justifiable if it can be oriented and stabilized in the horizontal position suitable for active swimming. Almost all cephalopods with a straight shell have cameral or/and endosiphuncular deposits allowing orientation and stabilization of the shell in the horizontal position. Such deposits are not recorded for the order Bactritida. Characters allowing the recognition of life-forms among cephalopods with a straight shell are listed below. Benthic life-form. The shell is medium-sized and large to very large (0.5–3.0 m). The siphuncle is marginal, wide, with massive endosiphuncular deposits. The cross-section is rounded, often compressed dorsoventrally, sometimes with a flattened venter, or lensshaped. A large protoconch suggests large eggs and direct development. Benthopelagic life-form. The shell is medium-sized. Main features: large apical angle (over 15°), the size of the body chamber equal or more than the size of the phragmocone, the aperture is narrowed, and often oblique. Endosiphuncular and cameral deposits may be present. The protoconch is usually absent. Nektobenthic life-form. This is the most common life-form. The shell is medium-sized, longiconic, with an apical angle less than 15°, the size of the phragmocone exceeds the size of the body chamber. The endosiphuncular and cameral deposits (used for stability and orientation control) are present. The initial chambers are usually small; some taxa have a subspherical protoconch suggesting small eggs and possible pelagic stages. The cross-section is rounded or compressed dorsoventrally, often with a flattened ventral side. In


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1

2 Benthopelagic

3

4

5

6

7

Nektobenthic

8

9

10

11

12

Planktonic

13

14

15

17

16

18

19

Fig. 2.3. Major type of life-forms of cephalopods with a curved exogastric (1, 2, 6–19) and endogastric (3–5) shell: (1) Burenoceras, O1; (2) Scyphoceras, P1; (3, 4) Protophragmoceras, S3; (5) Endoplectoceras, S3; (6) Blakeoceras, D2; (7) Conostichoceras, D2; (8) Oelandoceras, O1; (9) Lyeroceras, S; (10) Cyrtocycloceras, S2; (11) Bergoceras, C1; (12) Richardsonoceras, O1; (13) Phragmoceras, S2; (14) Inversoceras, S3; (15) Tetrameroceras, S; (16) Bolloceras, D2; (17) Pentameroceras, S2; (18) Mandaloceras, S3; (19) Cinctoceras, S3.

some taxa, the cross-section is compressed laterally, and these are considered as more likely to be nektonic. Planktonic life-form. Characters indicating adaptations to the planktonic lifestyle include a longiconic shell (suggests high buoyancy); presence of small iniPALEONTOLOGICAL JOURNAL

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tial parts of the shell with a protoconch suggesting numerous and possibly pelagic eggs and endosiphuncular or cameral deposits absent. The taxa with an annulated shell may also be assigned to plankton, because this type of shell is interpreted as an adaptation to the

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2c

2b

3‡

3c

1 Benthopelagic

Nektobenthic

Planktonic

7‡ 4

7b 5‡

5b

6

7c

8

Fig. 2.4. Major types of the life-forms of cephalopods with a straight shell: (1) Cameroceras, O2–3: (1a) cross section, (1b) longitudinal section; (2) Lambeoceras, O2–3: (2a) cross section, (2b) ventral view, (2c) fragment of a longitudinal section of the phragmocone; (3) Gonioceras O2: (3a) reconstruction of the shell, (3b) cross section, (3c) longitudinal section of the dorsal part of the siphuncle and chambers with cameral deposits; (4) Bridgeoceras, O1; (5) Virgoceras, S2: (5a) longitudinal section, (5b) shell, lateral view; (6) Dawsonocerina, S2; (7) Plagiostomoceras, O3–D1; (7a) longitudinal section of the phragmocone, (7b) cross section, (7c) external shell morphology; (8) Lobobactrites D3.

orientation with the aperture permanently facing downwards. Major types of the life-forms of cephalopods with a straight shell are shown in Fig. 2.4. 2.6. Life-Forms of Cephalopods with a Planispiral Shell The planispirally coiled shell with a gas-fluid buoyancy device considerably simplifies the problem of orientation and stability of the animal in a certain position: the center of gravity and the center of buoyancy may be positioned on the vertical line, or even almost coincide, as in the modern Nautilus. In that case the animal is in a state of indifferent stability. In contrast to the mol-

lusks with a straight or curved shell, there is no need for special mechanisms (deposits) to control stability or orientation of the shell in a position suitable for life. The morphological diversity of coiled shells is quite large. Attempts to describe and systematize this diversity have been made from the first half of the 20th century (Trueman, 1941; etc.). The foundation for the modern understanding of the diversity of construction of the planispiral cephalopod shell was laid by the classic works of Raup (1966, 1967). They suggested a simple and visually easy method of characterization of the planispiral shells using three measurable parameters: whorl expansion rate (W), the whorl overlap degree (D), and the proportions of the whorl cross-section (S)


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Table 3. Criteria for recognition of life-forms of cephalopods with a straight shell Life-form Benthic

Apical angle Any

Relative size of phragmocone and living chamber

Aperture shape

Frequently phragmocone is less than living chamber

Open, narrowed

Benthope- More Phragmocone is equal to or lagic than 15° less than living chamber, relatively short chambers Nektoben- Less Phragmocone is more than thic than 15° living chamber

Narrowed, oblique, closed, rarely open Open

Planktonic Any

Narrowed, oblique, closed

Phragmocone is more than living chamber, long chambers

(Fig. 2.5). Raup demonstrated the theoretically possible diversity of the cephalopod shell shapes and its realization by ammonoids. Raup’s data were based on measurement of 405 ammonoid genera including 44 Paleozoic, and 361 Mesozoic genera with a planispiral shell (Fig. 2.5). The major results of Raup’s study may be summarized as follows: (1) Ammonoid shells occupy a large morphospace with various combinations of whorl expansion rate and whorl overlap degree values, but virtually do not cross the line marking the area separating the shells with whorls that are not in contact (W = 1/D). 0

0.2

1

Cameral and endosiphuncular deposits

Other characters

Massive endosiphuncular deposits are present May be present

Usually large and very large size (50–150, up to 300 cm)

Present

Frequently dorsoventrally compressed section and/or flattened venter Usually small size and the presence of protoconch, annulated ornamentation

Absent

(2) The general distribution is unimodal: most ammonoid genera are grouped around one modal area (W ~ 2, D = 0.3–0.4). (3) Some taxa (as has been shown for Paleozoic goniatites and Mesozoic lithoceratins) occupy different morphospaces, suggesting different adaptations and habitats. (4) The combination of parameters of the modern Nautilus places it outside the morphospace typically populated by ammonoids, and it is not clear whether this is accidental or results from different adaptations

0.4

0.6

D

W = 1/D 2

a 3 b d

+

c 4 W

e (a)

(b)

Fig. 2.5. (a) Planispiral shell morphospace density contours showing the distribution of 405 ammonoid genera with various combinations of W (whorl expansion rate) and D (whorl overlap degree) (after Raup, 1967). The density contours are based on measurements of one specimen of the type species of the genera included; (b) measurements of the shell: W = (d/e)2, D = c/d, S = b/a (after Raup, (1967). A cross shows a position of the modern Nautilus pompilius. W = 1/D line separates the area of shells with whorls not in contact. PALEONTOLOGICAL JOURNAL

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0.1

0.2

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3

1.5

0.6

0.7

D 0.8

6 9

1A'

12

12 15 18

1A

2.0

0.5

2A 18

21

3A 15

15 9

6A

5A

2.5 4A

3.0 3.5 4.0

1

1

W = 1/D

4.5

5.0 W Fig. 2.6. The distribution of the shell shape in relation to their W (whorl expansion rate) and D (whorl overlap degree) values for the Paleozoic ammonoid genera. Solid lines are the density contours based on the W and D combinations of the type species of a genus. (1A–6A) major morphospaces.

achieved in the evolution of Nautilus, or perhaps all nautiloids (Raup, 1967, p. 51). Numerous studies have been published on the diversity of cephalopod shell geometry, its evolutionary changes, and functional and adaptive significance (Barskov, 1976, 1988, 1989; Ward, 1980; Chamberlain, 1976, 1980, 1981; Crick, 1983; Bayer and McGhee, 1984; Saunders and Swan, 1984; Saunders and Shapiro, 1986; Swan and Saunders, 1987; Nikolaeva and Barskov, 1994; Saunders and Work, 1996; Nikolaeva, 1999c; Saunders et al., 1999, 2004; Korn, 2000; Korn and Klug, 2003; Boiko, 2006; Konovalova, 2006; Kiselev, 2006; etc.). Data on Paleozoic ammonoids (597 genera of Agoniatitida, Goniatitida, Prolecanitida, and Ceratitida) published by Saunders et al., 2004, which we supplemented to the total of 648 genera, give the most complete information on the diversity of planispiral ammonoid shells in these orders. Data on planispiral shells of nonammonoid cephalopods (orders Tarphycerida, Barrandeocerida, Lituitida, Nautilida, and several genera of Oncocerida) are published by Barskov (1976, 1989) and Ward (1980). The main result of the above studies is the description of the irregular distribution of the shapes of planispiral shell is the morphospace defined by Raup’s parameters both among ammonoids and nonammonoid cephalopods.

The diagram of the W/D total distribution for Paleozoic ammonoids contains six areas of the predominant geometrical shell shape (Fig. 2.6), and five such areas for nonammonoid cephalopods (Fig. 2.7). Although their general morphological morphospaces coincide by the whorl overlap degree values (D), the areas defined by different values of W may be significantly different (Fig. 2.8). The peaks of modal distributions (morphospaces) of ammonoid and nonammonoid cephalopods were given different names by Saunders et al. (2004) and Barskov (1979, 1989), respectively. In this paper, to facilitate comparison, these morphospaces are renamed, and their approximate correspondence in the values of the parameter D is shown in Table 4. As justifiably assumed by previous workers, the presence of the modal peaks cannot be put down to optimization of a single function (Raup, 1967); however, it reflects a combined influence of hydrostatic and hydrodynamic properties, phylogeny, and external factors (Saunders et al., 2004) and differences in the habitats and lifestyle (Raup, 1967). Saunders et al. (2004, p. 33) accepted a certain degree of luck in the sense of Gould (1989), although acknowledging that this factor cannot be reliably recorded or documented. Thus, the fact that certain taxa belong to the same modal morphospace defined by similarity of shell geometry, in their adaptive character reflecting their affinity to the


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0.1

0.2

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1197

D

1.5 8 4

3N

6

2.0

10

1N

2.5

2N

10 8

3.0

8

4N

2

3.5 6

(a) 4.0

5N

W = 1/D

W 0 1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

D

1.5 3N

2.0 3

1N

2.5

3.0

3 4

2N

4N 1 2

3.5 1

(b)

4.0 1

W = 1/D

W

Fig. 2.7. The distribution of the shell shape in relation to their W (whorl expansion rate) and D (whorl overlap degree) for (a) all known genera and (b) Paleozoic genera of nonanmmonoid cephalopods. Solid lines are the density contours based on the W and D combinations of the type species of a genus. (1N–5N) major morphospaces.

same adaptive zone, i.e., to one of the life-forms in terminology accepted in this study. One of the certain indications that morphospaces reflect the affinity to certain life-forms is the fact that different taxonomic groups of various rank could occupy different morphological regions. This, in turn, supports the adaptive nature of evolution. PALEONTOLOGICAL JOURNAL

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Reyment’s (1973) experiments opened possibilities for adaptive interpretation of the shell shape described by Raup’s parameters. It was shown that the three major (geometric) shell types have different hydrostatic properties: involute, rapidly expanding (typified by Nautilus, modal space 4N of nautiloids); evolute, slowly expanding (typified by Dactylioceras, modal space 3N

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0.1

1.0 1.5

0.2

0.3

0.4

0.5

0.6

1A'

W = 1/D

2A

3A

1A

2.0

D 0.8

0.7

3N 6A

1N

2.5

5A 2N

4A

3.0

4N

3.5 4.0 4.5 5.0 W 1

2

3

Fig. 2.8. The distribution of the shell shape in relation to their W (whorl expansion rate) and D (whorl overlap degree) for Paleozoic cephalopod in total. Explanation: (1) ammonoids, (2) nonammonoid cephalopods, (3) position of major morphological peaks. (1A–6A) morphospaces of ammonoids, (1N–4N) morphospaces of nonammonoid cephalopods.

of nautiloids and 3A of ammonoids); and semi-involute moderately expanding (typified by Ceratites, modal space typical of Mesozoic ammonites, similar to modal space 2N of nautiloids and modal space 6A of Paleozoic ammonoids) (Fig. 2.9). Later it was shown (Chamberlain, 1980; Saunders and Swan, 1984; Saunders and Shapiro, 1986; Swan and Saunders, 1987; Jacobs, 1992; etc.) that Raup’s parameters allow evaluation of other shell parameters, such as the length and shape of the body chamber, position of the aperture, streamlin-

ing, strength, stability and the possibility of active swimming. Even a superficial comparison of the two graphs (Fig. 2.8) shows that the morphospaces populated by ammonoids and nonammonoids are significantly different. More than 70% of Paleozoic ammonoid genera have the value of the whorl expansion rate parameter (W) less than 2, whereas in the majority of the genera of nonammonoid cephalopods this value is over 2, and in post-Paleozoic taxa over 3, the value rare among

Table 4. Unification of the designations of the modal morphospaces of shell shape Saunders et al., 2004 A A B C D E F –

Used in this paper 1A' (D = 0–0.15; W = 1–1.75) 1A (D = 0–0.15; W = 1.71–2.3) 2A (D = 0.15–0.35; W = 1–2.1) 3A (D = 0.35–0.8; W = 1–2.3) 4A (D = 0–0.15; W = 2.31–3.3) 5A (D = 0.125–0.25; W = 2.1–3.3) 6A (D = 0.25–0.4; W = 2.1–3.3) –

Barskov, 1979, 1989 – C – A – – B D

Used in this paper – 1N (D = 0–0.15; W = 1.7–2.7) – 3N (D = 0.4–0.7; W = 1.5–2.5) 4N (D = 0–0.15; W = 2.7–3.5) – 2N (D = 0.15–0.4; W = 1.7–3.1) 5N (D = 0–0.1; W = 3.35–4.0)


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shells from morphospaces 3N, 4N, and 2N, because forms typifying these morphospaces were studied by Reyment. These are followed by shells from region 1N. The sequence of consideration of the types of the ammonoid shells follows the sequence of the morphospaces numbers: 1A to 6A.

(a)

(c)

(b)

Nonammonoid cephalopods. Note that for Paleozoic nonammonoid cephalopods morphospaces 1N and 4N were represented by only a few genera, whereas for the Mesozoic nautilid genera, including Triassic, these morphospaces became exclusive.

(d)

Fig. 2.9. Buoyancy and stability of spirally coiled shells with different whorl expansion rate and their whorl overlap degree; (a, b) evolute shells with a low whorl expansion rate (typified by Dactylioceras): (a) position of the shell in the water, if the body chamber is less than one whorl, (b) position of the same shell, if the body chamber is one and a half whorls; (c) involute shell with rapidly expanding whorls (typified by Nautilus); (d) involute shell with moderately rapidly expanding whorls (typified by Ceratites) (after Reyment, 1973).

ammonoids. Because the expansion rate is functionally connected with the shell hydrostatics (Reyment, 1973; Saunders and Shapiro, 1986), these differences, in our opinion, may be related to differences in the physiology of these two groups mainly related to buoyancy control (Barskov, 1999). Below we evaluate the adaptive significance of the functional possibilities of the shells from different morphospaces among ammonoids and nonammonoids. The sequence of the evaluation is as follows: first we discuss

1.0

0

0.1

0.2

0.3

0.4

Morphospace 3N (Fig. 2.10). These are evolute shells (D = 0.4–0.7) with a low whorl expansion rate (W = 1.5–2.5). As shown by Reyment (1973), when the body chamber is less than one whorl (Fig. 2. 9) buoyancy is so high and stability is so low that the shell can only float on the water surface in the subhorizontal position. To go deeper in the water and turn into the vertical position such a shell should have a body chamber of more than one and a half whorls or a similar volume of fluid in the phragmocone. In members of the order Tarphycerida, most of which populated this morphospace, in contrast to ammonoids from the same morphospace (see below), the body chamber did not exceed one whorl. In addition, in these taxa the apertural part of the body chamber was turned outwards (Trochoceras, Shumardoceras, etc.) from the remaining part of the last whorl, while the aperture was often narrow and even almost closed (Ophioceras) 0.5

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Morphospace 3N W = 1.5–2.5; D > 0.4 Planktonic life-form

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1N 2.5

3.0

3 (a)

4

2N 4N 1 2

3.5

(b) 1

4.0 1

W

W = 1/D (c)

Fig. 2.10. Major life-forms of nonammonoid cephalopods with a planispiral shell. Morphospace 3N: (a) Ophioceras S3, (b) Discoceras O2–3, (c) Millkoninckioceras C1–P. PALEONTOLOGICAL JOURNAL

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1 2

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Fig. 2.12. Major life-forms of nonammonoid cephalopods with a planispiral shell. Morphospace 2N: (a) Parastenopoceras P1, (b) Foordiceras P, (c) Heurekoceras P1, (d) Tylonautilus C1–P.

tion of the shell in any position needed for normal life did not require any additional mechanisms for orientation or stability control (such as endosiphuncular deposits, etc.). The shape and volume of the body chamber suggest the presence of relatively large soft body with a spacious mantle cavity. The presence (in most cases) of a hyponomic sinus suggested an advanced mechanism of active swimming. However, the shell geometry responsible for indifferent balance at the same time precludes fast prolonged swimming, although allowing short jerks. The lifestyle and swimming of Nautilus have been extensively observed and studied in captivity and in the wild and is largely a product of shell geometry and morphology, which are responsible for adaptive constraints. Judging from observations on Nautilus, fossil nautiloids, from a similar domain of the total cephalopod morphospace are considered to have been relatively deep-water near-bottom-dwelling pelagic animals (benthopelagic lifeform). They are likely to have been scavengers, carrionfeeders, incapable of active predation or hunting. Characteristically, the shell in this group does not possess strong ornamentation (Barskov, 1989; p. 60, text-fig. 27). In the Carboniferous and Permian, this morphospace contained several genera (Fig. 2.11), which had a discoconic laterally compressed shell. Presumably, these forms were more maneuverable than those with a pachyconic or subspheroconic shells. The modal area 4N for the first time appeared in the Carboniferous, and in the Paleozoic it was represented by seven or eight PALEONTOLOGICAL JOURNAL

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genera only. Beginning from the Triassic, such forms, similar to the forms from morphospace 5N became the sole morphotype used by nautilids. Morphospace 2N (Fig. 2.12). This morphospace includes moderately involute shells (D = 0.15–0.4), with a moderate or high whorl expansion rate (W = 1.7– 3.1, mean value of 2.5). Such shells have “intermediate” hydrostatic and hydrodynamic properties between the shells of types 3N and 1N. They had neutral buoyancy and were reasonably stable. This morphospace of nonammonoid cephalopods is close to one exploited by the majority of the Mesozoic ammonoids, but was almost entirely vacated by ammonoids throughout most of the Paleozoic. There are no extant equivalents of this shell type. It is possible to assume some functional and adaptive constraints determined by the shell geometry. The average whorl expansion rate, in contrast to shells from morphospace 3N, allows higher diversity in the morphology and size of the body chamber and, hence, of the soft body. These shells include representatives with both long and short body chambers. The most characteristic feature of these forms is the fact that 90% of them have a well-developed ornamentation. Among involute forms from morphospaces 1N and 4N, ornamented forms are rare (Barskov, 1989, p. 60, textfig. 27). The most widespread type of ornamentation is transverse (radial) ribs or nodes. Opinions of functional significance of ornamentation differ: from strengthening of the shells to withstand the external pressure or defensive structures (Ward, 1981) to indications of sex-

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ual dimorphism, and even lacking any immediate adaptive significance. Possibly, all these explanations make sense. In the context of buoyancy and stability, ornamentation (both radial and transverse) is explained as being used for stability control, which facilitates linear forward motion. The stabilizing role of the radial (spiral) ornamentation of the shell is obvious, and less so for transverse ornamentation. Transverse ribs, which are vertically orientated, reduce rocking in the forward– backward direction, while horizontal ribs may play the role of hydroplanes stabilizing the shell laterally. Cephalopods representing morphospace 2N, judging from the presumed shell hydrostatics and hydrodynamics, were capable of active swimming (representatives of this morphospace are known to have large muscle scars, suggesting the possibility of strong propulsive thrust), and were stable while moving. They were certainly more pelagic and nektonic than other morphotypes. However, the very presence of the outer conch and its planispiral geometry precludes efficient swimming in the sense of modern nektonic organisms and cannot therefore be interpreted as nektonic. It is possible that the taxa with the shell geometry allowing their placement in morphospace 2N had many adaptive opportunities. Depending on the extent of the development of the mantle cavity, shape of the soft body, and the cross-sectional shape, they could have been relatively active in the pelagic zone, and could be referred to the nektobenthic life-forms. Possibly, they were to a

lesser extent connected to the bottom than the benthopelagic taxa from morphospace 4N, and it is possible to suggest a large trophic range, allowing predation on small-sized swimming prey. Some shells representing this morphospace and demonstrating high whorl expansion rate values and wide cross-section (S < 0.8) can possibly be referred to the benthopelagic life-forms. About 18% of all nonammonoid cephalopods (order Nautilida) occupied this morphospace, beginning in the Carboniferous, where they represented two-thirds of the total number of genera, but only three genera existed in this morphospaces in the Permian. The decrease in the diversity among nonammonoid cephalopods may be explained by the appearance of ceratites, which also occupied this morphospace. Morphospace 1N (Fig. 2.13). This morphospace included involute (D = 0–0.15) shells with moderately and rapidly expanding whorls (W = 1.7–2.7). This is an extremely poorly represented morphospace among Paleozoic nonammonoid cephalopods. Altogether, it included two Carboniferous and three Permian genera. Morphologically this group is very close to the group occupying morphospace 1A of ammonoids, which at that time included more than 150 genera with a laterally compressed shell (S > 1.3). These shells had a relatively spacious body chamber, allowing for a large mantle cavity and large propulsive muscles and can therefore be interpreted as nektobenthic life-forms. Shells with isometric proportions of the cross-section (S = 0.8–1)


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possibly belonged to the benthopelagic life-forms, like the modern Nautilus, but were more shifted toward the nektonic lifestyle. They were most likely efficient scavengers and possibly predators of slow moving mediumsized prey. Large subspheroconic (S < 0.7) shells with a dorsoventrally compressed body (Permian Permonautilus, Triassic Sibyllonautilus) had poorer hydrodynamics, but were stronger (Chamberlain, 1980; Kiselev, 2006) than Nautilus. They had a less efficient mechanism for active swimming. They are interpreted as benthopelagic life-forms, possibly less connected with the bottom than the modern Nautilus. Barskov (1976, 1988, 1989) was the first to propose the above typification of the life-forms of nonammonoid cephalopods, which was later extended to ammonoids. The interpretation was mainly based on the parameters W and D. Differences in the parameter S, which determines the cross-sectional shape and therefore largely the soft body and potential adaptations to different lifestyles, received less study. If this approach is to some extent justified for nonammonoid cephalopods, the total generic diversity of which was about 100 genera and which are not much diverse in their cross-sectional shape: over 60% of all genera have almost isometric cross-section (S = 0.8–1.3), about 25% are compressed laterally (S over 1.3), and less than 15% have the shell compressed dorsoventrally) (S less than 0.7). For ammonoids, which are considerably more numerous (this analysis deals with more than 600 genera) and have large quantitative and qualitative variations of the cross-section, this approach is inadequate. In addition, the above papers did not explain the considerable difference in the position of morphological peaks in ammonoids and nonammonoid cephalopods, while this, as shown below, allows the diversity of the ammonoid shells within the same morphospace defined by parameter W and D only, which allows their assignment to different life-forms. Ammonoids The morphospaces occupied by the coiled shells of Paleozoic ammonoids and nonammonoid cephalopods are largely overlapped. This overlap indicates that the acquisition of similar shell geometry by completely different and phylogenetically unrelated higher taxa (over 10 orders) is to a large extent adaptive. At the same time, preferred (dominant) morphospaces of ammonoids and nonammonoid cephalopods for the same time spans have different position in the total all-cephalopod morphospace. As noted above all preferred morphospaces occupied by ammonoids, while coinciding with the areas occupied by nonammonoid cephalopods in the values of the parameter D (whorl overlap degree), have lower values of W. According to Saunders et al. (2004) and our own data, 72% of ammonoid genera have a value for parameter W of less than 1.75, whereas in nonammonoid cephalopods such genera are less than PALEONTOLOGICAL JOURNAL

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10%. In other words, in general “typical” ammonoids with the same values of the parameter D (which probably characterizes stability) have a lower whorl expansion rate. From the point of view of hydrostatics, this suggests that ammonoids had higher buoyancy, although some of them had shells that are morphologically completely identical to those of nonammonoid cephalopods. Therefore, ammonoids had more possibilities of buoyancy control and, hence, better mechanisms of filling and emptying the phragmocone chambers. For instance, it is suggested that in this process, in addition to the mechanism of the partial osmosis as in Nautilus (Denton et al., 1961), ammonoids used capillary absorption (Barskov, 1999). In addition, ammonoids in general have a higher diversity of proportions of the body chamber and of the whorl crosssection. For instance, among nonammonoid cephalopods, only six genera have narrow whorls with an arrow-shaped cross-section with a pointed ventral keel, and no cadiconic or spheroconic shells with a value of parameter S less than 0.5. This suggests a possibility of more diverse adaptations and, hence, higher morphological diversity of life-forms of ammonoids belonging, according to their Raup’s parameters, to the same peak region. Morphospace 1A (Fig. 2.14): involute (D = 0–1.5) slowly expanding forms (W = 1–2.3). This region is populated by 240 genera (37% of the total number of genera of Paleozoic ammonoids). A detailed analysis of the distribution of parameters W and D shows that this region can be split into two more or less separate morphological groups. The first (morphological group 1A') embraces the region with the lowest values of W: W < 1.7. The second (morphological group 1A), which includes the majority of forms from this region is characterized by the values of W around 1.7–2.3 with a mean value of W = 2.0. Below, each of these morphological groups are discussed in detail. Morphogroup 1A' (Fig. 2.14). These are involute, narrowly umbilicate shells (D ~ 0.01–0.15) with the lowest whorl expansion rate (W < 1.7) among coiled forms. Very low whorls and their almost incomplete overlap strongly change the shape of the body chamber and, hence, the soft body, which becomes compressed U- or V-like. An animal with this type of shell cannot be assumed to be capable of active swimming, or of considerable vertical migrations, while high stability could allow the existence above the fair-weather wave base. The upper pelagic zone was their most likely, if not the only possible, adaptive zone. This is supported by the appearance in this morphogroup of taxa with inflated, spheroconic chambers (Epiwocklumeria, Parawocklumeria), with specific constrictions (Prolobites, Renites) and, as usual, small-sized shells. This involute morphotype we interpret as a planktonic life-form (in classification accepted in this work, it is named “plankton 2”). Morphogroup 1A' contains forms with various propor-

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6

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(h) Morphospace 1A Morphogroups 1A' and 1A: W = 1–2.3; D < 0.15 Benthopelagic life-forms S < 0.8 shell more than 30 mm in diameter

(e)

(f) (i) Fig. 2.14. Major life-forms of Paleozoic ammonoids. Morphospaces 1A and 1A': (a–f) planktonic life-form: (a) Cheiloceras D3fm; (b) Raymondiceras D3fm; (c) Epiwocklumeria D3fm; (d) Prolobites D3fm; (e) Verancoceras C1s; (f) Physematites C2b, (g, h) nektobenthic life-form: (g) Homoceras C2b, (h) Hypergoniatites C1v; (i) benthopelagic life-form Goniatites C1v.

tions and shape of the whorl cross-section (Fig. 2.14), but it is dominated by pachyconic and subdiscoconic shells with an isometric cross-section S = 0.8–1.3

(about 58.7% from the total number of genera in the morphogroup). Subspheroconic and spheroconic shells are also present (24.6%). The number of the latter grad-


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CEPHALOPODS IN THE MARINE ECOSYSTEMS OF THE PALEOZOIC 1.0

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Morphospace 2A W < 2.1; D = 0.15–0.35 Planktonic life-forms W < 1.7, shell less than 30 mm in diameter

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Nektobenthic life-forms W = 1.7–2.1 S > 1.3 0.8 < S < 1.3

(e)

(d)

(f)

Fig. 2.15. Major life-forms of Paleozoic ammonoids. Morphospace 2A: (a–c) planktonic life-form: (a) Ferganoceras C1v, (b) Neoglyphioceras C1v, (c) Cravenoceras C1s; (d–f) nektobenthic life-form: (d) Hudsonoceras C2bs; (e) Metalegoceras P1; (f) benthopelagic life-form Eurites C1t.

ually increases throughout the Paleozoic, reaching its maximum in the second half of the Carboniferous. The discoconic shells are rare, and their number progressively decreases throughout the Paleozoic. In total, the number of planktonic taxa of this morphotype in the Paleozoic was about 10.3% of the total diversity of cephalopods, the majority of which belonged to the order Goniatitida. The maximum taxonomic diversity within this group was in the Carboniferous. Morphogroup 1A (Fig. 2.14). This morphogroup contains a higher number of genera (26.7% of the total). Shells with the parameters W = 1.71–2.3, D < 0.15 in general have the same characteristics as the shells from the nonammonoid region 1N. With their isometric proportions of the cross-section (S = 0.8–1.3) and W = 1.71–2.3, these cephalopods had a body chamber allowing the placement of reasonably well-developed propulsive muscles. This suggests that they were possiPALEONTOLOGICAL JOURNAL

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bly more active animals that can be interpreted as nektobenthic rather than planktonic life-forms. With a high degree of likelihood the nektobenthic life-forms also included laterally compressed shells (S > 1.3). Large (more than 3–5 cm in diameter), spheroconic (with values of S less than 0.8) shells from morphospaces 1A and 1A' with dorsoventrally compressed body, poor hydrodynamic properties but quite strong (Chamberlain, 1981, Kiselev, 2006). It is difficult to suggest that they had a developed mechanism for active swimming; therefore, we interpret them as belonging to the benthopelagic life-form inhabiting relatively deep near-bottom waters but, in contrast to benthopelagic Nautilus-like forms (morphospace 4N), they were less connected with the bottom. Their feeding habits were probably similar to those of the modern Nautilus. Morphospace 2A (Fig. 2.15). These are semi-involute and semi-evolute (D = 0.15–0.35) slowly expand-

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4.5

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Fig. 2.16. Major life-forms of Paleozoic ammonoids. Morphospace 3A. Planktonic life-form: (a) Prolecanites C1v; (b) Rhymmoceras C1s; (c) Alaoceras C1v.

ing (W ≤ 2.1) shells. This morphospace was populated by ammonoids only. Most genera (97 of 125, from this morphogroup have W ≤ 1.7. Animals with such low whorl expansion rate had a long soft body and very low whorl cross-section, excluding the possibility of developing mechanisms for active swimming. Cephalopods with such a shell shape are interpreted as planktonic (in the classification accepted in this paper “plankton 1”), although some these with W ~ 2 and S ≤ 0.8, with large size, similar to shell from morphospace 1A, perhaps belong to the type of benthopelagic life-forms (Fig. 2.15). Supposedly, animals with such a shell were slow-moving mollusks living near the substrate and feeding on microorganisms, scavenging, or hunting small prey. A small number of discoconic and oxyconic shells from this region with a higher whorl expansion rate (W ~ 2), may be considered as nektobenthic. This morphospace (2A) first appeared in the Middle Devonian. By the end of the Permian such forms had completely disappeared. Many species from this morphospace had well-developed and diverse ornamentation represented by transverse ribs, nodes, folds, spirals, or a combination of various types of ornamentation. The forms with a dorsoventrally compressed shell whorls (S < 0.7), pachyconic, subspheroconic, cadiconic (60% genera), and also cadiconic shells with S ~ 1 (24% genera) prevail. The number of pachyconic and subcadiconic shells increases throughout the Paleozoic.

Morphospace 3A (Fig. 2.16). These are evolute (D = 0.4–0.7) shells with a low whorl expansion rate (W = 1.3–2.3). This morphospace almost completely coincides with the nonammonoid morphospace 3N, although there are differences in the shell morphology. One essential difference between ammonoids and nonammonoid cephalopods in this morphospace was the presence of the considerably longer body chamber and, hence, a wormlike soft body and long, narrow mantle cavity, which completely excludes any possibility of an efficient mechanism for active swimming. Among nonammonoid cephalopods from this morphospace there are no known shells with a body chamber longer than one whorl. Hydrostatically, ammonoid morphology is more rational: a body chamber one and half whorls long and longer is occupied by the soft body and allows vertical orientation without putting an extra weight in the chambers. To achieve this orientation, nonammonoid cephalopods only had to fill with fluid phragmocone chambers in one whorl. Nonammonoid cephalopods (3N) and ammonoids from morphospace 3A also differ in shape and proportions of the cross-section. Among nonammonoid cephalopods from this morphospace, there were virtually no representatives with low whorls, which were common among ammonoids, especially at the end of the Paleozoic, in the Late Carboniferous and Permian. In total, evolute ammonoids of this morphospace, like nonammonoid cephalopods, belonged to the planktonic life-form (“plankton 1”), which is sup-


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5.0 W (c) Fig. 2.17. Major life-forms of Paleozoic ammonoids. Morphospace 4A. Nektobenthic life-form: (a) Propinacoceras P1; (b) Pinacites D2; (c) Irinoceras C1.

ported by their mass occurrence in various facies, from deep to relatively shallow. Morphospace 4A (Fig. 2.17). These were involute (D ≤ 0.15) forms with rapidly expanding whorls (W = 2.3–3.3; on average 2.5–2.8). A similar shell geometry is exhibited by nonammonoid cephalopods from morphospace 4N, but ammonoids show a more slowly expanding whorls, which allows the achievement of higher buoyancy, compared to the modern Nautilus, and at the same time higher stability. There are two major shell types within this morphospace. Discoconic and pachyconic shells, with an isometric whorl crosssection (S = 0.8–1.3) and laterally compressed: oxyconic or platyconic (medlicottiids and others). Most shells possess a hyponomic sinus, whereas some (Pinoceras and others) shell had an umbilical callus, a feature promoting stability and streamlining (Korn and Klug, 2002). At S ~ 1 the volume of the body chamber is sufficient to enclose organs necessary for swimming (hyponome and muscles). In addition, the shell shape is favorable for active swimming (Chamberlain, 1981). Such shell geometry allows the assignment to the nektobentic life-form. These mollusks were possible scavengers and predators hunting large, slow moving prey (many species have a large shell, with a diameter over 100 mm). PALEONTOLOGICAL JOURNAL

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At S > 1.3 the body of the animal becomes laterally compressed. This body shape does not allow welldeveloped propulsive musculature that could generate prolonged fast swimming. Animals with shells of this shape could not swim quickly, but could maneuver. They commonly have a complex suture (Medlicottia, etc.), i.e., they had a large surface of septa lined with organic membranes capable of absorbing considerable amounts of fluid, which may also be indicative of ability to rapidly change buoyancy (Barskov, 1999). Representatives of this morphospace are interpreted as belonging to the nektobenthic life-form. Species from this morphospace are known from the Middle Devonian throughout the entire Paleozoic. Their diversity remained low (about 6–7% of the total number of ammonoids) and somewhat increased at the end of the Devonian (clymeniids) and in the Permian (medlicottiids). Some ammonoids in this group had a relatively complex suture in oxyconic and platyconic shells. The numbers of the latter had increased by the end of the Paleozoic. Morphospace 5A (Fig. 2.18). These were moderately involute shells with rapidly expanding whorls. This morphospace contained species with parameters W = 2.1–3.1 and D = 0.15–0.25. They constituted 5% of the total number of genera in the Paleozoic, and the maximum of generic diversity was in the Middle Devonian. Shells from morphospace 5A had high stability

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(d)

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(e)

(f)

0.8 < S < 1.3

(h) (g) Fig. 2.18. Major life-forms of Paleozoic ammonoids. Morphospaces 5A and 6A; (a–f) nektobenthic forms: (a) Pronorites C1v; (b) Daraelites C1s; (c) Manticoceras D3fr; (d) Protoxyclymenia D3fm; (e) Epicanites C1v–s; (f) Mimagoniatites D1–D2ef; (g, h) benthopelagic forms: (g) Mimagoniatites D1–D2ef, (h) Mimagoniatites D1–D2ef.

and high buoyancy, while retaining a relatively large size of the body chamber, allowing the presence of the propulsive musculature and hyponome required for active swimming. By the shape of the cross-section, this morphospace contains two morphotypes: Disco-

conic and pachyconic shells with S = 0.8–1.3 and platyconic or oxyconic with S > 1.3. Due to the high number of genera, oxyconic shells dominated this morphogroup beginning in the Carboniferous. Ammonoids with shells of this shape, like those from morphospace 4A,


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6A

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(b)

Fig. 2.19. Morphotypes of Paleozoic ammonoids (of ammonoids with rapidly expanding whorls; benthopelagic life-form): (a) Parentites D2ef; (b) Mimagoniatites D1–D2ef.

are interpreted here as belonging to the nektobenthic life-form. Morphospace 6A (Fig. 2.18). These are moderately evolute shells with rapidly expanding whorls (W = 2.0– 3.1, D = 0.25–0.4). These values are characteristic of morphospace 2N of nonammonoid cephalopods, and similar to those, ammonoids populating morphospace 6A are interpreted as nektobenthic. Most shells of Paleozoic ammonoids, populating this morphospace had a laterally compressed whorl cross-section (S > 1.3), in contrast to nautiloids, which typically had isometric (S = 0.8–1.3) or dorsoventrally compressed (S < 0.8) whorls. Ammonoids with Nautilus values of S (< 0.8) formed a small group, which can be interpreted as a benthopelagic life-form (Fig. 2.18). In total, the number of genera in morphospace 6A was low (5–6% of the total number of Paleozoic genera). In contrast to nautiloids, which show a maximum taxonomic diversity in this morphospace in the Carboniferous, ammonoids had there only a few genera of the orders Prolecanitida, Goniatitida, and Anarcestida in the most of the Paleozoic. Morphospace 6A becomes clearly delineated only from the end of the Permian, mainly because of the diversification of the order Ceratitida. In addition, there is small number of ammonoids that were not within any of the recognized morphospaces. These typically had very high whorl expansion rate W = 3.3–4.97 and narrow or moderately narPALEONTOLOGICAL JOURNAL

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row umbilicus (D = 0.01–0.3). These are mainly Devonian genera and species (Parentites, Kimoceras, Celaeceras, some anarcestids). Most of these had a shell with parameters characteristic of morphogroup 4N of nautiloids and can be interpreted as a benthopelagic life-form (Nautilus-like), similar to nautiloids from group 4N (Fig. 2.19). 2.7. Life-Forms of Cephalopods with Planispiral Shell in which the Whorls Were Not in Contact This morphogroup includes planispiral shells, in which the last, outer whorl was not in contact with the preceding whorls. There are only a handful of such species (in total less than 20 genera known from five orders): Order Nautilida: Chouteauoceras (C1), Rineceras (C1), Homaloceras (D2), Halloceras (D1), Goldringia (D2), Pleuronoceras (D2). Order Barrandeocerida: Bickmorites (O2–S), Gasconsoceras (S2), Wilsonoceras (O3), Cumingsoceras (S2). Order Tarphycerida: Tragoceras (O1), Tallinoceras (O1), Alaskoceras (O1), Aphetoceras (O1). Order Oncocerida: Gyronaediceras (D3). Order Anarcestida: Anetoceras (D1), Erbenoceras (D1), Palaeogoniatites (D1). Almost all these shells have a slowly expanding shell with a short body chamber, suggesting high buoy-

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1

3b

2 3a

4a

4b

5a

5b

Benthopelagic life-form

6

7a

7b

Fig. 2.20. Major life-forms of cephalopods with a planispiral shell with whorls not in contact. (1) Goldringia, D2; (2) Estonioceras, O1; (3) Tragoceras, O1; (4) Bickmorites, S2; (5) Rineceras, C1; (6) Cumingoceras, S2; (7) Nephriticeras, D2.

ancy and limited stability. Transverse ornamentation present in some of these genera may have had adaptive significance (to increase stability). The cross-section is round and more or less isometric. Such shell geometry almost excludes the possibility of effective swimming adaptations allowing only for passive existence in the pelagic zone below the fair-weather wave base. Hence, we consider the representatives of this morphological group as a planktonic life-form. Some taxa with a rapidly expanding shell, such as Gasconsoceras and Cumingsoceras (Barrandeocerida), that had lower buoyancy, could belong to relatively deep slow moving benthopelagic life-forms (Fig. 2.20). 2.8. Life-Forms of Cephalopods with a Conispiral Shell (Fig. 2.21) The forms with a spiral (tiricinic and throchoid) shells are uncommon among Paleozoic nonammonoid cephalopods. There are altogether 15 genera in three

orders: Oncocerida (6 genera), Barrandeocerida (6 genera), and Nautilida (2 genera). No such shells are found among Paleozoic ammonoids. Two morphotypes are recognized: those with a spire not projecting above the last whorl and those in which it does project. All the taxa with such a shell shape have a relatively short body chamber, which, from the hydrostatic point of view, suggests high buoyancy. Those without a projecting spire have the parameters W, D, and S belonging to groups 3N and 2N. Species with slowly expanding whorls, such as the genera Trochoceras Barrande (Nautilida), Peismoceras Hyatt, Catyrephoceras Foerste, with high buoyancy, are interpreted as planktonic lifeforms. This is supported by the presence of a narrowed aperture in the latter genus. More slowly expanding shells of Naediceras Hyatt, (Oncocerida), Hercoceras Barrande (Nautilida) can belong to the nektobenthic life-form. The asymmetry in arrangement of inner whorls in representatives with a trochoid and torticonic shell con-


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1211

Planktonic life-form

3 1

2 Nektobenthic life-form

4

5

6

Benthic or benthopelagic life-form

7

8

9

10

Fig. 2.21. Major life-forms of cephalopods with a spirally conical shell: (1) Catyrephoceras, S2; (2) Peismoceras, S2–3; (3) Hercoceras, D2; (4) Lechritrochoceras, S2; (5) Leurotrochoceras, S2; (6) Naedyceras, D2; (7) Lorieroceras, D1; (8) Foersteoceras, S3; (9) Mitroceras, S3; (10) Sphyradoceras, D1.

tributes to the shell hydrostatics. The rearrangement of fluid in the asymmetric chambers allows orientation and stabilization of the shell both in vertical and inclined positions. Shells with a projecting spire (genera Mitroceras Hyatt, Foersteoceras Ruedemann, and Lorieroceras Foerste—Oncocerida), are very similar to gastropod shells in their appearance. The presence of the phragmocone, providing high buoyancy allows the assignment of taxa with such shell shape to the benthic or benthopelagic life-forms, capable of slow swimming or floating just above the bottom. PALEONTOLOGICAL JOURNAL

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2.9. Life-Forms of Cephalopods with a Heteromorphic Shell This group includes taxa with a shell that changes considerably throughout ontogeny. In some cases this lead to significant changes in lifestyle. For instance, most ammonoids at the early postlarval stage undoubtedly belong to the planktonic life-form, although later, as suggested above, could be very different in the adopted lifestyle. Likewise, most Carboniferous nautilids at the postlarval stage had a curved cyrtoceraconic shell and can be interpreted as belonging to benthopelagic life-forms, whereas at the adult stages their lifestyle was essentially different. Here we are discussing

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1a

1b

1c

2

3

4

5a

5b

Fig. 2.22. Nektobenthic life-form of cephalopods with a heteromorphic shell. Order Lituitida: (1) Lituites, O2: (1a) longitudinal section, (1b) shape of aperture, (1c) shell, lateral view; (2) Ancistroceras, O1; (3) Rhynchortoceras, O2; (4) Angelonoceras, O2; (5) Jolietoceras, S2: (5a) cross section, (5b) lateral view.


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1213

Nektobenthic life-form 2a 1a

1b

2b

1c

2c

2e

3

Planktonic life-form

4a

4b

5a

6a

6b

5b

6c

Fig. 2.23. Life-forms of cephalopods with a heteromorphic shell. Order Ascocerida: (1) Aphragmites, S3: (1a) dorsal side, (1b) septal view, (1c) lateral view; (2) Glossoceras, S2–3 (2a) longitudinal section, (2b) lateral view, (2c) dorsal side, (2e) reconstruction of the mollusk; (3) Ascoceras, S2–3; (4) Billingsites, O3: (4a) lateral side, (4b) dorsal side; (5) Probillingsites, O2–3: (5a) ventral side, (5b) lateral side; (6) Schuchertoceras, O3: (6a) lateral view, (6b) ventral view, (6c) dorsal view.

taxa in which shell morphology changed during the adult stage. In the Paleozoic such shell shape was characteristic of representatives of the orders Lituitida and Ascocerida, and only of a few genera of other orders. Order Lituitida (Fig. 2.22). In the members of this order the initial part of the shell is planispiral with conPALEONTOLOGICAL JOURNAL

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tacting or not contacting whorls, and later the shell is straight. In one genus (Rhynchorthoceras Remele, 1881) the initial part is cyrtoconic. A very short living chamber and very long phragmocone are typical features of the body plan in this order. This morphology suggests high buoyancy, which allows the interpreta-

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Table 5. Ecological structure of the taxocoenosis of modern cephalopods (after Nesis, 1975) Taxa Octopoda Sepiida Teuthida Total

Myopsida Oegopsida

Life-forms

Number of genera

Benthic

42 21 84 10 157

22 (50%) – – 1 (10%) 23 (16%)

Benthopelagic Nektobenthic 6 (15%) 1 (5%) – – 7 (5%)

tion of this shell shape as belonging to the planktonic life-form. This may be supported by the presence in some of them of a narrowed, almost closed aperture (genus Lituites). However, the main part of the shell is long and straight. The coiled part is very small. The phragmocone chambers contain massive cameral deposits. This suggests that the living animal’s longitudinal body axis was orientated horizontally and, hence, active swimming was possible. Isometric proportions of the soft body and relatively large size also allows the development of the propulsive muscles and feeding on the relatively large prey. Therefore, these representatives are herein interpreted as belonging to the nektobenthic life-form. At the early ontogenetic stages they apparently inhabited the pelagic zone and were less connected with the bottom. Order Ascocerida (Fig. 2.23). Essential changes in the shell morphology throughout the ontogeny are the fundamental features of the body plan in this order. The order contains two groups of genera, with no apparent genetic connections. One group is Middle–Late Ordovician, another group is Late Silurian. Members of the Late Silurian group are known to have had a weakly curved or straight shell with a normally developed ortho- or cyrtochoanitic siphuncle. At the adult stage the shell became a sausage-shaped, phragmocone chambers were formed on the dorsal side only, the siphuncle disappeared and therefore the buoyancy control was low, as the phragmocone became a passive floating device (Fig. 2.23a). The construction of adult shell of the Silurian ascocerids with a dorsal gas-filled chambers, which acted as a passive floating device and relatively spacious living chamber allowing the development of the active propulsive mechanisms, allow the interpretation of such shell as belonging to the nektobenthic life-form. The Ordovician members (Fig. 2.23b) are only known from the adults, the shells of which are eggshaped. The phragmocone chambers were also all positioned dorsally and were not connected to the soft body, not permitting efficient buoyancy control. They acted as a passive floating device and orientated the longitudinal axis of the body at an angle to horizontal plane. A considerably changed shape of the body chamber (and soft body) could not allow for the development of an active propulsive mechanism in these forms. This and the

– 18 (85%) – 9 (90%) 27 (19%)

Nektonic

Planktonic

– 1 (5%) 42 (50%) – 43 (25%)

14 (35%) 1 (5%) 42 (50%) – 57 (35%)

presence of the soft body support that this taxon belonged to a planktonic life-form. CHAPTER 3. ECOLOGICAL SPECIALIZATION AND ECOGENESIS OF PALEOZOIC CEPHALOPODS 3.1. Ecological Structure of the Modern Cephalopod Taxocoenosis The widespread belief that modern cephalopods are mainly active pelagic predators competing with pelagic fish is far from true (Table 5), and even less true when fossil cephalopods are considered. Up to now, ammonites are portrayed as active nektonic predators in many textbooks and even scientific publications (see Chapter 2). Shevyrev (2005) published a review of existing knowledge on the lifestyle of ammonoids. Barskov (1989) considered the occurrence of Paleozoic nonammonoid cephalopods in various adaptive zones. The above data show that only a quarter of modern cephalopods are active pelagic nektonic animals. This ecological structure (Figs. 3.1, 3.2) of the cephalopod fauna developed in the post-Paleozoic time, and its formation is related to the appearance and expansion of endocochliates. In the Paleozoic, from which interval this group is virtually unknown, the ecological structure of the cephalopod taxocoenosis was different and changed with time. Nevertheless, at some stages of geological time the ecological structure of the cephalopod taxocoenosis resembled that of today’s in the proportion of taxa living near the bottom (benthic, or benthopelagic) and living in the water column, pelagic taxa (planktonic, or nektobenthic), which will be discussed below in the framework of the discussion of the ecological structure of the cephalopod taxocoenosis in different geological epochs. Below we discuss details of the ecological specialization (ecological structure) of the orders of Paleozoic ectocochliate cephalopods, and its changes throughout time (ecogenesis). The total number of genera in the ecological classification may exceed the number of described taxonomic genera in an order, since the species in one genus may belong to different life-forms, and it is sometimes difficult to make an ecological


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classification, because in some cases insufficient preservation prevents their assignment to any life-forms.

b 16%

pl 35%

1215

3.2. Ecological Structure of Paleozoic Cephalopods In this section we discuss the analysis of the ecological specialization of Paleozoic cephalopods in individual orders based on the criteria chosen for life-forms in Chapter 2, changes in the ecological structure of the orders (their ecogenesis) during different geological epochs, and their possible causes.

bp 4%

nb 19%

n 25% Fig. 3.1. Ecological structure of the taxocoenosis of modern cephalopods. Explanations: here and further in the Chapter for Figs. 3.1–3.32: (n) nektonic, (nb) nektobenthic, (bp) benthopelagic, (b) benthic, (pl) planktonic life-forms.

interpretation for some ancient taxa. In these cases it is accepted that the genus belongs to two life-forms. The number of genera may be lesser than that in the taxonomic

3.2.1. Order Ellesmerocerida This is the earliest cephalopod order giving rise to almost all Paleozoic cephalopods, except for nautilids, ascocerids, bactritids, and ammonoids. As shown in Chapter 1, the adaptation to pelagic existence without necessarily interrupting the initial close connection with the bottom is the ecological basis for cephalopod origin, related to the formation of the gas-fluid buoyancy device. Therefore the initial life-form of ellesmerocerids and all other cephalopods was certainly benthopelagic. As early as the Early Cambrian, some representatives acquired characters suggesting more active swimming and of their affinity to the life-form. In the Early Ordovician, ellesmerocerids reached their maximum diversity, both taxonomic (with more than 50 genera) and ecological, indicating adaptations to various environments (with different success rates).

Number of genera 60

(b)

50 (a)

Myopsida 6%

40 30 20 Octopoda 27%

Oegopsida 54%

Sepiida 13%

10 0 % 100 90 80 70 60 50 40 30 20 10 0

(c)

b

bp

nb

n

pl

Fig. 3.2. Taxonomic structure of life-forms of modern cephalopods. Explanations: (a) generic diversity of the orders of modern cephalopods; (b–c) distributions of life-forms of modern cephalopods: (b) absolute, (c) relative. PALEONTOLOGICAL JOURNAL

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(b)

50 40

pl 5%

30 (a) b 14%

20 10 0

nb 46%

bp 35%

% 100

(c)

80 60 40 20 0 Cambrian

Early Ordovician

Middle Late Ordovician Ordovician

Fig. 3.3. Order Ellesmoceratida. Explanations for Figs. 3.3–3.20. (a) Ecological structure; changes in the ecological structure during the time of their existence, (b) absolute values, (c) percentage.

By the end of the Early Ordovician, they included all known life-forms (Fig. 3.3). Interestingly, both planktonic and benthic life-forms were formed at this time. A sharp reduction in taxonomic diversity (down to seven genera) of ellesmerocerids in the Middle Ordovician was certainly related to the competition with the six new descendant orders, and the three orders that appeared at the end of the Early Ordovician. A few ellesmerocerid genera retained their presence in all adaptive zones. In the Late Ordovician they were represented only by one genus in each of the three life-forms: benthic, benthopelagic, and nektobenthic. The total ecological structure of the order is shown in Fig. 3.3a. At the beginning of the Middle Ordovician, new orders inherited their ecological specialization from the ancestral families of ellesmerocerids. Benthopelagic specialization of the family Plectronoceratidae is clearly observed at the early stages of the evolution of their descendants, in the order Discosorida. The trend toward nektonization observed in the orders Orthocerida and Pseudorthocerida was inherited from the ancestral family Baltoceratidae.

acquired as a result of the longer chambers of the phragmocone was to a large extent cancelled out by the retention of the primitive state of the siphuncle (it is quite wide, sometimes extending more than a half of the shell diameter, its apical parts filled with endosiphuncular deposits—endocones). The latter feature, however, facilitated orientation control. Animals with a straight shell could, therefore, place their body and hyponome in a horizontal position to facilitate more intense motion. Species with a relatively breviconic and annulated shell are interpreted as belonging to the benthopelagic life-form. Gigantic species reaching 4 m in length and weighing several tons, are interpreted as benthos. More than half of all endocerids with a relatively longiconic shell and small size are nektobenthic. Endocerids include no taxa with shell morphology allowing their assignment to plankton. The total ecological structure of the endocerid taxocoenosis is shown in Fig. 3.4a. From the ecological point of view the decrease in taxonomic diversity in the Middle and Late Ordovician (Fig. 3.4b) was accompanied by a decrease in the proportion of nektobenthic taxa and an increase in benthopelagic and benthic taxa (Fig. 3.4c).

3.2.2. Order Endocerida

3.2.3. Order Actinocerida Despite their considerable taxonomic diversity (about 50 genera of 10–11 families), actinocerids are, from the morphological point of view, the most homog-

This order is morphologically similar to ellesmerocerids. Endocerids evolved from the early members of the family Bassleroceratidae. The high buoyancy


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CEPHALOPODS IN THE MARINE ECOSYSTEMS OF THE PALEOZOIC Number of genera 18

1217

(b)

15 12 9 (a)

3

p 12% nb 50%

6

0 % 100

bp 38%

(c)

80 60 40 20 0

Early Ordovician

Middle Ordovician

Late Ordovician

Fig. 3.4. Order Endocerida. Explanations as in Figs. 3.1. and 3.3.

enous cephalopod order. Only two genera exhibit an exogastrically curved shell, and only one had annulated ornamentation. Genera and families in this order are based on variations in the morphology and structure of the siphuncle. These variations apparently did not have much effect on the adaptations of these mollusks. The presence of the marginal siphuncle filled by deposits in the apical parts indicates the horizontal position of the living animal. Large size and dorsoventrally compressed cross-section, often with a flattened ventral side (genera Kochoceras, Selkirkoceras, many species of Actinoceras, etc.)—all these are adaptations to benthic lifestyle (Fig. 3.5). The genera Gonioceras and Lambeoceras, exhibiting a flattened shell with a lensshaped cross-section, are good examples of benthic adaptations. These mollusks probably had a lifestyle similar to that of some modern slow moving sepiids. A more active motion may be suggested for actinocerids with a relatively narrow siphuncle and a shell with a rounded or laterally compressed cross section (Early Paleozoic Ormoceras, Sactoceras, Elrodoceras, or Carboniferous Loxoceras). They can be considered to represent the nektobenthic life-form. The genera Ellinoceras and Magadanoceras with fluted septa represent a unique phenomenon among all known cephalopods with an orthoceraconic shell. If it is supposed that one of the functions of the fluted septum was to withstand external pressure, this would suggest that such forms were capable of relatively fast and energetic vertical movements. Probably these were actively PALEONTOLOGICAL JOURNAL

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moving animals with a large range of vertical and horizontal migrations, similar to some neritic squids. Only a few actinocerids with the shortest breviconic shell (apical angle more than 15°) can be interpreted as benthopelagic life-forms. The most nektonized forms (Middle–Late Ordovician Troedssonoceras with a longitudinally-ribbed shell and Carboniferous Loxoceras (L. sagitta)) had a subcylindrical shell and narrow marginal siphuncle. Most actinocerids had a large initial part of the shell with a conical first chamber suggesting large bottom laid eggs and reproduction on the sea floor. There were not planktonic (as it is understood in modern ecology) at any stage of their ontogeny. The only Early Ordovician genus with a wide marginal siphuncle, Polydesmia, is interpreted as benthic. In the Middle Ordovician, at the time of the maximum taxonomic diversity, nektobenthic forms were dominant. A decrease in diversity before the Devonian occurred because of the relative reduction in the number of nektobenthic forms in the taxocoenosis; however, one or two nektobenthic genera persisted into the Devonian and Carboniferous. A slight increase in the diversity of actinocerids in the Mississippian (up to 5−6 genera) resulted in the appearance of species with a breviconic shell, which are interpreted as benthopelagic, and very large species (Rayonoceras giganteum), which were probably benthic (Fig. 3.5).

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(b)

25 20 15 (a)

10 5

b 14% bp 13% nb 73%

0 % 100

(c)

80 60 40 20 Pennsylvanian

Mississippian

Middle Devonian Late Devonia

Early Devonian

Silurian

Middle Ordovician Late Ordovician

Early Ordovician

0

Fig. 3.5. Order Actinocerida. Explanations as in Figs. 3.1. and 3.3.

3.2.4. Order Orthocerida Major morphological features of orthocerids, including a longiconic orthoceraconic shell, a relatively narrow siphuncle, and long chambers, suggest high buoyancy and adaptation to life in the water column (not benthic). High buoyancy and presence of cameral deposits improving orientation and maintenance in a horizontal position are adaptations to active motion in both horizontal and vertical directions. The morphology of the initial part of the shell with a small subspherical protoconch and the microstructure of its walls suggest the presence of small, numerous, possibly pelagic eggs, which could indicate an incomplete embryonic development and a planktonic larva. Around three-quarters of orthocerid genera were nektobenthic (Fig. 3.6a). An ability to decollate the apical part of the shell in the genus Sphooceras facilitated active swimming. Species of such genera as Kionoceras or Polygrammoceras with a shell circular or laterally compressed in cross section, long chambers, and well-developed cameral deposits and longitudinal ornamentation were apparently more active swimmers resembling modern neritic squids. The benthopelagic life-form included relatively bradyconic representatives with a narrowed aperture (Ordovician Whitfieldoceras, Whiteavesites, and Clinoceras; Silurian Paraphragmitidae with an exogastrically curved annulated shell and narrowed aperture; and also Late Paleozoic Brachycycloceras). Suppos-

edly, some representatives with an annulated shell (Leurocycloceras, Metaspyroceras), small-sized shell, or shells with no or little cameral deposits are interpreted as planktonic. Over half of orthocerid genera existed in the Middle Ordovician–Silurian. At that time their ecological diversity was the highest. The remaining three or four long-surviving orthocerid genera remaining after Late Devonian are interpreted as mainly nektobenthic lifeforms (Fig. 3.6). 3.2.5. Order Pseudorthocerida The body plan of pseudorthocerids was distinguished from that of orthocerids mainly by the presence of endosiphuncular deposits and a conical protoconch. The endosiphuncular deposits suggest that pseudorthocerids were from the beginning able to finely calibrate their orientated position with a horizontal longitudinal axis of the body, the conical protoconch suggests that they had bottom eggs and direct development. However at the very end of their ontogeny, they possibly had pelagic larvae (Permian genera Shikhanoceras and Simorthoceras). It is interesting that although orthocerids and pseudorthocerids appeared at the same time and had similar proportions of life-forms in the overall diversity (Figs. 3.6, 3.7), these orders are significantly different in the dynamics of taxonomic diversity, and in the change of the ecological structure of their taxocoenoses in the evolution. In orthocerids the maximum


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1219

Number of genera 25 20

(b)

15 bp pl 12% 10%

(a) 10 5 0 % 100

nb 78%

(c)

80 60 40

Pennsylvanian

Middle Permian

Late Permian

Pennsylvanian

Early Permian

Late Permian

Mississippian

Early Carboniferous

Late Devonian

Middle Devonian

Early Devonian

Early Silurian

Late Ordovician

Middle Ordovician

0

Early Ordovician

20

Fig. 3.6. Order Orthocerida. Explanations as in Figs. 3.1. and 3.3.

pl 1% bp 8%

(a)

nb 91%

(b)

16 14 12 10 8 6 4 2 0 % 100

(c)

80 60 40

Mississippian

Late Devonian

Middle Devonian

Early Devonian

Silurian

Late Ordovician

Middle Ordovician

0

Early Ordovician

20

Fig. 3.7. Order Pseudorthocerida. Explanations as in Figs. 3.1. and 3.3. PALEONTOLOGICAL JOURNAL

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(a) bp 11%

pl 59%

nb 30%

Number of genera 16 14 12 10 8 6 4 2 0 % 100

(b)

(c)

80 60 40 20 0

Early Middle Ordovician Ordovician

Late Ordovician

Silurian

Fig. 3.8. Order Tarphycerida. Explanations as in Figs. 3.1 and 3.3.

diversity was in the Middle Ordovician–Silurian, whereas in pseudorthocerids it was in the second half of the Devonian and Early Carboniferous, when orthocerids included less than ten genera. In the Ordovician orthocerids were represented by all known life-forms, including benthopelagic and planktonic, whereas pseudorthocerids were represented solely by nektobenthic taxa. The benthopelagic life-form of pseudorthocerids became widespread only in the Devonian, whereas plankton in this order appeared only in the Permian (compare Figs. 3.6b, 3.6c and Figs. 3.7b, 3.7c). All this suggests that the representatives of these two closely related orders were in a competing relationship, which may also support their independent origin. 3.2.6. Order Tarphycerida As mentioned in Chapter 1, the archetype of cephalopods for the first time showed a trend toward planispiral coiling. Tarphycerids are distinguished from representatives of other orders with a planispiral shell by two major features. The ophioconic shell with hardly contacting whorls is very short, compared to morphologically similar shells in other orders. In addition, in most taxa in this order, the body chamber was deviating from the last whorl, and the aperture was narrowed or even closed. These characters suggest high buoyancy and mainly hypostome position in life. Tarphycerids showed the highest taxonomic and ecological diversity in the Early Ordovician, where they represented for the first time a real planktonic life-form, which constitutes a large part of the tarphycerid taxocoenosis (Fig. 3.8a). Representatives with a more involute shell are inter-

preted as benthopelagic, and those with a discoid shell, as nektobenthic life-forms. The decrease in the number of genera in the Middle Ordovician–Silurian included benthopelagic taxa, while the proportions of the planktonic and nektobenthic life-forms remained unchanged (Figs. 3.8b, 3.8c). It is shown below that from the Middle Ordovician the morphospace of the benthopelagic forms with a coiled shell were occupied by barrandeocerids. 3.2.7. Order Lituitida This is a small order (less than 10 genera), existing only in the Ordovician (Fig. 3.9). The shell is heteromorphic, with the initial part planispirally coiled in 1.5–2 whorls, and the larger part straight. The body chamber is short, in some taxa the aperture is narrowed, with lobes. Middle Ordovician lituitids of this shape (three genera) are interpreted as plankton. Other taxa, with an open aperture, are interpreted as nektobenthic life-forms. The evolution of this order displays a kind of rejection of the advantages of the coiled shell (orientation control without additional stabilizing mechanisms). A secondary development of the straight shell, required the appearance of such deposits (cameral deposits on the ventral side). 3.2.8. Order Barrandeocerida Members of this order have a coiled shell like in tarphycerids. Some workers include them within tarphycerids. As shown in Chapter 1, their body plans are dif-


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(a)

pl 38% nb 62%

Number of genera 5 4 3 2 1 0 % 100

1221

(b)

(c)

80 60 40 20 0

Early Ordovician

Late Ordovician

Middle Ordovician

Fig. 3.9. Order Lituitida. Explanations as in Figs. 3.1. and 3.3.

b 3%

(b)

15 (a)

10 5 0

Middle Devonian

Early Devonian

Silurian

nb 3%

(c)

Late Ordovician

% 100 80 60 40 20 0

Middle Ordovician

bp 46%

pl 48%

Fig. 3.10. Order Barrandeocerida. Explanations as in Figs. 3.1. and 3.3.

ferent in the way they maintained buoyancy. Barrandeocerids had a more advanced exchange between the siphuncle and chambers (thin connecting rings) and originally larger apical angle, whereas coiled forms had a higher whorl expansion rate. These features of shell geometry considerably contributed to the differences in the ecological structure of the order and its changes throughout time (compare Figs. 3.10, 3.8). About half of barrandeocerids belonged to the benthopelagic lifeform and almost the same number belonged to the planktonic life-forms. Only one genus in the Early Devonian (Sphyradoceras with a trochoid gastropodlike shell) is referred to the benthic life-form, and one PALEONTOLOGICAL JOURNAL

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Silurian genus (Jolietoceras with a heteromorphic shell, imitating the geometry of the Early Ordovician lituitid) are interpreted as nektobenthic life-forms. Once they appeared in the Middle Ordovician, barrandeocerids completely replaced tarphycerids in the benthopelagic habitats. They reached maximum taxonomic and ecological diversity in the Silurian, when the number of planktonic forms exceeded that of benthopelagic. However, with the appearance of new planktonic forms (nautilids and ammonoids) in the Devonian, the benthopelagic life-forms of barrandeocerids became very important and were the only ones that survived into the Middle Devonian (Figs. 3.10b, 3.10c).

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b 6% pl 25%

(b)

30 0

Late Devonian

Middle Devonian

Early Devonian

Silurian

nb 34%

(c)

Late Ordovician

% 100 80 60 40 20 0

Middle Ordovician

bp 35%

Fig. 3.11. Order Discosorida. Explanations as in Figs. 3.1. and 3.3.

Early Carboniferous

Late Devonian

(c)

Middle Devonian

% 100 80 60 40 20 0

Early Devonian

bp 45%

Silurian

nb 30%

b 10%

Late Ordovician

pl 15%

(b)

Middle Ordovician

(a)

60 50 40 30 20 10 0

Fig. 3.12. Order Oncocerida. Explanations as in Figs. 3.1. and 3.3.

3.2.9. Order Discosorida The initial body plan of this order results from this order being a descendant of the ellesmerocerid family Plectronoceratidae. At the beginning of their evolution, discosorids had an endogastrically curved shell and thick multi-layered connecting rings suggesting relatively limited ability to control buoyancy. This shell geometry indicates that they probably belonged to the benthopelagic life-form, the ecological affinity inherited from their ancestors and typical of the ecological specialization of the taxocoenosis in general (Fig. 3.11). However, as early as the Late Ordovician the order contained species with an exogastrically

curved and almost straight shell. Beginning in the Early Devonian, discosorids more or less evenly occupied all adaptive zones. At the end of their evolution, in the Late Devonian, they reached their maximum taxonomic diversity, and in the Famennian, together with clymeniids, constituted 80% of all cephalopods in existence. 3.2.10. Order Oncocerida In contrast to discosorids, the body plan in this group was based on it origin from the exogastrically curved ellesmerocerids. At the same time, in general ecological structure the orders are very similar


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1223

(b)

6 (a)

4 2 0

pl 46%

% 100

nb 54%

(c)

80 60 40 20 0

Middle Ordovician

Late Ordovician

Silurian

Fig. 3.13. Order Ascocerida. Explanations as in Figs. 3.1. and 3.3.

(Figs. 3.11, 3.12) and supplemented each other by the proportions of life-forms throughout their co-existence. Until the Late Devonian, the generic diversity of oncocerids was more than that of discosorids. In the second half of the Silurian forms appeared among both oncocerids and discosorids exhibiting a new, peculiar planktonic morphotype with a barrel-shaped shell and closed aperture (oncocerid families Hemiphragmoceratidae and Trimeroceratidae, and discosorid families Phragmoceratidae, Mandaloceratidae, and Mesoceratidae). Another example of such parallel emergence of forms with similar shell geometry in both orders was the appearance of nektobenthic life-forms with an almost straight shell in the Late Devonian 3.2.11. Order Ascocerida This is a small order, represented by one and a half dozen genera, the main feature of the body plan of which was a natural truncation of the posterior end of the orthoconic phragmocone and development at the final orthogenetic stages of an ellipsoidal, sausageshaped, or barrel-shaped shell with chambers on the posterior and dorsal sides. In these forms, the phragmocone acted as a passive floating device. The members represented two life-forms: (1) nektobenthic, which characteristically displayed an open aperture and horizontal position of the longitudinal body axis (for instance, the Ordovician Probillingsites, or Silurian Ascoceras) and (2) planktonic, shell of which had a narrowed aperture and was inclined or of hypostome orientation (the Ordovician Billingsites, Schuchertoceras, or Silurian Glossoceras and Aphragmites). The proportions of these life-forms in the community remained virtually the same throughout their existence (Fig. 3.13). PALEONTOLOGICAL JOURNAL

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3.2.12. Order Nautilida The basic geometry of nautilids is a planispiral shell, but in the modern system the order includes several genera with a trochoid, curved shell (Middle Devonian Rutoceras, Centrolitoceras, and others and the Early Permian Sphooceras) and even with a straight shell (Casteroceras). Nautilids originally appeared in the Early Devonian as benthopelagic and planktonic forms. Later the group contained members of the nektobenthic life-form and even benthic representatives. In general the ecological structure of the order contains planktonic, benthopelagic, and nektobenthic life-forms, with their proportions considerably changing throughout ecogenesis. In the evolution, the proportion of the planktonic life-form decreases, whereas the proportions of nektobenthic and benthopelagic life-forms increased (Fig. 3.14). To a large extent, the order of development was determined by competition with ammonoids, which appeared in the Early Devonian and were a large group with a planispiral shell. In the pre-Permian epochs, especially in the Carboniferous, most nautilids were represented by nektobenthic and benthopelagic forms with a morphotype of semi-evolute moderately or rapidly expanding shells (morphogroup 2N) (Fig. 3.14c). Among ammonoids such forms appear and predominate in the second half of the Permian. From that time onward, the proportion of benthopelagic forms with a morphotype of the modern Nautilus began to increase among nautilids, apparently because of the replacement by ammonoids, whereas ammonoids in this adaptive zone were represented by another morphotype: spheroconic and cadiconic shells with slowly expanding whorls.

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Late Permian

Middle Devonian

Early Devonian

nb 39%

(c)

Early Permian

bp 30%

pl 30%

(b)

Pennsylvanian

(a)

Number of genera 50 40 30 20 10 0 % 100 80 60 40 20 0

Mississippian

b 1%

Late Devonian

1224

Fig. 3.14. Order Nautilida. Explanations as in Figs. 3.1. and 3.3.

(a) pl2 10%

pl1 37%

bp 11%

nb 42%

70 60 50 40 30 20 10 0 % 100

(b)

(c)

80 60 40

Famennian

Frasnian

Givetian

Emsian

0

Eifelian

20

Fig. 3.15. Order Anarcestida. Explanations as in Figs. 3.1. and 3.3.

Ammonoidea Seven orders of ammonoids existed in the Paleozoic. Their main acquisition was a planispiral shell divided into chambers by a complexly folded septum. Once they had appeared, at the end of the Early Devonian, ammonoids quickly occupied a dominant position in the cephalopod communities in all adaptive zones, with the exception of benthic. 3.2.13. Order Anarcestida The first members of this order appeared in the Emsian (Early Devonian). They had a gyroconic, advo-

lute, or thinly discoidal planispiral shell with loosely coiled, non contacting or contacting, but not embracing whorls, a combination of features allowing their positive assignment (like their ancestral bactritids) to the planktonic life-form. But shortly afterwards, from the Middle Devonian, the taxocoenosis was dominated by the involute shells with rapidly expanding whorls and narrow umbilicus (morphogroup 4A) belonging to the nektobenthic and benthopelagic adaptive types (not unlike the modern Nautlius). The proportion of the latter in the overall diversity constituted about 10% of all genera and progressively decreased in the ecogenesis (Fig. 3.15). The proportions of planktonic and nekto-


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CEPHALOPODS IN THE MARINE ECOSYSTEMS OF THE PALEOZOIC 70

1225

(b)

60 50 40 30 bp 10%

(a)

10

pl2 24% pl1 3%

20 0 % 100

nb 63%

(c)

80 60 40

0

Eifelian Givetian Frasnian Famennian Tournaisian Visean Serpukhovian Bashkirian Moscovian Kasimovian Gzhelian Asselian Sakmarian Artinskian Kungurian Roadian Wordian Capitanian Wujiapingian Changhsingian

20

Fig. 3.16. Order Tornoceratida. Explanations as in Figs. 3.1. and 3.3.

benthic forms in this order over the course of evolution remained more or less constant, but with a tendency towards an increase in the proportion of planktonic forms: in the Eifelian these represented 32%, in the Givetian 55%, and in the Frasnian 43%. In the Famennian this order is represented only by planktonic forms, dominated by the involute shells with slowly expanding whorls (Prolobites and related genera). The change in the ecological specialization of Anarcestida was probably related to events of anoxia in the Late Devonian, which reduced the suitable near-bottom habitats, and possibly as a result of competition with other groups (tornoceratids and discosorids). 3.2.14. Order Tornoceratida This order is characterized by an undivided ventral lobe and unstable position of the siphuncle. Tornoceratida existed from the Middle Devonian to the Late Permian, although their taxonomic diversity was not high. This order was represented by three life-forms: benthopelagic, nektobenthic, and planktonic (Fig. 3.16), but the ecological structure of the order changed considerably in the course of ecogenesis. In the Middle Devonian and in the Frasnian Tornoceratida were dominated by morphotypes 1A, 4A, and 5A (see Chapter 2), which are exclusively involute nektobenthic forms. In PALEONTOLOGICAL JOURNAL

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the Famennian, Tornoceratida colonized both benthopelagic and planktonic adaptive zones. In the benthopelagic zone they replaced anarcestids. In the planktonic zone, they, similar to anarcestids, are mainly represented by the morphotype of involute (Cheiloceras) shells with slowly expanding whorls in contrast to clymeniids, which appeared in the same adaptive zone, and with mostly evolute shells (morphogroup 3A) of the planktonic life-form. Tornoceratida were the only ammonoid order to survive the Devonian–Carboniferous boundary. And these were species, possibly of nektobenthic lifestyle. Despite an increase in diversity in the Early Tournaisian, Tornoceratida remained not particularly numerous in the Carboniferous and Permian and belonged to mainly nektobenthic, and to a lesser extent benthopelagic life-forms, while the number of supposedly planktonic genera was low. In the mid-Permian several benthopelagic forms appeared in this group, but by the end of the Permian only one involute planktonic form survived. Several long-ranging and often cosmopolitan genera (Tornoceras, Irinoceras, Neoaganides, and Agathiceras) are known among tornoceratids. 3.2.15. Order Clymeniida Clymeniids are possibly the most bizarre Paleozoic cephalopods. The main distinctive feature of this group

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were nektobenthic, some (e.g., Kazakhstania) had evolute shells, which may suggest that these belonged to the planktonic life-form.

bp 2% pl2 10% nb 24%

3.2.17. Order Goniatitida

pl1 64%

Fig. 3.17. Order Clymeniida. Explanations as in Figs. 3.1. and 3.3.

is their dorsal siphuncle. They lived in the second half of the Famennian. This order is characterized by an extremely high evolutionary rate (over 70 genera appeared and became extinct in about 10 million years) and a clear-cut planktonic specialization (74% of clymeniids genera likely to have been planktonic) (Fig. 3.17). It is possible that the first clymeniids were planktonic, while nektobenthic and some benthopelagic forms appeared later. Most planktonic forms belonged to morphogroup 3A, whereas others are represented by involute, slowly expanding shells, among which there are some bizarre shells with inflated whorls and triangular coiling (Wocklumeria, Synwocklumeria). Nektobenthic forms, mainly semi-involute, rapidly expanding shells forms (morphogroups 5A, 6A) constituted 24% of the generic diversity in the order, whereas benthopelagic forms are represented by only 2 genera. Because of their ecological specificity clymeniids often occupied the deep shelf. Their maximum diversity was around the levels marking global transgressions. The proportion of planktonic forms increased in the course of evolution of this order reaching 100% at the end of the Famennian. As mentioned above, one of the possible reasons for the increasing proportions of planktonic forms, observed in the Famennian in all ammonoid orders, could be anoxic events affecting the near-bottom zones of marine basins. Repeated large-scale anoxic events (at least four) are documented in the Famennian sections by deposition of black shale (Becker, 1993). The last (Hangenberg) event if was not the main reason, contributed greatly to the extinction of most ammonoids and many other organisms at the Devonian–Carboniferous boundary. 3.2.16. Order Praeglyphioceratida A few representatives (about 10 genera) of this order are known from the Famennian and Tournaisian. In the Devonian they were characterized by only nektobenthic forms, whereas in the Tournaisian, while most genera

Goniatitida is the most numerous and morphologically diverse order of Paleozoic cephalopods with a planispiral shell. Ecologically it contains nektobenthos (37%), plankton (35%), and benthopelagic forms (28%) (Fig. 3.18). From the time of their appearance in the Middle Tournaisian and during the entire Carboniferous and most of the Permian goniatitids played a dominant role in the cephalopod communities, determining their morphological and ecological structure. The first representative of the order, genus Goniacyclus, is interpreted as nektobenthic, but numerous and diverse genera appeared almost at the same time, and are interpreted as belonging to the benthopelagic lifeforms or to somewhat late appearing planktonic forms. The main feature of the ecological structure of the order is high taxonomic diversity of benthopelagic forms. The structure of the order changed significantly in the course of its evolution (Fig. 3.18). The main trend was towards a gradual decrease in the diversity of nektobenthic genera (45% in the Early Carboniferous, 25% at the end of the Permian) due to the increase in the proportions of planktonic and benthopelagic forms. The most significant decrease in number of nektobenthic goniatitids is recorded for the Early and Late Permian. Partly, this could be related to the competition of other ammonoid groups: prolecanitids in the Early Permian (their number increased from 12 genera in the Late Carboniferous to 24 genera in the Early Permian), and ceratitids in the Late Permian. 3.2.18. Order Prolecanitida Prolecanitids appeared in the Early Tournaisian, almost immediately after a major extinction at the Devonian–Carboniferous boundary. This ammonoid group is characterized by a unique type of increased sutural complexity and a distinctive trend in the evolution of shell morphology. The first prolecanitids had an evolute, widely umbilicate shell with flattened flanks, which in the course of evolution became more and more involute and platyconic. At the same time the whorl expansion rate increased, which indicates a change in the ecological specialization of the order (Fig. 3.19) from mainly planktonic in the Early Carboniferous to nektobenthic in the Late Carboniferous and Early Permian, when they were the most diverse. At that time specific platyconic nektobenthic forms with ventrolateral keels and a number of other characters appeared. A complex, strongly fluted septum typical of this group suggests the ability of the animal to quickly change its buoyancy, and, in addition, it served to increase the strength of the shell allowing it to exist at various


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(b)

100 80 (a)

60 40

pl2 16%

bp 28%

20 0 % 100

pl1 19% nb 37%

(c)

80 60 40

0

Tournaisian Visean Serpukhovian Bashkirian Moscovian Kasimovian Gzhelian Asselian Sakmarian Artinskian Kungurian Roadian Wordian Capitanian Wujiapingian Changhsingian

20

Fig. 3.18. Order Goniatitida. Explanations as in Figs. 3.1. and 3.3.

Number of genera 20

(b)

16 12

pl1 bp 5% 3%

8

(a)

4 0 % 100 nb 92%

(c)

80 60 40

0

Tournaisian Visean Serpukhovian Bashkirian Moscovian Kasimovian Gzhelian Asselian Sakmarian Artinskian Kungurian Roadian Wordian Capitanian Wujiapingian Changhsingian

20

Fig. 3.19. Order Prolecanitida. Explanations as in Figs. 3.1. and 3.3. PALEONTOLOGICAL JOURNAL

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1227

1228


(b)

16 (a) bp 8%

pl1 64%

12 8

nb 28%

4 0 % 100

(c)

80 60 40

Changhsingian

Wujiapingian

Capitanian

Roadian

0

Wordian

20

Fig. 3.20. Order Ceratitida. Explanations as in Figs. 3.1. and 3.3.

depths. A streamlined shape with keels indicates good maneuverability in various shelf environments. 3.2.19. Order Ceratitida Ceratitids first appeared at the Early–Middle Permian boundary. In the Permian, this group, which reached its maximum diversity in the Mesozoic (Triassic) was not yet widespread. The first Ceratitids with an evolute shell and slowly expanding whorls are interpreted as belonging to the planktonic life-form, while nektobenthic and benthopelagic genera appeared later, Capitanian and Wujiapingian, respectively. Predominantly planktonic specialization is characteristic of the Paleozoic history of the order, although at the end of the Permian the proportion of nektobenthic species, represented by the morphotype of semi-evolute, discoconic forms with rapidly expanding whorls (morphogroup 6A), increased to 34% (Figs. 3.20b, 3.20c). In the Carboniferous and Early Permian this morphospace was mainly occupied by nautilids. Later, in the Permian, ceratitids pushed nautilids out of this adaptive zone. 3.3. Morphological Diversity of Life-Forms and Ecogenesis of Cephalopod Taxocoenosis in the Paleozoic As discussed in Chapter 2, cephalopods with initially different types of shell morphology (straight, curved,

spirally coiled) were adapted to existence in the same adaptive zone, i.e., while developing the same life-form, could acquire both similar and different morphology. To summarize the data from Chapter 2 and preceding sections of Chapter 3, several morphotypes can be recognized within each of the four major life-forms of cephalopods. Benthic. Two morphotypes can be recognized within the benthic life-form: (1) breviconic cyrtocones or secondarily orthoconic shells, typified by the genera Scyphoceras and Burenoceras (Fig. 2.3); longiconic, orthoconic, large to gigantic shells typified by the endocerid and actinocerid genera: Cameroceras, Endoceras, and Gonioceras (Fig. 2.4). Benthopelagic. Five morphotypes can be recognized within the benthopelagic life-form: (1) breviconic cyrtoconic and orthoconic shells, typified by the genera Protophragmoceras and Conostichoceras; (2) loosely coiled shells with very rapidly expanding whorls, typified by Lyrioceras and Nephriticeras (Fig. 2.20); (3) spirally conical shells with projecting whorls typified by the genera Lorieroceras and Foersteoceras (Fig. 2.21); (4) planispiral shells with rapidly expanding whorls, typified by modern Nautilus; and (5) slowly expanding coiled forms, typified by cadiconic and spheroconic goniatitids. Nektobenthic life-form: (1) Straight or slightly curved longiconic shells, typified by Virgoceras


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Endocerida

Ellesmerocerida

Ellesmerocerida

Tarphycerida

Tarphycerida

11% pl

12% b

31% bp

46% nb

Endocerida

Tarphycerida

Ellesmerocerida Endocerida

Tarphycerida

Ellesmerocerida

Fig. 3.21. Ecological structure of cephalopods in the Early Ordovician. Small diagrams show the taxonomic structure of the lifeform (for Figs. 3.21–3.32). Explanations: (nb) nektobenthic, (bp) benthopelagic, (b) benthic, (pl) planktonic life-forms.

(Fig. 2.4); (2) shells with a dorsal gas-filled chambers, typified Ascoceras (Fig. 2.23); (3) shells with trochoid, non-projecting whorls, typified by Cumingoceras and Leurotrochoceras (Fig. 2.21); (4) Planispiral shell moderately evolute with moderately rapidly expanding whorls, usually with well-developed ornamentation— morphogroup 6A, 2N (Figs. 2.12, 2.18); and (5) planispiral involute discoconic and pachyconic shells, with rapidly and moderately expanding whorls, smooth ammonoids—morphogroups 1A, 4A (Fig. 2.14). Planktonic life-form: (1) Cyrtoconic or secondarily orthoconic breviconic shell with a narrowed or closed aperture, typified by Phragmoceras, Pentameroceras, or Cinctoceras (Fig. 2.3); (2) longiconic, straight or weakly curved shells without mechanism of orientation control, typified by Bactrites (Fig. 2.4); (3) planispiral shell evolute, loosely coiled (typified by Estonioceras and Bickmorites) or tightly coiled, slowly expanding whorls—morphogroups 3A, 3N (Figs. 2.10, 2.16, 2.20); and (4) planispiral involute shell with slowly expanding whorls, morphogroup 1A` (Fig. 2.14). PALEONTOLOGICAL JOURNAL

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Below the ecological structure of the cephalopod community is discussed for each stage of the Paleozoic. About 100 cephalopod genera are known from the Early Ordovician; half of these were ellesmerocerids, about 20% are tarphycerids, and about 15% endocerids. Another four orders: orthocerids, actinocerids, pseudorthocerids, and lituitids were represented by only a few genera. The ecological structure of the cephalopod community (Fig. 3.21) was mainly composed of taxa of the former three orders (show in Fig. 3.21). Nektobenthic forms constituted about half of the total taxocoenosis and were the most diverse taxonomically: this adaptive zone was populated by all then existing orders, with ellesmerocerids representing over half of the total number of taxa, while 20% belonged to endocerids, and the number of genera of the remaining five orders constituted only a third of the total diversity. The morphotype of straight longiconic shells dominated, while only five genera were coiled tarphycerids. About 30% of the taxocoenosis was represented by benthopelagic life-forms, which included three orders,

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Ellesmerocerida

Oncocerida

Endocerida Oncocerida

Discosorida Tarphycerida

Actinocerida

Orthocerida Lituitida Barrandeocerida

7% b

Pseudorthocerida Ellesmerocerida Tarphycerida Lituitida Barrandeocerida

12% pl Oncocerida

Discosorida Ellesmerocerida Ascocerida Endocerida

Oncocerida

Tarphycerida Lituitida Barrandeocerida

28% bp 53% nb

Actinocerida Orthocerida

Discosorida Pseudorthocerida

Fig. 3.22. Ecological structure of cephalopods in the Middle Ordovician. Explanations as in Fig. 3.21.

with three different morphotypes. Planktonic and benthic forms each represented approximately onetenth of the content of the taxocoenosis. Of ten planktonic genera, eight belonged to the orders Tarphycerida and two to Ellesmerocerida. The benthic life-forms included ellesmerocerids (nine genera) and endocerids. Middle Ordovician. About 140 genera are recorded from the Middle Ordovician. The taxonomic composition of the assemblage is fundamentally different from that discussed above. Ellesmerocerids, endocerids, and tarphycerids, which dominated previously, were sharply reduced in diversity. The number of genera of orthocerids, pseudorthocerids, actinocerids, and lituitids increased significantly, while four new orders emerged. At the same time, the proportions of the lifeforms changed only insignificantly (Fig. 3.22). The proportion of benthic and benthopelagic forms decreased by 8% compared to the Early Ordovician. This may be a reflection of the increasing transgression in the mid-Ordovician, which resulted in the pelagic adaptive zone increasing in size and becoming more

strongly differentiated. The adaptive vacancies that emerged created an opportunity for new groups, with a new body plans and large adaptive potential, which replaced more primitive groups (ellesmerocerids, endocerids, and tarphycerids), and increased in diversity. The taxonomic composition of all life-forms in the Middle Ordovician became more diverse. Despite the increase in numbers and proportion of the total, new genera of actinocerids and oncocerids appeared alongside the continuing Early Ordovician endocerids and ellesmerocerids. Orthocerids and actinocerids became the dominant benthopelagic life-forms. Among planktonic forms, ellesmerocerids considerably reduced their presence, but six new orders appeared. To a varying extent, the nektobenthic adaptive zone was occupied by 11 of the Middle Ordovician orders. In the Late Ordovician the general ecological structure of the community (Fig. 3.23) changed only slightly: the proportion of the benthopelagic forms increased, while that of the nektobenthic decreased by 6%. This might have resulted from large scale reduction


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Endocerida Oncocerida Ascocerida

Tarphycerida

Actinocerida

Discosorida Barrandeocerida Orthocerida

Tarphycerida

7% b

Endocerida

14% pl

Orthocerida Discosorida Oncocerida

Ascocerida

32% bp

Tarphycerida Endocerida

47% nb

Oncocerida Discosorida


Pseudorthocerida

Fig. 3.23. Ecological structure of cephalopods in the Late Ordovician. Explanations as in Fig. 3.21.

of the shelf areas, which were the main habitats of nektobenthic organisms. The taxonomic composition of all life-forms changed significantly. Although the same 11 orders continued from the Middle to the Late Ordovician, the number of representatives of different orders within each of the life-forms decreased. Among the representatives of the benthopelagic life-form, pseudorthocerids were replaced by the twofold increased number of oncocerids. Ellesmerocerids disappeared from the benthic communities, while oncocerids became more prominent. Oncocerids took a leading position in the composition of the benthopelagic life-form, and together with Discosorids constituted 75% of the taxocoenosis. The number of orders representing the nektobenthic life-form (the most diverse life-form) decreased, while lituitids, ellesmerocerids, and barrandeocerids disappeared. Representatives of the latter order constituted about 20% of the planktonic life-form, which by then did not contain lituitids. All this suggests the replacement of less PALEONTOLOGICAL JOURNAL

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advanced order groups by more advanced ones, and the ecological differentiation of the more advanced forms. The ecological structure of the Silurian cephalopod community (Fig. 3.24) is very generalized, since during the Silurian the taxonomic diversity and proportions of the life-forms were very changeable from province to province (Barskov and Kiselev, 1995). This structure mainly reflects the interval of the mid-Silurian–maximum transgression, diversification of reef communities and graptolites, which were possibly the links in the same trophic chain. In the Silurian in the ecological structure of the community, the proportion of pelagic forms (planktonic and nektobenthic) increased and now for the first time constituted 70%, and has not gone below this level ever since. Ecologically this may mean the beginning of the development of a balanced community, with the structure changed only insignificantly, despite the fundamental changes in the composition of life-forms and the community in general.

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Discosorida Actinocerida Oncocerida

Tarphycerida Barrandeocerida Actinocerida Orthocerida

4% b Discosorida

Pseudorthocerida

Tarphycerida Ascocerida

Barrandeocerida

31% pl

26% bp

Oncocerida Orthocerida Discosorida

39% nb Ascocerida

Oncocerida

Tarphycerida Barrandeocerida Actinocerida

Orthocerida


Fig. 3.24. Ecological structure of cephalopods in the Silurian. Explanations as in Fig. 3.21.

Another feature of the Silurian community was the appearance of a morphotype with a unconventional shell morphology. Benthic communities were dominated by pseudorthocerids, which were represented by forms with a gastropod-like shell, and the newly appeared orthocerids. 80% of the benthopelagic lifeform was represented by discosorids and pseudorthocerids with a curved shell and narrowed or closed aperture, which for the first time occupied this adaptive zone. The nektobenthic life-form was for more than a half represented by the morphotype of straight shells with an advanced mechanisms of horizontal orientation control (orthocerids, pseudorthocerids, actinocerids, although they included representatives with a weakly curved, secondarily straight, or planispiral shell and above-mentioned forms with a low trochoid and ascoceroid (apically decollating). A quarter of the planktonic life-forms were represented by coiled barrandeocerids and tarphycerids with slowly expanding, often loosely coiled shells, and among the latter by low trochoid

shells. Half of the planktonic life-forms were breviconic, often barrel-shaped shells with a narrowed or closed aperture. It was the first time when the planktonic life-form included orthocerids with a small shell without mechanisms for orientation control (ancestors of bactritids). Early Devonian. A considerable decrease in the generic diversity and fundamental change in the dominant taxonomic group (appearance of first ammonoids and or nautilids) did not affect the total ecological structure of the cephalopod community. The proportion of pelagic forms reached 73% because of an increase in the number of nektobenthic genera (Fig. 3.25). A few benthic forms were represented by orthocerids, pseudorthocerids, and actinocerids with a bradyconic and trochoid shell. About half of benthopelagic life-forms were discosorids. The other half comprised orthocerids, oncocerids, and actinocerids with a breviconic-cyrtoconic or secondarily orthoconic shell. The


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Endocerida

Pseudorthocerida

Orthocerida

Actinocerida

Oncocerida

Actinocerida Actinocerida Nautilida

6% b

Pseudorthocerida Discosorida Oncocerida

Discosorida

Orthocerida

Anarcestida

21% bp

28% pl Barrandeocerida

45 nb


Pseudorthocerida Anarcestida Discosorida Oncocerida

Fig. 3.25. Ecological structure of cephalopods in the Early Devonian. Explanations as in Fig. 3.21.

nektobenthic life-form was dominated for the first time by the morphotype of coiled semi-involute shells, represented by anarcestids. The remaining five orders, which belonged to this life-form, were represented by the morphotype of primarily orthoconic (orthocerids, pseudorthocerids, actinocerids) and secondarily straight shells (oncocerids and discosorids). The composition of the planktonic life-form changed fundamentally, both taxonomically and morphologically. Taxonomically, the planktonic life-form in the Early Devonian comprised almost 70% barrandeocerids, and 25% newly appearing groups of the order rank: ammonoid order Anarcestida and nautilids. Two morphotypes of coiled forms dominated: evolute shells with slowly expanding whorls and spirally coiled shells with loosely coiled whorls, whereas involute, slowly expanding forms were represented by only a few genera. Among planktonic forms, a somewhat larger role was played compared to the Silurian by the morphotype of orthoconic subcylindrical shells. The morphotype of the cyrtoconic shells with a closed aperture (oncocerids, discosorids), dominant in the Silurian, was represented by a few genera. PALEONTOLOGICAL JOURNAL

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Middle Devonian. The ecological structure retained the same proportions of life-forms as in the Early Devonian (Fig. 3.26). Benthic forms were dominated by oncocerids with a breviconic, cyrtoconic, and secondarily orthoconic shell, by representatives of barrandeocerids, discosorids, and nautilids, which were represented by one genus each. The benthopelagic group had the morphotypes of coiled shells with rapidly expanding whorls equally represented, but taxonomically, apart from anarcestids and nautilids the community contained newly appeared barrandeocerids. The nektobenthic life-form also became more diverse than in the Middle Devonian: but while retaining equal proportions of the morphotypes of orthoconic and coiled involute shells, the latter contained newly appeared tornoceratids, apart from the continuing anarcestids and nautilids. In the Middle Devonian planktonic living forms the number of anarcestids and nautilids increased. Among the coiled forms the number of taxa with an involute shell, with slowly expanding whorls increased. The proportion of the morphotypes with a coiled shell and orthoconic shells (bactritids) remained the same.

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Barrandeocerida Anarcestida

Oncocerida

6% b

Barrandeocerida

Nautilida


Bactritida

29% pl

Nautilida

21% bp

Discosorida

Anarcestida

44% nb

Anarcestida

Barrandeocerida Nautilida Actinocerida

Oncocerida Oncocerida

Orthocerida Discosorida

Tornoceratida Pseudorthocerida

Fig. 3.26. Ecological structure of cephalopods in the Middle Devonian. Explanations as in Fig. 3.21.

The number of breviconic forms with a closed aperture somewhat increased (oncocerids, discosorids). Late Devonian. In the late Devonian, the generic diversity was the highest in the entire Paleozoic history of the class. It increased due to the increase in the number of planktonic forms, as a result of which the proportion of pelagic life-forms reached 85%. In the Late Devonian the taxonomic composition and ecological structure of the cephalopod community was profoundly changed (Fig. 3.27). The major transformations are recorded in the Famennian, after the crisis of cephalopods at the Frasnian/Famennian boundary, whereas the structure of the Frasnian community in general was very similar to that in the Middle Devonian. The taxonomic identity of the Late Devonian cephalopods was determined by the appearance of the ammonoid order Clymeniida, which existed only in the Famennian and by the increase in the generic diversity of tornoceratids (ammonoids), among which the planktonic and benthopelagic forms appeared for the first time, and of discosorids. Most clymeniids (two-thirds of all existing genera), occupied the planktonic adap-

tive zone, representing the morphotype of evolute shells (morphogroup 3A), and in the end of their evolution also involute shells (morphogroup 1A) with slowly expanding whorls. Beginning in the Famennian, a distinct planktonic specialization is also recorded in the order Anarcestida. Among tornoceratids, planktonic forms also appear in the Famennian. Discosorids were more flexible and adaptively diverse and displayed the entire spectrum of life-forms, although being mostly connected with benthic environments. Discosorids, as well as clymeniids, completed their evolution at the end of the Famennian. The taxonomic composition of life-forms. Benthic forms included only seven discosorid genera. Discosorids also dominated the benthopelagic forms displaying a morphotype of breviconic cyrtocones with a narrowed aperture, whereas the morphotype of coiled forms was less than 20%, in contrast to the Middle Devonian, when anarcestids, nautilids, and barrandeocerids constituted not less than half of all benthopelagic forms. Coiled ammonoids (tornoceratids, anarcestids, and newly appeared clymeniids) began playing a more


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Nautilida Clymeniida Orthocerida Tornoceratida

Discosorida

Anarcestida Oncocerida

2% b

Praeglyphioceratida

43% pl

Discosorida

Clymeniida

Discosorida

14% bp

Bactritida Nautilida

1235

Actinocerida Nautilida Orthocerida Pseudorthocerida

Clymeniida

41% nb

Änarcestida

Tornoceratida

Discosorida

Anarcestida

Oncocerida

Tornoceratida

Fig. 3.27. Ecological structure of cephalopods in the Late Devonian. Explanations as in Fig. 3.21.

noticeable role in the composition of nektobenthic lifeform. About a third of all nektobenthic ammonoids were represented by the morphotype of the secondarily straight shells of oncocerids and discosorids. In contrast to the Middle Devonian, discosorids were dominant. Orthocerids, pseudorthocerids, and actinocerids with an initially straight shell, comprising more than 25% of the diversity in the Middle Devonian, in the Late Devonian, were represented by only a few genera. The taxonomic structure of the planktonic life-form for over 80% was dominated by ammonoids of the orders Tornoceratida, Anarcestida, and Clymeniida. By the number of genera, more than a half of these were clymeniids, representing the morphotype of the evolute shells with slowly expanding whorls. The morphotype of involute shells with slowly expanding whorls was represented by tornoceratids and, to a lesser extent, by anarcestids and late clymeniids. Less than 20% of the planktonic cephalopods were represented by the morphotype of straight (bactritids) and breviconic cyrtocones with a narrowed or closed aperture. The maximum in the Paleozoic generic diversity and a distinct PALEONTOLOGICAL JOURNAL

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ecological structure of the cephalopod community of the Late Devonian reflects specific climatic and marine conditions of that time, and, to a lesser extent, may be an indicator of these conditions. A warm climate and abundant land vegetation, which, when washed into the water, was not effectively utilized, resulted in an accumulation of non-oxidized organic matter on the sea bottom. The absence of the glacial cover and, hence, psychrosphere, precluded the convection of marine water and resulted in the appearance of anoxic conditions in the deep regions of the marine basins. Periodical spilling of anoxic waters onto shallow shelf areas (“black shales events”) potentially causing mass extinctions (Kellwasser Event at the Frasnian–Famennian boundary, Hangenberg Event at the Famennian–Tournaisian boundary). The latter event was catastrophic in the evolution of cephalopods, and its consequences were as dramatic as the outcome of the biotic crisis at the Permian–Triassic boundary. Evidently, such global events were responsible for fundamental changes in the cephalopod communities, which represented the most numerous and important part of the Paleozoic marine

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Nautilida Oncocerida Pseudorthocerida

Goniatitida

Bactritida Tornoceratida

Nautilida

Goniatitida

29% pl

25% bp

Tornoceratida

Oncocerida

46% nb

Prolecanitida

Oncocerida

Nautilida

Pseudorthocerida

Goniatitida

Orthocerida Actinocerida Prolecanitida

Fig. 3.28. Ecological structure of cephalopods in the Mississippian. Explanations as in Fig. 3.21.

biota. Apparently, the reduction of the proportions of the coastal benthic forms in the total cephalopod diversity during the late Devonian was a direct result of the reduction of coastal habitats. The benthic life-form was represented solely by discosorids. The benthopelagic adaptive zone was dominated by the morphotype of breviconic cyrtocones of discosorids. In the Late Devonian this adaptive zone was also occupied by the ammonoid order tornoceratids. Benthopelagic tornoceratids were represented by relatively large forms with an involute, spheroconic or subspheroconic shell with slowly expanding whorls. Apparently this shell shape was more successful in competition with anarcestids, some of which were benthopelagic forms with rapidly expanding whorls. In the Mississippian (Early Carboniferous) changes in the total ecological structure were caused by the relative reduction in the number of planktonic forms and by ectocochliates completely abandoning the benthic adaptive zone. Slightly less than a half of the diversity was represented by nektobenthic forms, while planktonic and benthopelagic forms were in almost equal proportions. In addition, the composition of all life-forms became dominated (up to 90%) by morphotypes of ammonoids, among which goniatitids, which appeared around this time, constituted over half of the total diversity of cephalopods (Fig. 3.28). Of the benthopelagic life-forms, goniatitids represented around 70% and mainly had large, slowly

expanding involute and semi-involute shells with a compressed whorl cross section. The development of this morphotype, which in the Carboniferous and Permian became dominant among benthopelagic ammonoids, was the major change in the morphological structure of this life-form at the Devonian–Carboniferous boundary. Together with nautilids and tornoceratids (morphotype of involute shells with relatively rapidly expanding whorls), they almost completely replaced the morphotype of orthoconic pseudorthocerids and the last oncocerids. At the beginning of the Carboniferous a new ammonoid order (Prolecanitids) took the place of extinct anarcestids. The first prolecanitids (Tournaisian) were represented by evolute shells with relatively slowly expanding whorls interpreted as adaptive forms. Their further development resulted in the appearance of involute, rapidly expanding nektobenthic forms. The nektobenthic life-form was more diverse taxonomically and was represented by eight orders, but was dominated by the coiled, involute shells, with moderately or rapidly expanding whorls (morphological regions 1A, 4A, 5A) (goniatitids, tornoceratids, and prolecanitids). The second nektobenthic morphotype was represented by semi-evolute moderately expanding forms (mostly nautilids and a few genera of prolecanitids and goniatitids). Half of all nektobenthic genera were represented by goniatitids. The planktonic morphotype of straight shells was represented by the last of oncocerids genera and by bactritids (10%). The coiled


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Nautilida Goniatitida

Bactritida

Nautilida

25% bp

32% pl Goniatitida

Prolecanitida

43% nb

Orthocerida Pseudorthocerida Actinocerida


Tornoceratida

Fig. 3.29. Ecological structure of cephalopods in the Pennsylvanian. Explanations as in Fig. 3.21.

planktonic forms were dominated by the morphotype of evolute, slowly expanding shells, although the proportion of the involute and semi-involute slowly expanding shells increased (morphospace 2A). Pennsylvanian assemblages (Middle and Late Carboniferous together). The general ecological structure remained unchanged (Fig. 3.29). Ammonoids diversified, while nonammonoid cephalopods, except for nautilids, were represented by only a few genera. Actinocerids and orthocerids were represented by no more than five genera, and pseudorthocerids by less than ten genera. The benthopelagic adaptive zone was now populated only by goniatitids and nautilids, mainly represented by involute and semi-involute, cadiconic, spheroconic, and subspheroconic shells with slowly expanding whorls. Involute shells with rapidly expanding whorls were less common. Nektobenthic life-forms were more diverse taxonomically, although 80% of these were represented by ammonoids and nautilids with a coiled semi-involute and semi-evolute shells with moderately expanding whorls. Thus, the cephalopod community became more morphologically homogeneous. Taxa with orthoconic, cyrtoconic, and other shell shapes virtually disappeared from all adaptive zones. Early Permian. The ecological structure of the cephalopod taxocoenosis changed by a slight increase in the number of nektobenthic genera and slight PALEONTOLOGICAL JOURNAL

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decrease in the number of planktonic genera with an evolute slowly expanding shell (Fig. 3.30). The taxonomic composition of the life-forms changed as follows. Among the benthopelagic life-forms the proportion of nautilids increased, while the number of goniatitids genera decreased (number of goniatitids decreased across all adaptive zones); a few benthopelagic tornoceratid genera were present. Tornoceratids were represented by one morphotype of involute shells with rapidly expanding whorls. The number of goniatitids significantly decreased among nektobenthic forms, whereas prolecanitids reached an equal proportion with them. Nautilids and tornoceratids increased their proportion in the nektobenthos. Less than 20% were represented by the morphotype of orthoconic shells of orthocerids and pseudorthocerids. Among planktonic forms, the proportion of goniatitids also decreased, due to the proportional increase of nautilids, and nautilids were represented by the morphotype of evolute shells with slowly expanding whorls, whereas goniatitids were dominated by representatives of the second morphotype (involute forms with slowly expanding whorls). Middle Permian. The overall structure of the taxocoenosis changed only slightly. The proportion of benthopelagic life-forms decreased to 24%, whereas the proportion of planktonic forms increased to 32% (Fig. 3.31). Among benthopelagic life-forms the proportions of the taxonomic groups somewhat changed:

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Tornoceratida


Goniatitida

Prolecanitida

27% pl

26% bp 47% nb

Tornoceratida


Orthocerida

Pseudorthocerida

Prolecanitida

Fig. 3.30. Ecological structure of cephalopods in the Early Permian. Explanations as in Fig. 3.21.

nautilids decreased, while goniatitids prevailed (25 genera). Among nektobenthic forms, goniatitids constituted 45%, while prolecanitids, tornoceratids, and nautilids were less common. The proportion of the morphotype of orthoconic shells of orthocerids and pseudorthocerids constituted only about 5%. In plankton, the proportion of nautilids decreased, whereas ceratitids appeared (5 genera). Both these groups were represented by evolute shells with slowly expanding whorls. Among goniatitids, which contributed the largest proportion to plankton, were equally represented both evolute and involute morphotypes. The proportion of straight shells (bactritids) decreased. Late Permian. The Late Permian shows an extremely unusual ecological structure of the cephalopod community (Fig. 3.32), which somewhat resembles that of the Late Devonian. The taxonomic composition of the life-forms fundamentally changed, primarily because of the Late Permian extinction of previously dominant goniatitids and the early evolution of ceratitids. In the benthopelagic life-form nautilids became dominant for the first time (more than 60%), whereas in the Mesozoic and Cenozoic this adaptive zone became their only ecological domain. The number of goniatitid genera, which had previously contributed more than 80% to the benthopelagic life-form, experienced a four-

fold decrease. Ceratitids for the first time colonized this adaptive zone. Ceratitids, together with nautilids, in equal proportions constituted two-thirds of the generic composition of the nektobenthic life-form. In plankton, the main contributors were ceratitids, represented by an evolute morphotype; whereas goniatitids, tornoceratids, and bactritids were represented by a few genera only. The end-Permian extinction affected all of the then existing orders. Bactritids, pseudorthocerids, and goniatitids became completely extinct. Only two genera of ceratitids, belonging to the planktonic life-form, continued into the Triassic to give rise to the Triassic ceratitid radiation. Among nautilids, the Permian–Triassic crisis was survived by two or three benthopelagic genera, including one prolecanitid genus, which shortly afterwards became extinct without descendants, and the only orthocerid genus, which survived until the end of the Triassic. The above discussion of the changes in the proportions and taxonomic composition of the life-forms in the cephalopod taxocoenosis throughout geological epochs during the Paleozoic allows the following conclusions: There were several time spans of absolute duration of tens of million years, during which the ecological structure of the cephalopod community changed insig-


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Nautilida

Goniatitida

Tornoceratida Bactritida Nautilida

24% bp

32% pl

Orthocerida + Pseudorthocerida

Ceratitida

44% nb

Goniatitida

Nautilida Ceratitida

Prolecanitida

Goniatitida

Tornoceratida

Fig. 3.31. Ecological structure of cephalopods in the Middle Permian. Explanations as in Fig. 3.21.

Ceratitida Goniatitida


36% pl

Goniatitida

22% bp 42% nb

Tornoceratida

Nautilida

Ceratitida

Nautilida

Ceratitida

Orthocerida + Prolecanitida Pseudorthocerida Goniatitida

Fig. 3.32. Ecological structure of cephalopods in the Late Permian. Explanations as in Fig. 3.21. PALEONTOLOGICAL JOURNAL

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nificantly. The proportions of the benthic (benthic and benthopelagic life-forms) and pelagic forms (nektobenthic and planktonic) fluctuated insignificantly. Periods when the ecological structure was stable (homeostatic) included the Ordovician, Early and Middle Devonian, and Frasnian (Late Devonian), and Carboniferous to Middle Permian. When the proportions of the benthic (benthic and benthopelagic) and pelagic (nektobenthic and planktonic) life-forms, are compared for each of these periods, a distinct trend towards a gradual increase of the proportion of the pelagic sector may be observed. As early as the Early Ordovician, despite the predominance of the most primitive groups, the resulting ecological structure contained all life-forms. In the Middle Ordovician, when five new orders appeared and another three orders that were previously represented by a few genera, became widespread, a general ecological structure of this new community, which was twice as numerous and more diverse taxonomically, changed less than might have been expected. The pelagic part of the community increased to 60% due to the appearance of new groups originally adapted to the pelagic adaptive zones (orthocerids, pseudorthocerids, actinocerids—all nektobenthos; barrandeocerids—plankton). However in the Late Ordovician among these groups, there appeared benthic and benthopelagic forms. Thus, the relative stability of the ecological structure of the cephalopod community in the Ordovician, despite the appearance of new groups and increase in taxonomic diversity, was determined by the stability in the marine adaptive zones. The Ordovician community characteristically shows a greater taxonomic diversity for each of the existing life-forms, compared to any other Paleozoic epoch. All then existed cephalopod orders had their representatives in all adaptive zones. In the Ordovician communities, the proportion of benthic (benthic and benthopelagic) forms constitutes 43 and 39% in the Early and Late Ordovician, respectively, and 35% in the Middle Ordovician. The reduction of benthic forms in the Middle Ordovician correlates with the maximum transgression. At that time a structure of the marine ecosystem similar to that of today began developing. Beginning in the Devonian, the proportion of the benthic communities became stabilized at a level of 25%. (In the Recent community, this proportion is about 20%.) In the Devonian, there remained some benthic forms, but beginning in the Carboniferous no benthic forms among ectocochliates ever appeared. Periods of stability in the ecological structure were interrupted by shorter periods when the ecological structure of the community was significantly different (epoch of disrupted adaptive homeostasis). These included Silurian, Famennian (Late Devonian), and Late Permian. All these periods show an increase in the proportion of planktonic forms. Theoretically there are three groups of reasons responsible for upheaval in the ecological structure of the community. The internal reasons: appearance of new body plans, of new taxonomic groups, the physiol-

ogy of which (reflected by the shell geometry and construction) was adapted to a certain lifestyle. To some extent, this may explain the increase in the proportion of planktonic forms in the second half of the Permian, when the first ceratitids appeared. Among the external reasons it is possible to assume increased pressure of competition with other, more successful bottom-dwelling organisms. For instance, changes in the taxonomic structure of the cephalopod community in the Silurian due to more than twofold increase in the proportion of planktonic forms mainly resulted from the reduction in the nektobenthos, the proportion of which in the community was the lowest in the entire Paleozoic. It is the most likely that this, like the reduction in the number of bottom-dwelling cephalopods, could be related to the appearance of new groups of bottom-dwelling fauna. For some of the new colonists, cephalopods could become prey (e.g., for large arthropods). Other new inhabitants (fish) could have been better hunters. Other external reasons could include changes in the oceanic environment leading to the reduction in the number of suitable niches in some adaptive zone. Possibly, this may explain the largest change in the ecological structure of the community in the second half of the Late Devonian. Anoxic events in the Late Devonian, leading to the extinction of the groups of benthic biota and groups connected with the benthos, affected cephalopod communities by almost twofold reduction of the proportion of benthic and benthopelagic forms and almost threefold increase in the number of the inhabitants of the planktonic life-form. Half of the planktonic life-form was represented by the order Clymeniida (Ammonoidea), more precisely by a morphotype of evolute shells, which are interpreted as inhabitants of the open pelagic zone. Probably, all of these reasons played their roles, to varying extents, at times of upheaval of the ecological structure of the cephalopod community. CHAPTER 4. ECOLOGICAL STRUCTURE OF PALEOZOIC AMMONOID COMMUNITIES IN THE URALIAN PALEOBASIN The study of the evolution of ammonoid assemblages over a prolonged time interval within one large paleobasin allows detailed tracking of the nature and mechanism of the morphological and ecological change in communities and their relationship with abiotic events. 4.1. General Background The Uralian Late Paleozoic basin is one of the most favorable objects for such studies. This ancient ocean existed continuously during the second half of the Paleozoic. In the southeast it was connected, through the Aral Region, with the marine basins of the Tien Shan and Pamir, in the east with central and eastern Kazakhstan, in the west with the epicontinental seas of the Rus-


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sian Platform, and in the north with the basins of Novaya Zemlya and Timan-Pechora Province. Almost the entire Middle–Upper Paleozoic sequence of the Urals, from the Emsian (Lower Devonian) to the Artinskian Stage of the Lower Permian contains numerous ammonoids. Some assemblages (Famennian, Late Tournaisian, Late Visean, Serpukhovian, Early Bashkirian, Gzhelian, Asselian, Sakmarian, Early and Late Artinskian) are unique both in taxonomic diversity and in the number of individuals. During the time span under consideration this basin was gradually closing and shallowing as the Siberian and Kazakhstan continents collided with the Russian craton. Beginning in the Late Devonian, the Uralian Paleo-ocean, which separated Baltica from Siberia and Kazakhstania, was gradually closing. This process was accompanied by orogeny and volcanism on the eastern coast of the basin, and by extensive seismic events resulting from the sideways sliding of plates on both the eastern and western coasts. By the end of the Devonian, in the Frasnian and Famennian a series of north-south structures was formed in the territory of the modern Urals, including microcontinents and island arcs separating marine basins with different sedimentary settings (Puchkov, 2000). In the west, between the margin of Baltica and the microcontinent Uraltau, there was a deep epicontinental basin, which formed in the Early Paleozoic (Mizens, 1997, 2000; etc.). The submerged north-south tectonic structures of the Uralian paleocean were the main source of terrigenous material. In its northwestern part there existed deep troughs, separated by uplifts. In the Frasnian and Famennian Domanik-like sediments and thick series of mainly terrigenous and terrigenouscarbonate beds accumulated in the troughs. In the Late Devonian, in the eastern regions there remained a deep oceanic basin, Magnitogorsk island arc, and a subduction zone. At the Devonian and Carboniferous boundary, a considerable change in geodynamic settings occurred in the Urals, resulting from the interruption of the subduction in the Magnitogorsk Megazone (Puchkov, 2000). Thus, the Uralian Basin was gradually loosing its oceanic characteristics, as Laurasia became more consolidated by the beginning of the Carboniferous. In the Early Carboniferous and at the beginning of the Middle Carboniferous, a deep marine basin remained in the north, which was bordered on the west by a shallow Carboniferous shelf of the Timan-Pechora Province. Deep marine basins remained in the territory presently occupied by the Middle and South Urals. These basins had the appearance of narrow, asymmetrical troughs in the west and east, and were freely connected in the south. In the deepest zones of the southern regions of the basin (Zilair Megasynclinorium) sedimentation, as in the Late Devonian, was mainly performed by gravity flows. The rising Urals Mountains were the main source of the terrigenous material, and in PALEONTOLOGICAL JOURNAL

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the southeast of the basin there were also volcanic islands of the Magnitogorsk arc. In the south regions of the basin, the depth increased from north to south from Bashkortostan to the Aktyubinsk Region. In the second half of the Early Carboniferous and at the beginning of the Middle Carboniferous a shallow shelf sea was present on the eastern slope of the Urals, with accumulation of mainly carbonate series, and only in its western part had relatively deep settings (Mizens, 2002; etc.). In the Bashkirian, the Uralian Fore-Deep began developing, which led to a considerable change in the sedimentary and paleogeographic settings in the basin. The amount of polymictic material washed from the slopes of the Mountainous Urals increased resulting in the accumulation of thick flyschoid series on the eastern slope of the depression. At the same time the area of the Carboniferous shelf in the west decreased. In the Moscovian, the Uralian Basin had a number of submeridional facial zones. In the center, there was a chain of islands, symmetrically separating the basin into the western and eastern areas. These areas were isolated in the Moscovian in the North, Middle, and most of the South Urals and were connected at the periclinal region, which is supported by several faunal groups in common (Ivanova, 2002). In the eastern slope of the South Urals the marine basin represented a gulf with no exit in the north and a connection in the south with the European part of the Basin and, via the Aral Region, with seas of Tien Shan, Pamir, central and eastern Kazakhstan (Chuvashov et al., 1984; Ivanova and Chuvashov, 1990). The European part of the Urals was a more open deep epicontinental basin, which in the west was separated by a chain of islands and shoals from the Moscovian Sea of the Russian Platform. In the Late Carboniferous, orogeny began and marine conditions remained mainly on the western slope of the Urals. At that time and during the Early Permian (Asselian, Sakmarian), the Uralian Basin was a meridional asymmetrical strait, surrounding the East European inner sea in the east and connecting the basins of Paleotethys and northern Panthalassa. The strait was separated from the platform regions of the East European sea by a chain of shoals, islands, and reefs. The latter were interpreted as barrier reefs (Antoshkina, 2003; etc.). The Permian reef bodies extended as a submeridional belt along the entire western slope of the Urals from the south to the north, with a branch to the east of the Timan. The hydrological, geographical, and geological conditions were very different on either side of the barrier reef. A that time a large carbonate-evaporite platform existed on the East European plain. Thick molasse and flyschoid series, which accumulated in the fore-deep, were built of terrigenous material from the Hercynian Mountain chains in the east and at the same time carbonate material from the slope of the platform in the west. During the entire Late Carboniferous and Early Permian the fore-deep had a distinct transverse asymmetry. From the east to

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(a)

the west, its structure looked as follows: steep slope, covered by a thick terrigenous trail, bottom of the trough, in the central part covered by the carbonate flysch (the proportion of the terrigenous rocks decreased to the west, as the proportion of carbonates increased). The western slope was a steep cliffy platform slope on which carbonate mud and debris of shelled benthic organisms brought from the edge of the platform accumulated (Khvorova, 1961). A large part of the carbonate was brought to the Uralian strait from the west, at least, until the Artinskian, when shallowing permitted the carbonate sedimentation. In the Artinskian the south termination of the strait began closing, as a result of the collision of the Russian and Siberian plates and of the northward movement of Cimmerian blocks, and eventually, the closure of the Paleotethys. From the end of the Sakmarian–beginning of the Artinskian, the Uralian Strait has no connection with the Paleotethys, gradually closing from south to north, where the connections with the boreal Panthalassa apparently continued, until the complete closure of the Uralian basin at the end of the Middle Permian (Fig. 4.1). The evolution of the biota was also certainly affected by climate. In the Late Paleozoic the Uralian Basin was mainly confined to the tropical and subtropical zones. This partly explains considerable taxonomic diversity of Paleozoic ammonoids in the South Urals. Only at the end of the Sakmarian and in the Artinskian in the north there was a maritime humid climate, which was related to the development of the large South Pole glaciation, which reached its peak in the Asselian (Chumakov and Zharkov, 2002).

D3fm Sib

Panthalassa

Kaz

Ur

Ur

ali

Uu

0°

an

Oc

ean

Ba Lau Ar Av

Tt

Afr

Sam

(b) C1–2 Sib

Panthalassa

Ur Kaz

0°

Laurentia

Uu

Ba

Tt

Lau

Paleothetys

Ar

Av

Al

Afr

Sam

(c)

C3–P1

Panthalassa Sib Tt Kaz Lau

Ur Ba

0°

Al

Uu

Av Ar

Paleothetys

Sam

Afr

1

2

3

4

5

6

4.2. Ecological Structure of the Paleozoic Ammonoid Communities in the Urals The analysis of the ammonoid assemblages is based on the measurements of Raup’s classical parameters of 756 species of the Devonian, Carboniferous, and Early Permian ammonoids from localities in the Urals, PaiKhoy, Novaya Zemlya, and Timan. We measured holotypes or other specimens from the type series, mostly adult shells, because the shell parameters may vary widely throughout growth. Methods, morphogroup and life-form recognition are extensively discussed in Chapter 2. The morphospace defined by the ranges of the parameters W, D, and S of ammonoids studied is confined to the following values of the parameter D ≈ 0– 0.71; W ≈ 1.4–4,19; S = 0.3–2.8 and describes almost all possible morphological variations for monomorphic

Fig. 4.1. Scheme of paleotectonic reconstruction (after Puchkov, 2000, modified): (a) for the Famennian (Upper Devonian); (b) for Mississippian (Early Carboniferous)–beginning of Pennsylvanian (Middle Carboniferous); (c) from the end of the Pennsylvanian (Late Carboniferous)–Early Permian. (1) areas with the continental crust, (2) continents, microcontinents, island arcs, (3) remaining areas with continental crust, (4) rifts, (5) zones of subduction, (6) ammonoid localities in the Uralian Paleobasin. Land: (Av) Avalonia, (Al) Alai Massif, (Ar) Armorican Massif, (Afr) Africa, (Ba) Baltica, (Kaz) Kazakhstan, (Lau) Laurentia, (Sib) Siberian Continent, (Tt) Tajik-Tarim Massif, (Uu) Ust-urt, (Ur) Uraltau, (Sam) South America.


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shells (Fig. 4.2). The most widespread (over 40% of the total species diversity) were the nektobenthic lifeforms from morphospace 1A (morphogroup 1A). This morphogroup is one of the most stable and is present in assemblages throughout the time span studied. Typical representatives include Tornoceras, Dzhaprakoceras, Goniatites, Dombarites, Proshumardites, Delepinoceras, Bilinguites, Agathiceras, Gonioloboceras, Thalassoceras, etc. Species with a discoconic, platyconic, or oxyconic shell with moderately or rapidly expanding whorls (W ~ 2.0–3.5)—morphogroup 4A, for instance Timanites, Kazakhoceras, Girtyoceras, Carinoceras, Medlicottia, Artinskia, etc. also belonged to the nektobenthic life-form, but, in contrast to group 1A, they were scarce throughout the Paleozoic. Their species diversity is about 10% of the total number of forms. A usual shell size is Dm = 40–60 mm, whereas in some species the shell reached size considered large for the Paleozoic ammonoids: Dm = 100–200 mm and over. Planktonic evolute, widely umbilicate forms with a varying cross-sectional shape of the whorl from morphogroup 3A (plankton-1) constitute about 22% of the total taxonomic diversity. Typical representatives of this morph include Hexaclymenia, Trigonoclymenia, Clymenia, Eonomismoceras, Rhymmoceras, Alaoceras, Dombarigloria, Cancelloceras, Eoasianites, Svetlanoceras, Paragastrioceras, etc. Small involute and moderately involute shells with slowly expanding whorls (Prolobites, Epiwocklumeria, Quasicravenoceras, Mirilentia, Lyrogoniatites, Ferganoceras, Emilites, Protopopanoceras, Crimites, etc.), included in morphogroup 1A' and 2A and characterizing the second type of the planktonic life-form (P-2), are less diverse—9% of the total number of species. Benthopelagic forms were not numerous. They included subspheroconic, spheroconic, and cadiconic shells, which were relatively large—with a diameter of 35 mm and more, and with a slowly or moderately slowly expanding shell: the genera Eurites, Glaphyrites, some species of the genera Goniatites, Dombarites, Gastrioceras, Cravenoceras, etc. We refer relatively large (Dm = 35 mm and over) Pachyconic and subdiscoconic shells with a moderately narrow or medium-sized umbilicus from morphogroup 2A to the benthopelagic forms. In total, the diversity of benthopelagic forms was low—17% of the total number of species. The proportions of the life-forms, taxonomic and morphological diversity of the ammonoids from the Uralian Basin changed considerably throughout the course of their evolution (Figs. 4.3a, 4.3b). It is possible to recognize several stages of intense morphogenesis separated by episodes of crises or stagnations, largely related to global or regional abiotic events (transgressions and regressions, change in sedimentary settings, tectonic disruption, climatic changes, etc.). PALEONTOLOGICAL JOURNAL

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1.0 1.5 2.0 2.5

0

0.1

0.2

0.3

0.4

0.5

1243 0.6

0.7

D

1A'

3A

2A

1A

6A 5A 4A

3.0 3.5 4.0 W = 1/D 4.5 W Fig. 4.2. Distribution of the shell shape of 756 species of Paleozoic ammonoids from the localities in the Urals, PaiKhoy, and Novaya Zemlya in relation to the values of the whorl expansion rate (W) and whorl overlap degree (D). Morphospaces recognized for Paleozoic ammonoids are indexed (see Chapter 3).

Below the evolution of ammonoids is discussed by stratigraphic stages from the Devonian to Permian. 4.3.1. Early Devonian Emsian The evolution of ammonoids in the Uralian basin began from the Emsian (Early Devonian). The oldest ammonoid assemblages are known from the eastern slope of the North and Middle Urals (Sverdlovsk Region), and also Novaya Zemlya. Ammonoids were studied by B.I. Bogoslovsky (1962a, 1963, 1969, 1972). Ammonoid collections are medium sized, including about 200 specimens housed in the Paleontological Institute of the Russian Academy of Sciences (PIN). Ammonoids come from several localities. In the north Urals they were found in the region of the Krasnotur’insk (right bank of the Zabolotnaya River, tributary of the Bolshya Volchanka). From darkgray to black, bedded cherty limestone with thin laminations of calcareous sandstone and marl Bogoslovsky (1969) identified Erbenoceras advolvens, Mimosphinctes tenuicostatus, Teicherticeras lissovi, and Convoluticeras erbeni. Ammonoids from the Upper Emsian beds are found in two localities, situated on the left bank of the Pyshma River (eastern slope of the Middle Urals). The limestone inclusions from tuff interbeds and xenolithes from porphyrites yielded Teicherticeras pyshmense. In the number of specimens the Uralian assemblages are dominated by representatives of the genus

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D3fm1

0

D3fm1

Prolobites– Platyclymenia Clymenia– Gonioclymenia Kalloclymenia– Wocklumeria Eocanites– Gattendorfia Goniocyclus– Protocanites Pericyclus– Progoniatites Fascypericyclus– Ammonellipsites Bollandites– Bollandoceras Berychoceras– Goniatites Hypergoniatites– Ferganoceras Uralopronorites– Cravenoeras Fayettevillea– Delepinoceras Homoceras– Hudsonoceras Reticuloceras– Bashkortoceras Bilinguites– Cancelloceras zones not identified zones not identified zones not identified zones not identified zones not identified zones not identified Aktasty Assemblage Baigendzhinian Assemblage

Cheiloceras

0

Prolobites– Platyclymenia Clymenia– Gonioclymenia Kalloclymenia– Wocklumeria Eocanites– Gattendorfia Goniocyclus– Protocanites Pericyclus– Progoniatites Fascipericyclus– Ammonellipsites Bollandites– Bollandoceras Beyrichoceras– Goniatites Hypergoniatites– Ferganoceras Uralopronorites– Cravenoceras Fayettevillea– Delepinoceras Homoceras– Hudsonoceras Reticuloceras– Bashkortoceras Bilinguites– Cancelloceras zones not identified zones not identified zones not identified zones not identified zones not identified zones not identified Aktasty Assemblage Baigendzhinian Assemblage

Cheiloceras

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% 100 (a)

80

60

40

20

D3fm2–3

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1 C1tn1

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2 C1tn2–3

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C1v1

3 C1v2 C1s1 C1s2

Number of species 100

C1v2 C1s1 C1s2

Teicherticeras, which at the adult stage had a narrow evolute shell with contacting but not embracing whorls. Representatives of Erbenoceras advolvens with loosely coiled whorls were subdominant. In Novaya Zemlya, Late Emsian ammonoids are represented by the species Gracillites svetlanae, C2b1

C2b1

C2b2 C2m C3k C3gh P1as P1s

C2b2 C2m C3k C3gh P1as


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90 (b)

80

70

60

50

40

30

20

10

P1ar

4

Fig. 4.3. Famennian, Carboniferous, and Early Permian ammonoid assemblages of the Urals: (a) proportions of life-forms and (b) dynamics of species diversity. Explanations: (1) evolute planktonic forms (P-1), (2) involute planktonic forms (P-2), (3) nektobenthic forms, (4) benthopelagic forms.

Metabactrites formosus, and Latanarcestes boreus (Bogoslovsky, 1972), which also had a shell with loosely coiled whorls.

All known Emsian species belonged to the planktonic life-form (plankton-1, evolute).

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4.3.2. Middle Devonian Eifelian Scarce ammonoids are known from the Middle Devonian of the Uralian Basin (23 species of 13 genera are known from the Eifelian). Ammonoids were studied by Bogoslovsky (1958a, 1958b, 1961, 1969). They are found in several localities, situated on the eastern slope of the North and Middle Urals. The collections (about 150 specimens) are housed in the Paleontological Institute, Russian Academy of Sciences. Early Eifelian ammonoids have been described from the light-gray, fine-grained micritic limestones on the eastern slope of the North Urals: Gyroceratites glaber, Laganites tenuis, Parentites praecursor, Augurites mirandus, and Latanarcestes kakvensis (Bogoslovsky, 1961). Together with ammonoids there are found occasional brachiopods, gastropods, tetracorals, crinoids, and nautiloids. Ammonoid assemblages are dominated by oxyconic and discoconic shells of morphogroups 4A and 6A, nektobenthic forms—3 species, planktonic life-form is represented by one evolute species Gyroceratites glaber. The species Parentites praecursor has a discoconic shell with very rapidly expanding whorls (W = 3.8) and is provisionally interpreted as a benthopelagic form. A diverse ammonoid assemblage is described from the Lower Eifelian on the left bank of the Bobrovka River (eastern slope of the Middle Urals, Artemovskii District). The following species were identified from the cherry-red, thickly bedded cherty limestones, overlain by calcareous sandstones: Gyroceratites gracilis, Fasciculoceras uralense, Mimagoniatites obesus, M. angulostriatus, Agoniatites uralensis, Latanarcestes pronini, L. ventroplanus, Subanarcestes macrocephalus, S. bisulcatus, Mimanarcestes nalivkini, and Werneroceras bobrovkense (Bogoslovsky, 1969). Apart from ammonoids, the limestones include bivalves and nautiloids, and diverse benthic fauna: tetracorals, brachiopods, gastropods, crinoids, trilobites, and tentaculites. Ammonoids are scanty (only 132 specimens in our collections). The assemblages (both in number of specimens and in specific diversity) are dominated by oxyconic, discoconic, and subdisconic nektobenthic species (7 of 11 species). Planktonic forms are represented by evolute shells with varying whorl height (three species). The benthopelagic life-form is represented by one species only. Givetian Givetian ammonoids are known from one locality (left bank of the Bolshoi Elets River, upstream of the village of Eletskaya, near Vorkuta) on the western slope of the Polar Urals. A few ammonoids have been described from the thickly layered dark-gray limestones with foraminifers, brachiopods, and trilobites: Werneroceras uralicum, Pseudofoordites hyperboreus, and Wedekindella psittacina (Bogoslovsky, 1959). The PALEONTOLOGICAL JOURNAL

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host rocks are dated Late Givetian. The species Werneroceras uralicum and Pseudofoordites hyperboreus may be referred to the nektobenthic life-form, because they have a discoconic medium-shelled shell (Dm = 30– 50 mm) with a narrow umbilicus and relatively rapidly expanding whorls. Wedekindella psittacina have a discoconic, narrowly umbilicate small shell (less than 20 mm in diameter), with slowly expanding whorls, characteristic of the planktonic (plankton-2) life-form. The development of the Early and Middle Devonian ammonoids from the Uralian Paleocean reflects the general features of the evolution of this group at this stage. Only planktonic forms are characteristic of the Emsian. In the Eifelian, the diversity of life-forms and morphotypes increased, and nektobenthic and benthopelagic forms appeared. Ammonoids predominantly belong to the nektobenthic life-form and are found in limestones including diverse benthic fauna. An extremely impoverished ammonoid assemblage from the Givetian of the Subpolar Urals is also mainly represented by nektobenthic forms. Givetian ammonoids, similar to Eifelian, come from the relatively shallow carbonate beds, including diverse benthic fauna. 4.3.3. Late Devonian Frasnian In the Late Devonian, the diversity of ammonoids in the Uralian Paleocean increased considerably. The main peak of morphogenesis was in the Famennian and was to a large extent related to the appearance and evolution of the order Clymeniida. The taxonomic and morphological diversity of Frasnian ammonoids were relatively low. They are mainly known from the localities on the western slope of the Middle and Subpolar Urals, in Novaya Zemlya, Pai-Khoy, and South Timan. In the second half of the Frasnian, the ammonoids began their evolution in the South Urals. The Lower, Middle, and (possibly) Upper Frasnian (Manticoceras Zone) contain numerous ammonoids. However, fine correlations with the Western European zones is often complicated, because the majority of the Uralian species are endemic or have a wide stratigraphic distribution. The assemblage includes 27 species, 11 genera, and four families. Collections include about 2900 specimens housed in the Paleontological Institute of the Russian Academy of Sciences, Chernyshev TsNIGR-Museum, St. Petersburg Mining Institute, and in All-Russia Scientific Research Geological Oil Institute (the taxonomic composition of Frasnian ammonoids was studied by many workers (Bogoslovsky, 1957, 1958b, 1969, 1971; Bogoslovsky et al., 1982; Yanischewsky, 1926; Lyashenko, 1956, 1957; etc.). The morphospace defined by the parameters W, D, and S of Frasnian ammonoids is restricted by the values of W = 1.54–3.3; D = 0.01–0.57; and S = 0.67–1.9. The fauna was dominated by the nektobenthic ammonoids with discoconic and oxyconic shells, with a narrow or

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moderately narrow umbilicus (23 species, 85%). Evolute ophioconic or platyconic shells are also present (4 species, 15%). Ammonoids are known from various parts of the basin, characterized by different sedimentary settings and geology. In total, the assemblages are impoverished both taxonomically and morphologically. Frasnian ammonoids of the Urals show a high degree of endemism at the species level. In the north of the Uralian Paleobasin, ammonoids are found in several localities, on the western slope of the Subpolar Urals and in the Chernyshev Range (basin of the Usa River). Ammonoids were found in gray and dark-gray, almost black, limestones (Kynov-Sargaevo Beds). The assemblage included Hoeninghausia uchtensis, Koenetites uralensis, Timanites keyserlingi, Tornoceras typum, Komioceras stuckenbergi, Ponticeras sp., and Manticoceras sinuosum. This assemblage is typical of the Lower Frasnian (Manticoceras Zone, possibly an equivalent of Zone C, based on Timanites keyserlingi). All species have a discoconic, narrowly umbilicate shell, are often large in size and represent the nektobenthic life-form. An equivalent ammonoid assemblage is known from the localities in the basin of the Kara River (eastern slope of the Pai-Khoy Range). Timanites keyserlingi and Tornoceras typum are recorded from black pyritized limestone (Bogoslovsky, 1969). On the South Timan, the Early Frasnian ammonoids (Zones B–E) are found in the localities in the Ukhta River basin. Ammonoids come from the carbonate interbeds in the series of the greenish-gray clays with rare layers of light-gray and greenish-gray marl of the Ust-Yarega Formation, and also from the carbonate interbeds of the Domanik series. The Ust-Yarega Formation is composed of clay, siltstone with marl interbeds, and of sandy, clayey, and detrital limestone. The deposits of the Domanik Formation represent organicrich carbonate-cherty shale with clayey interbeds. Together with ammonoids, there were found rare bactritoides, nautiloids, and tentaculites, and in addition, numerous remains of brachiopods and ostracodes. The assemblage includes Tornoceras typum, Timanites keyserlingi, Ponticeras uralicum, P. uchtense, P. tshernyschewi, P. bisulcatum, P. lebedeffi, P. auritum, Probeloceras keyserlingi, P. domanicense, Komioceras stuckenbergi, Aulotornoceras keyserlingi, Uchtites syrjanicus, Manticoceras ammon, etc. The assemblages are dominated by nektobenthic discoconic and oxyconic forms (11 species), and also narrow evolute planktonic species of the genera Ponticeras and Probeloceras (4 species). The Late Frasnian ammonoids come from the localities in the Sed’yu River basin, along the rivers Lyaiol and Vezha-Vozh from the Lyaiol Formation (Bogoslovsky, 1969). This formation is represented by the sediments of the slope of the fore-reef depression, including turbidites: interbedding of organic-rich shale and limestone. Ammonoids come from the limestone, which also contains numerous remains of brachiopods, ostracodes, and conodonts, whereas clay contains

miospores. Predominantly algal reef was located in the south and southeast of the region (Yudina and Moskalenko, 1997). The assemblages are dominated by numerous species of the genus Manticoceras, with subdominant Carinoceras ljaschenkovae, C. menneri, Tornoceras typum, and Timanoceras ellipsoidale. Most species have a discoconic or oxyconic shell with a moderately narrow or narrow umbilicus and a moderate or high whorl expansion rate, characteristic of the nektobenthic life-form. Shells frequently reach large size (up to 200 mm in diameter and over), while the average diameter of the shells of the genus Manticoceras is 60– 70 mm). In the Middle Urals, ammonoids are found in the deposits of the Lower and Upper Frasnian (Manticoceras). A few (53 specimens) of the Early Frasnian ammonoids come from the Kynov-Sargaev Beds (locality on the left bank of the Khoroshevka River, near the town of Gubakha, Perm Region). Bogoslovsky (1969) recorded Koenetites uralensis, Hoeninghausia koswensis, and Timanites keyserlingi from the compact light gray, ferruginous limestone. These species have a narrow, discoconic or oxyconic, large (Dm = 50– 140 mm) involute shell, characterized by the nektobenthic life-form. The Late Frasnian ammonoids are recorded from the localities on the Kos’va and Vil’va Rivers (western slope of the Middle Urals, Perm Region). Manticoceras intumescens, M. cordatum, M. sinuosum, M. sp., Tornoceras typum, and Aulotornoceras sp. have been recorded from the light gray, grained limestone beds (Bogoslovsky, 1969). All species have a discoconic involute shell with a narrow or moderately narrow umbilicus, high whorl expansion rate, and belong to the nektobenthic life-form. In the south of the Uralian Paleobasin the evolution of ammonoids began somewhat later. The earliest occurrences are found in the Upper Frasnian. The Upper Frasnian ammonoids come from the localities on the western slope of the South Urals (Bashkortostan, basins of the Zilim and Zigan Rivers). This territory, situated near the margin of Baltica and separated from the open ocean by the Uraltau microcontinent, at the end of the Devonian represented an epicontinental basin. At the end of Frasnian, various facies accumulated in this basin, including those characteristic of the shallow shelf: limestones, calcareous-clayey shales, siltstones, clayey shales, etc., containing few ammonoids. The following species have been recorded from these beds: Manticoceras sinuosum, M. cordatum, M. intumescens, and Tornoceras typum (Bogoslovsky, 1969). All these species belonged to the nektobenthic life-form. An impoverished taxonomic composition and homogenous ecological structure of the communities apparently suggest unfavorable conditions in the usual ammonoid habitats. Thus, the assemblages of the Frasnian ammonoids are characterized by the predominance of large involute


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discoconic nektobenthic shells both in the number of individuals and taxonomic composition. Planktonic evolute, platyconic shells recorded from the Early Frasnian of the South Timan were distributed to a much lesser extent. Ammonoids inhabited the region of the Carboniferous shelf and upper part of the slope. The absence of benthopelagic forms, which became widespread later, is very characteristic of the Late Frasnian. Famennian In the mid-Frasnian–Tournaisian, the TimanPechora system of troughs was developing in the western part of the North Uralian Basin (Malyshev, 2000). As a result, carbonate shoals were formed on the local uplifts, where the accumulative subaqueous structures e.g., carbonated banks, or small carbonate buildups surrounded by the zones of uncompensated sedimentation carbonate banks and small isolated organic buildups (Antoshkina, 2003). Mainly clayey-carbonate and clayey-cherty beds of the Domanik type accumulated in deep troughs. Marine water surrounding the carbonate plateau, characterized by quiet hydrodynamic conditions, abundant decayed organic matter or presence of volcanic material, which affected the chemical composition and facilitated the appearance of the anoxic conditions and hydrogen sulfide accumulation (Maksimova, 1970; Antoshkina, 2003). These conditions were apparently unfavorable for ammonoids. During the most of the Famennian, in the territory of the North and Subpolar Urals they are almost completely absent. The only discovery of the Early Famennian ammonoids in the North Urals (Yaiva River) is Cheiloceras verneuili listed by Krotov (1888). A few Late Famennian (Kalloclymenia–Wocklumeria Genozone) ammonoids are known from the Kozhim section (western slope of the Subpolar Urals). The ammonoid assemblage contains Kalloclymenia kozhimensis and Rectimitoceras obsoletum and R. angustilobatum collected from the overlying beds (Bogoslovsky and Kusina, 1980). Ammonoids come from the series of black clayey-cherty limestones with lenses and massive cherty beds. A character of faunal preservation indicates a possible post-mortem transfer. The assemblage is dominated by evolute planktonic shells of Kalloclymenia kozhimensis occasionally forming layers, whereas involute, narrowly umbilicate species of the genus Rectimitoceras, characteristic of the nektobenthic life-form, are represented by only a few specimens. The taxonomically richer Late Famennian ammonoid localities, containing Posttornoceras contiguum, Maeneceras sulciferum, Uraloclymenia volkovi, Falciclymenia uralica, Protxyclymenia dubia, Cymaclymenia costata, and Kalloclymenia glabra are known from the eastern slope of the Polar Urals (Man’ya and Loz’va rivers). This assemblage is dominated by evolute planktonic forms. PALEONTOLOGICAL JOURNAL

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A regression over the territory of the South Urals at the Frasnian–Famennian boundary reached its peak at the beginning of the Famennian (triangularis conodont zone), and led to a crisis in the ammonoid communities. No ammonoids are found from the basal Early Famennian of the South Urals. The regression was followed by a transgression resulting in the increased outer shelf, which was the main ammonoid habitat in the Famennian (Becker, 1993; etc.); and this was probably the main reason for the rapid growth of their taxonomic and morphological diversity. The Famennian was a unique time in ammonoid evolution. At this time ammonoids displayed the highest taxonomic diversity in their Paleozoic history. Famennian ammonoids included in this analysis come from the interval corresponding to the total of the four ammonoid genozones: Cheiloceras, Prolobites–Platyclymenia, Clymenia–Gonioclymenia, and Kalloclymenia–Wocklumeria. A collection of more than 25 thousand shells is housed at the Paleontological Institute, Russian Academy of Sciences (coll. nos. 1263, 1266, 1447, 2688, 2755, 3754). The systematic composition of the Famennian ammonoid fauna of the South Uralian-Kazakhstan Region has been studied by many authors (Tokarenko, 1903; Perna, 1914; Kolotukhina, 1938; Kind, 1944; Nalivkina, 1953; Bogoslovsky, 1969, 1971, 1981; Nikolaeva and Bogoslovsky, 2005; etc.). The Famennian beds of the South Urals yielded 193 ammonoids species of 60 genera and 21 families. The morphospace for this interval is restricted by the values of W = 1.4–2.7; D = 0–0.65, and S = 0.33–2.0. The assemblages of Famennian ammonoids were dominated by planktonic forms—49% of species, most of which have evolute ophioconic or platyconic shells—116 species (Fig. 4.4). Most members of this morphogroup belong to the order Clymeniida, including the families Cymaclymeniidae, Gonioclymeniidae, Hexaclymeniidae, Cyrtoclymeniidae, Rectoclymeniidae, Clymeniidae, Carinoclymeniidae, Pachyclymeniidae, Gonioclymeniidae, etc. of tornoceratids, this morphogroup includes species of the families Pseudoclymeniidae and Tornoceratidae (genus Posttornoceras). Other planktonic forms, 24 species, were represented by involute, slowly expanding shells, characteristic of the representatives of the orders Anarcestida and Tornoceratida (species of the family Prionoceratidae, Sporadoceratidae, Dimeroceratidae, Cheiloceratidae, Posttornoceratidae, and Prolobitidae). Among clymeniids, some representatives of the family Parawocklumeridae with a triangular coiling in early whorls belonged to this group. Nektobenthic forms were also widespread (33% of the total number of species). Two-thirds of these (52 species) were represented by pachyconic and discoconic involute and semi-involute shells. Representatives of the nektobenthic life-form were present in all orders, with the majority in the order Tornoceratida (families Cheiloceratidae, Tornoceratidae, Posttornoc-

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49% 116 species

33% 78 species 10% 24 species 1

2

3

4

Fig. 4.4. Proportions of life-forms in the assemblage of Famennian ammonoids of the Uralian basin. Explanations as in Fig. 4.3.

eratidae, etc.), Praeglyphioceratida, and Anarcestida. To a lesser extent (26 species) this group is represented by clymeniids, by species with a discoconic semi-involute shell. The nektobenthic oxyconic forms with rapidly expanding whorls were uncommon. Species, belonging to the benthopelagic life-form were uncommon (8% of the total taxonomic diversity). Characteristically Famennian ammonoid communities displayed a trend towards gradual increase in the proportion of planktonic forms and reduction of benthopelagic, and later nektobenthic forms. Morphological and taxonomic diversity and ecological structure of the communities of Famennian ammonoids varied greatly in time and different facial environments (Figs. 4.3, 4.5). Famennian ammonoid localities in the South Uralian region are subdivided into three major groups based on their type of preservation, diversity, and the number of shells, on the one hand, and on a facial type of host rocks, on the other. The latter facial types are recognized in: (1) Bashkortostan, (2) western Kazakhstan, (3) eastern slope of the South Urals. These types of localities belong to different tectonic zones and different types of basins. The Famennian ammonoid facies in Bashkortostan were accumulated in an epicontinental basin of varying depth, with mostly carbonate sedimentation. Localities of Famennian ammonoids in western Kazakhstan (Aktyubinsk and Orenburg regions) belong to the deep regions of the western shelf of the Uralian Ocean. Famennian carbonates found in the Chelyabinsk Region, with mass accumulations of ammonoid shells, were apparently deposited in the back-arc basin in the Magnitogorsk tectonic zone. Despite the geographic isolation of the above localities

and considerable differences in the lithology of the host rocks assemblages of Famennian ammonoids are similar in taxonomic composition and are considerably different from the synchronous assemblages of the RhenoHercynian zone (Nikolaeva and Bogoslovsky, 2005). At the same time each type of locality displays an individual ecological structure of ammonoid communities. The Famennian section in Bashkortostan contains clymeniids and goniatitids of all four genozones. Deposits of the Cheiloceras genozone are well represented on the Basu, Inzer, Ryauzyak, Zilim, Takata, Mendym, Terekly, and Ishikai rivers. They are composed of gray and light gray bedded limestone with numerous ammonoids, often in association with brachiopods, trilobites, and corals. In the Inzer and Basu rivers, and in some other localities, the basal beds of the genozone are composed of a series of shale and clay with interbeds of cherty rocks and limestone, with a total thickness of 3 m (Tyazheva, 1961). In the middle and upper parts of the series, thinly bedded gray limestones contain shells of straight nautiloids and ammonoids. The middle part of the genozone is represented by a series of gray thinly bedded micritic limestones, whereas the overlying beds are composed of light gray medium-bedded limestones with accumulations of ammonoid shell of the genus Cheiloceras. In many neighboring regions of Bashkortostan the terrigenous member at the base of the genozone is absent, whereas light gray and gray limestones compose the entire genozone and contain a rich fauna of ammonoids and brachiopods. In total, these deposits may be interpreted as having been accumulated in a shallow basin. The assemblages is dominated by numerous ammonoids of the genus Cheiloceras (Cheiloceras circumflexum, C. subpartitum, C. inversum, C. globosum dorsatum, C. sacculum, C. amblylobum, Sporadoceras latilobatum, etc.). The communities are dominated by involute pachyconic and discoconic nektobenthic forms (55% of species), whereas benthopelagic species to a lesser extent (27%). Planktonic forms constitute 18% of the total species diversity (Figs. 4.5a, 4.5b). They are mainly represented by small involute shells. Discoconic forms, with moderately narrow umbilicus and slowly expanding whorls (Pseudoclymenia) are less common. The overlying gray and light gray bedded limestone, with interbeds of black calcareous-cherty shales and with chert lenses contain numerous ammonoids of the Prolobites–Platyclymenia Genozone, often together with brachiopods. In general beds with ammonoids can be interpreted as medium-deep thin carbonates. The total thickness of the genozone is 2 to 9.5 m. Numerous ammonoids: Prolobites delphinus, Sporadoceras rotundum, Platyclymenia tschernyschewi, Rectoclymenia cf. subflexuosa, and Protactoclymenia krasnopolski from the localities on the Terekly, Ishikai, and Sikashty rivers are found in the carbonate interlayers (Karpinsky, 1869; Chernyschev, 1887; Bogoslovsky, 1981). In general, the assemblage is uniform, noticeably dominated


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CEPHALOPODS IN THE MARINE ECOSYSTEMS OF THE PALEOZOIC % (b) 100 90 80 70 60 50 40 30 20 10 0

Number of species 40 Cheiloceras Genozone (a) 30 20 10 0

Bashkortostan

Western Eastern Urals Kazakhstan

Bashkortostan

Number of species 90 Prolobites–Platyclymenia Genozone % (d) 100 80 (c) 90 70 80 60 70 60 50 50 40 40 30 30 20 20 10 10 0 0 Bashkortostan Western Eastern Urals

Bashkortostan

Kazakhstan

% (f) 100 90 80 70 60 50 40 30 20 10 0

Number of species 40 Clymenia–Gonioclymenia Genozone (e) 30 20 10 0

Bashkortostan

Western Kazakhstan

Bashkortostan

Number of species Calloclymenia– 14 Wocklumeria Genozone 12 (g)

% (h) 100 90 80 70 60 50 40 30 20 10 0

10 8 6 4 2 0

Bashkortostan

Western Kazakhstan

Bashkortostan

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Western Kazakhstan

1 2 3 4 Western Kazakhstan

Fig. 4.5. Famennian ammonoid assemblages in different facial types of sections of the South Urals: (a, c, d, g) species diversity and (b, d, e, f) proportions of life-forms: (a, b) Cheiloceras Genozone, (c, d) Prolobites–Platyclymenia Genozone; (e, f) Clymenia– Gonioclymenia Genozone, (g, h) Kalloclymenia–Wocklumeria Genozone. Explanations as in Fig. 4.3. PALEONTOLOGICAL JOURNAL

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by the compressed shells of Platyclymenia tschernyschewi. The average size of shells is about 25 mm. The remains are unsorted. The shells are moderately well preserved, occasionally with remains of fine ornamentation, but in the majority there is no terminal aperture preserved, whereas some shells are rounded, suggesting short-distance transportation. In this period the species diversity was reduced considerably (5 species instead of 11 in the preceding genozone) and the ecological structure of the community changed. The proportion of planktonic forms increased, with evolute compressed clymeniid shells prevailing in number of individuals. The diversity of nektobenthic forms considerably decreased, and the number of benthopelagic forms was also somewhat reduced (Figs. 4.5c, 4.5d). These changes happened while the basin was gradually becoming deeper due to a transgression, with reached its maximum flooding interval in the annulata phase. The impoverished taxonomic diversity suggests that the environment in the basin was probably unfavorable for ammonoids. The taxonomic diversity progressively decreased (Fig. 4.5e) in the later epochs. The assemblage of the Clymenia– Gonioclymenia genozone contains almost exclusively evolute shells of Clymenia laevigata. Beds of this age are represented by gray and light gray, in places cherty limestones with ammonoids, often in association with brachiopods, trilobites, bivalves, corals, and ostracodes (Sultanaev, 1973; Kochetkova et al., 1986). In the Zigan River basin (see Nalivkin, 1926, 1937, 1945) beds of this age contain interbeds of algal limestones, suggesting that the basin was relatively shallow. The youngest assemblage (Kalloclymenia–Wocklumeria Genozone) is exclusively represented by planktonic evolute and involute shells, some of which had triangular coiling of early whorls or at all stages: Parawocklumeria paradoxa, Cymaclymenia evoluta, and Synwocklumeria baschkirica (Nalivkin, 1945; Sultanaev, 1973; Popov, 1975). The evolution of ammonoids in Bashkortostan was mainly restricted to a shallow carbonate shelf. Numerous remains of benthic organisms suggest the nearby presence of a carbonate platform or several such platforms. In some localities the rocks of this age contain terrigenous beds, indicating periodic influx of clastics. In the early Famennian this part of the basin became deeper, while the shallow marine habitats of the earliest Famennian were gradually diminished. Ammonoids are considerably less diverse than in western Kazakhstan, with communities dominated by one or two endemic taxa. A short-distance post-mortem transportation of shells is possible. Famennian ammonoids of western Kazakhstan are diverse and numerous. They mainly come from carbonates containing almost exclusively conodonts and cephalopods and no or very little benthic fauna. In addition to ammonoids, numerous remains of straight nautiloids are present in the rocks. Ammonoid shells form huge

accumulations, often forming cephalopod shellstone. The shells are unsorted and not oriented. Many shells have fine structures preserved (terminal apertures, spines, and keels). The shell matrix is often present. Body chambers of large shells often contain smaller shells, indicating post-mortem transportation at short distances. This ammonoid fauna was studied by Kind (1944), Nalivkina (1953), Bogoslovsky (1955; 1960a, 1960b; 1962a, 1962b; 1965; 1975; 1976; 1977; 1979a, 1979b; 1981; 1982; 1983), Nikolaeva and Bogoslovsky (2005). Ammonoids come from the Kiinskaya Formation (localities–Shiyli-Sai, Ornektotas-Sai, Aral-TyubeBakai, Kiya 1, Kiya 2, etc.), which represents a chertyclayey terrigenous serious interbedded with gray cephalopod limestones (Bogoslovsky, 1969; Nikolaeva and Bogoslovsky, 2005). Limestones of the Kiya Formation are irregularly bedded, often clayey, with scanty crinoid debris, containing small thin-shelled planktonic ostracodes, single-chambered foraminifers (Parathurammina spp.), few Septaglomospiranella sp., and numerous conodonts (Akhmetshina et al., 2004). The total thickness does not exceed 15–20 m. The reduced thickness, specific microfacies and virtually complete absence of benthic organisms suggest sedimentary settings of a deep shelf. In most localities the most diverse and numerous ammonoid assemblages are found in the Cheiloceras and Prolobites–Platyclymenia genozones. The number of shells and diversity of ammonoids noticeably decrease upward in the Famennian section (Fig. 4.5). The ammonoid assemblage of the Cheiloceras Zone is represented by numerous species of Cheiloceras, Sporadoceras, Dimeroceras, etc. (section on the Aral– Tyube–Bakai River). Upward in the section, the rocks contain mass accumulations of Sporadoceras clarkei, S. muensteri, S. equale, Pseudoclymenia pseudogoniatites, etc. The communities were dominated by nektobenthic species—47 %, in most cases with a discoconic or pachyconic shell, with moderately and even slowly expanding whorls (W ~ 1.7). Benthopelagic forms are represented to a lesser extent (19% of species). Planktonic forms are considerably more diverse taxonomically compared to those from localities in Bashkortostan, in total they constitute 34% of the species diversity (Figs. 4.5a, 4.5b). At the base of the genozone they are represented by involute tornoceratid species, with compressed evolute Pseudoclymenia pseudogoniatites appearing upward in the section. Beginning from the Prolobites–Platyclymenia Genozone, ammonoid assemblages of western Kazakhstan are noticeably dominated by clymeniids (about 80% of the total number of species). The appearance of clymeniids was apparently not related to changes in the sedimentary setting because neither lithological changes, nor discontinuities are observed in the sections at the level with the earliest clymeniids. The first clymeniids (genera Cyrtoclymenia, Platyclymenia, Genuclymenia, Pleuroclymenia) appear after the complete disappearance of the tornoceratid genus Pseudoclymenia,


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which were similar to clymeniids in their shell (compressed evolute shell). Simultaneously with clymeniids, assemblages contain many shells of Prolobites which may be interpreted as planktonic. The shells of Prolobites are small, spheroconic, with very slowly expanding whorls and with deep constrictions, and with the last (terminal) constriction almost completely closing the aperture. The highest ammonoid diversity coincides with the maximum of the large-scale transgression (zones delphinus and annulata). Beds at this level contain large accumulations of Prolobites delphinus, P. nanus, Renites striatus, Sporadoceras cf. inequale, Sporadoceras cf. clarkei, S. pisum, numerous representatives of Maeneceras, Falcitornoceras, Araneites, Nothosporadoceras, Praeglyphioceras, compressed ribbed and smooth clymeniids Pricella stuckenbergi, Platyclymenia pompeckii, P. cf. subnautilina, Rectoclymenia subplicata, Rectoclymenia roemeri, Genuclymenia aktubensis, G. frechi, Posttornoceras contiguum, Platyclymenia subnautilina, P. valida, Genuclymenia frechi, Trigonoclymenia protacta, T. spinosa, Protoxyclymenia dubia, Genuclymenia sp., etc. Juvenile and adult shells are present, many with body chambers. Average size of the shells is 2.5–3 cm, whereas the maximum size is almost 25 cm. In the overlying beds similar limestone contained Uraloclymenia nodosa, Trigonoclymenia tigra, Pleuroclymenia costata, Protactoclymenia sp., Protoxyclymenia carinata, and Sporadoceras discoidale. In the Prolobites–Platyclymenia Genozone the ecological structure of ammonoid communities changed fundamentally. The communities (58% of species) are composed of planktonic, mainly evolute forms. The proportion of nektobenthic forms (25% of species) and benthopelagic forms (17% of species) considerably decreased (Figs. 4.5c, 4.5d). The latter are represented mainly by pachyconic shells with a narrow or moderately narrow umbilicus and slowly expanding whorls. These changes and the appearance and diversification of clymeniids apparently could not be solely related to the deepening of the basin during the local transgression, but rather are the product of the evolution of the ammonoid taxocoenoses of the deep shelf, on the global scale. In the Clymenia–Gonioclymenia Genozone the number and diversity of ammonoids decreased considerably. The assemblages are dominated by large shells of the genus Clymenia, which represent over a half of the total number of specimens. The assemblage includes Rectimitoceras pompeckji, Sporadoceras kiense, Mimimitoceras liratum, Rectimitoceras kiense, R. cf. substriatum, Progonioclymenia aff. acuticostata, Cymaclymenia barbarae, Protoxyclymenia rotundata, Cyrtoclymenia angustiseptata, Clymenia laevigata, Discoclymenia cucullata, Renites kiensis, Costaclymenia binodosa, Gonioclymenia levis, Sphenoclymenia sp., Biloclymenia aktubensis, Maeneceras inflexum, Gonioclymenia hoevelensis, Ornatoclymenia ornata, PALEONTOLOGICAL JOURNAL

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Gonioclymenia hoevelensis Wedekind, Sphenoclymenia maxima, Protoxyclymenia pseudoserpentina, Biloclymenia dubia, Kiaclymenia semiplicata, etc. Despite the considerable change in the taxonomic composition, the overall ecological structure of the communities did not change (Fig. 4.5). The number of nektobenthic forms somewhat increased (30% of species instead of 25% in the preceding genophase) due to the reduction in the diversity of benthopelagic forms. The proportion of planktonic forms remained unchanged. This may be explained by stable conditions in the ammonoid habitats through the genophase. At the end of the Famennian, in the Kalloclymenia– Wocklumeria Genophase the diversity decreased even more. The number of shells also considerably decreased. The shells are scattered in the rocks (clayey limestones), and are poorly preserved. The communities are mainly composed of planktonic forms (84% of species) (Figs. 4.5g, 4.5h). The newly appeared wocklumeriids are most abundant, and they had small (1.2– 2 cm in diameter) inflated and spheroconic, or triangularly coiled shells. The diversity and abundance of kosmoclymeniids, cyrtoclymeniids, and cymaclymeniids very much decreased. This is an unusual situation because in the underlying beds with a similar lithology (Clymenia–Gonioclymenia Genozone) kosmoclymeniids, cyrtoclymeniids, and cymaclymeniids are very diverse (Nikolaeva and Bogoslovsky, 2005a; Nikolaeva and Bogoslovsky, 2005b). The absence of representatives of these families in the assemblages of the sphaeroides Zone in western Kazakhstan distinguishes it from synchronous communities of Great Britain, Germany, Morocco, Poland, and Northern Caucasus, where the evolution of kosmoclymeniids, cyrtoclymeniids, and cymaclymeniids continued, and wocklumeriids existed together with evolute cymaclymeniids until the very end of the Famennian. In the Late Famennian, only a few genera were endemic to western Kazakhstan, but there were many endemic species. In addition, in comparison with other regions of the Uralian-Kazakhstanian region, the basin of western Kazakhstan retained a high diversity of ammonoids during the entire Famennian. Planktonic evolute shells of clymeniids were the key players in the ammonoid communities until the end of the Famennian. In the Clymenia–Gonioclymenia Genophase, pachyconic, involute, narrowly umbilicate clymeniids, belonging the nektobenthic life-form, became considerably more widespread. At the very end of the Famennian, among the planktonic forms, narrow evolute shells of clymeniids almost disappeared, being replaced by small, inflated, triangularly coiled wocklumeriids. A decrease in the ammonoid communities in western Kazakhstan in the Late Famennian could be connected with the total reduction of the shelf areas in the course of progressive collision of plates and microcontinents in the area of the future Ural mountains. The

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reduction of the epicontinental part of the basin, certainly led to structural changes in the communities and affected the food resources of cephalopods. Communities, almost entirely composed of highly specialized forms, probably could not adapt to environmental changes and new habitats. Several localities of Famennian ammonoids are situated on the eastern slope of the Urals, in the Verkhneuralsk Region to the southwest of the town Verkhneuralsk (Karpinsky, 1884; Tokarenko, 1903; Perna, 1914; Bogoslovsky, 1969, 1971. 1981; Nikolaeva and Bogoslovsky, 2005a). The lower part of the section is composed of reddish limestone with greenish-gray interbeds, with numerous shells of ammonoids, nautiloids, and small corals, remains of trilobites and gastropods. The overlying beds are represented by light gray and brownish-gray compact, medium-grained limestone, containing a rich fauna of brachiopods, bivalves, and to a lesser extent gastropods and ammonoids. The main source of the carbonate could be local subaqueous uplifts inhabited by small-sized shelled benthos. These are relatively shallow water deposits probably representing the back-arc facies. Well-preserved ammonoid shells do not suggest significant post-mortem transportation. Therefore, it is possible that the simultaneous presence of remains of relatively deep-sea organism (ammonoids) and shallow-sea benthos suggest that the material was transported by a flow of debris from the edge of a shelf or the boundary of a carbonate platform to the deep shelf. Ammonoid assemblages in these localities belong to the Cheiloceras and Prolobites– Platyclymenia genozones. The ammonoid assemblage of the Cheiloceras Genozone is diverse. It includes goniatitids and tornoceratids: Sporadoceras muensteri, Sporadoceras rotundum, Dimeroceras mamilliferum, Dyscheiloceras latilobus, a variety of Cheiloceras and Tornoceras species. Similar to other regions the communities are mainly composed of nektobenthic forms—61% of species. Benthopelagic (16%) and planktonic (23%) forms are represented to a lesser extent (Figs. 4.5a, 4.5b). This ecological structure of the communities, apparently resulted from a relatively shallow ammonoid habitats. In the Prolobites–Platyclymenia Genophase the diversity of planktonic forms (47% of species) increased, mostly due to the evolution of evolute clymeniids (Figs. 4.5c, 4.5d). The number of nektobenthic and benthopelagic forms decreased (45 and 8%, repsectively). The assemblage is composed of various species of the genera Prolobites, Maeneceras, Sporadoceras, Pernoceras, Protornoceras, Kirsoceras, Armatites, Platyclymenia, Cyrtoclymenia, Pricella, Genuclymenia, Rectoclymenia, and Protoxyclymenia, typical of the lower and upper parts of the Prolobites–Platyclymenia Genozone (delphinus–annulata and dunkeri zones). The taxonomic diversity, increased at the beginning of the genozone later rapidly decreased, which, apparently, was

related to the overall decrease of the shelf areas. The overlying beds do not contain ammonoids. Two large stages with different ecological structure of the ammonoid communities are recognized in the evolution of the fauna of the Upper Devonian ammonoids of the Uralian Paleobasin. The first (Frasnian–Middle Famennian) shows low taxonomic diversity, predominance of nektobenthic forms (85%), while planktonic forms were uncommon and benthopelagic forms are almost entirely absent. At the second stage (Late Famennian) that taxonomic diversity was high. All major life-forms are represented in the communities. The main trend may be identified as a focused evolution of planktonic forms in all three orders, which rapidly became dominant in the communities. Despite different habitats of Famennian ammonoids (shallow shelf near a carbonate platform, open deep shelf, etc.) leading to the formation of communities different ecologically, it is possible to recognize some general patterns in the development of ammonoid fauna in this territory. (1) The richest assemblages come from the deep-sea sediment suggesting that Famennian ammonoids, like modern Nautilus, preferred deep sea. This is confirmed by the fact that the peaks of ammonoid diversity in each of the regions studied coincided with the annulata phase, a time of a global-scale highstand (Becker, 1993; etc.). After a flooding interval the diversity and abundance of ammonoids in all studied localities of the Uralian region gradually decreased. (2) The major trend in the evolution of the ammonoid communities in the Famennian was an increase in the diversity of planktonic life-forms and reduction of the diversity of the benthopelagic lifeform, followed by the decrease in the diversity of nektobenthic forms (Fig. 4.3). In the second half of Famennian, with the appearance of the order Clymeniida, the proportion of planktonic species increased in all four types of localities and represented more than 50% of species diversity. (3) The increase in the number of planktonic forms was happening at the time of the maximum flooding interval with a peak in the annulata phase. At the same time, it is apparent from the analysis of the western Kazakhstan ammonoids that this reason cannot alone account for the increased abundance of the planktonic forms, nor for the explosive radiation of clymeniids. (4) The decrease in the ammonoid diversity in the Uralian region by the end of the Famennian was gradual. It is unlikely that it was related solely to global eustatic fluctuations. It is most likely the decrease was contributed to considerably by local tectonic events resulting in a decrease of the shelf, which was a traditional ammonoid habitat and possibly by a decrease in food resources. The extinction primarily affected the planktonic evolute forms, and their species diversity sharply decreased. At the end of the Famennian the community was dominated by planktonic narrowly


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umbilicate, slowly expanding forms. Representatives of this morphogroup, were apparently the best adapted to the unfavorable conditions established from the end of the Famennian.

7% 2 species 27% 8 species

The Devonian–Carboniferous boundary is marked by a deep crisis in the ammonoid communities, which is often explained by a series of large-scale transgression-regression pulses recorded in many places on the globe and accompanied by the development of shortterm anoxic events (so-called Hangenberg Event) followed by sea level drops. As a result of the mass extinction at the Devonian–Carboniferous boundary, only a few prionoceratid species survived to cross the boundary (with an involute, narrowly umbilicate forms, with slowly and moderately expanding whorls, representatives of the nektobenthic and planktonic (P-2) life-forms.

1

In the Urals and adjacent territories of Pai-Khoy, ammonoids are apparently present in all genozones (here we used the European zonation) of the Tournaisian: Eocanites–Gattendorfia, Goniocyclus–Protocanites, Pericyclus–Progoniatites, Fascipericyclus–Ammonellipsites. Because the Tournaisian–Visean boundary is based on foraminifers, it divided the Fascipericyclus– Ammonellipsites ammonoid genozone into two parts. However, in the opinion of most ammonoid workers, the Fascipericyclus–Ammonellipsites Phase represents a single definitive stage in the evolution of the group and is not divided into the Tournaisian and Visean substages, and therefore this phase is analyzed here as a single unit. The degree of knowledge of the assemblages is different. Ammonoid collections include more than 750 specimens, housed in the Paleontological Institute, Russian Academy of Sciences, in Chernyshev TsNIGR Museum, and Department of Paleontology of St. Petersburg State University. The systematic composition of ammonoids was studied by many authors (Librovitch, 1941; Balashova, 1953; Kusina, 1971, 1973, 1974, 1980, 1983, 2000; Popov, 1975; Popov and Kusina, 1997; Kusina and Konovalova, 2004; etc.). The Tournaisian ammonoid assemblage includes 54 species of 31 genera and 7 families. The morphospace of the used combination of the parameters W, D, and S is delineated by the values D = 0.02–0.4; W = 1.54–2.7, and S = 0.35–2.4. In general, communities of Tournaisian ammonoids have involute pachyconic and subdiscoconic nektobenthic forms (49% of species). Spheroconic and pachyconic involute forms with a narrow or medium-sized umbilicus are less common— benthopelagic (27%). Evolute and involute planktonic forms constituted 24% of species (Fig. 4.6). The ammonoid diversity remained low throughout the Tournaisian and only slightly increased at the very end of the Tournaisian. (Fig. 4.3b). No. 11

2

3

4

Fig. 4.6. Proportions of life-forms in the Tournaisian ammonoid assemblages in the Urals. Explanations as in Fig. 4.3.

Tournaisian

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49% 15 species

4.3.4. Mississippian (Early Carboniferous)


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Early and Middle Tournaisian were not epochs of high diversity of ammonoids in the Uralian Basin. The earliest Tournaisian ammonoids are known from localities in western Kazakhstan (Berchogur) (Kusina, 1985). The assemblage includes a few weakly ornamented, involute, narrowly umbilicate species: Acutimitoceras subbilobatum, A. carinatum, A. mugodzarense, A. yatskovi, A. pulchrum, Rectimitoceras aff. substriatum, and R. bertchogurense. Ammonoids are found in small lens-shaped accumulations in yellowish-gray, clayey, bedded limestone, deposited in a shallow shelf (Kusina, 1985, Barskov et al., 1984). The remains are moderately well preserved, and the shell matrix is preserved fragmentarily. The average shell size is 12– 25 mm. The taphonomy and preservation of these ammonoids suggest considerable post-mortem transportation. Together with ammonoids, conodonts and sometimes brachiopods are found. Ammonoid assemblages contain a few species, characterizing nektobenthic and, to a lesser extent, planktonic (1 species) life-forms. Such ecological structure is apparently characteristic of the post-crisis or crisis phase of community evolution. Occasional finds of Middle Tournaisian ammonoids, so called Goniocyclus fauna, are known from the western slope of the South Urals (Orenburg Region) and Subpolar Urals (Popov and Kusina, 1997). In the South Urals ammonoids are found in terrigenous-carbonate beds on the Kamsak River. The assemblage contains the following species: Goniocyclus subtilis, G. dombarovensis, G. vodoresovi, Muensteroceras modestum, Aquilonites uralensis, and Gattendorfia uralica, characterizing mainly nektobenthic and, to a lesser extent, benthopelagic life-forms. In the Subpolar Urals, ammonoids of this age are found in the clayey-carbonate beds (Vangyr River),

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% 100

Number of species 14 (b)

90

12

80 70

10

60

8

50 6

40 30

4

20 2

10 0

0

Subpolar Urals, South Urals, terrigenous facies carbonate facies 1

2

Subpolar Urals, South Urals, terrigenous facies carbonate facies 3

4

Fig. 4.7. Late Tournaisian (Fascipericyclus–Ammonellipsites) ammonoid assemblages in various facial sections the Urals: (a) proportions of life-forms and (b) species diversity. Explanations as in Fig. 4.3.

deposited in the upper part of the slope of the deepwater shelf depression, in association with conodonts and deep water ostracodes (Sobolev, 2005; Kusina and Sobolev, 2005). Ammonoids are represented by imprints. The assemblage contains only species of the genus Goniocyclus, representatives of which have a pachyconic narrowly umbilicate ornamented shell with moderately or rapidly expanding whorls, characteristic of the nektobenthic life-form. At the beginning of the Late Tournaisian (Pericyclus–Progoniatites Zone = Russian Protocanites–Pericyclus Zone), ammonoids were very uncommon in the Urals. With some degree of doubt a few specimens of the genus Pericyclus from the deep water clayeycherty-carbonate series of the Kara Formation on the eastern slope of the Pai-Khoy Range may be dated as the Pericyclus–Progoniatites Zone (Kusina, 1999). At the end of the Late Tournaisian–beginning of the Visean (Fascipericyclus–Ammonellipsites Zone) the taxonomic and morphological diversity of ammonoids considerably increased (Fig. 4.3). New morphotypes among benthopelagic and nektobenthic forms appeared, which was connected with the appearance of new genera in the families Muensteroceratidae, Intoceratidae, Prolecanitidae, Kozhimitidae, and Pericyclidae. Localities of the ammonoid Fascipericyclus– Ammonellipsites Zone are known from the western slope of the South and Subpolar Urals and eastern slope of the Pai-Khoy. The most diverse ammonoid assemblage was found in the northern part of the Uralian Paleobasin, on the western slope of the Subpolar Urals (basin of the Kozhym River). Ammonoids are found in calcareous and carbonaceous shale with numerous sid-

erite nodules, which were formed in relatively deep waters on the slope of an intrashelf depression. Ammonoids are found in association with straight and coiled nautiloids, a few columnals, and ostracodes of the deep water type (Sobolev, 2005). The assemblage includes species of the genera Dzhaprakoceras, Muensteroceras, Intoceras, Aquilonites, Hammatocyclus, etc. The community is dominated by nektobenthic forms (48% of species) and benthopelagic forms (30% of species). Planktonic species constitute 22% (Fig. 4.7). Ammonoids are found in the siderite nodules and are usually represented by molds. The shell layer and body chambers are usually not preserved. The shells possess signs of partial rework and are rounded, which suggests post-mortem transportation. Ammonoids, found on the eastern slope of the PaiKhoy Range, in the basin of the Kara and Silovaya Yakha rivers come from clayey-cherty-carbonate beds with phosphoritic nodules, formed in the outer, deep shelf or upper slope. Species found include Eurites latus, Ammonellipsites nikitini, Ortocyclus polaris, Muensteroceras hibernicum, etc., and mainly belong to the benthopelagic (80% of species) and nektobenthic life-forms. In the South Urals, the Late Tournaisian–Early Visean ammonoids of the Fascipericyclus–Ammonellipsites Zone accumulated in the shallower environment, in the shelf. They are known based on a few finds in carbonate beds (marginal parts of bioherms) and terrigenous-carbonate beds on the western slope (basins of the Zilim and Tanalyk rivers) (Popov, 1975). The ammonoid assemblage is impoverished compared to the North Uralian and contain Fascipericyclus fascicu-


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latus, Pericyclus princeps, Muensteroceras sp., Ortocyclus worki, and Polaricyclus rileyi, mainly belonging to the benthopelagic (32% of species) and nektobenthic (47%) life-forms. A few evolute and involute planktonic species are also present (Fig. 4.7). A few general conclusions may be made based on the analysis of the ammonoid evolution in the Tournaisian in the Urals. (1) As in the Late Devonian, Tournaisian ammonoids preferred deep-water regions of the outer shelf: the most abundant (in the number of specimens) and taxonomically diverse ammonoid assemblages are found in deep-water terrigenous beds of the North Urals. (2) A distinctive feature of the Tournaisian communities is the predominance of nektobenthic (49%), while benthopelagic species are less diverse (27%), and planktonic forms are uncommon. (3) Two large stages may be recognized in the evolution of ammonoid communities of the Uralian basin, like in the Famennian: Early Tournaisian (Eocanites– Gattendorfia Zone) and a later interval from the Middle Tournaisian to Early Visean. Each of these stages, separated by a crisis, had its own morphological and ecological community structure. At the first stage, morphological diversity impoverished (only involute forms are present) with predominance of nektobenthic forms and slight presence of the planktonic species. Imitoceratids with an undivided ventral lobe prevailed. In the Uralian, communities of this stage do not contain evolute forms from the order Prolecanitidae (genus Eocanites), widely distributed in the deeper water deposits of the Rheno-Hercynian Basin, North Africa, and Pamir. Characteristically most Early Tournaisian ammonoid communities show a little presence of benthic forms, which, is apparently explained by the reduction of these ecotypes as a result of the crisis at Devonian–Carboniferous boundary. At the second stage, goniatitids with a bipartite ventral lobe appeared and began their evolution. At that time communities with an increased diversity of nektobenthic and benthopelagic forms and a few planktonic taxa, appear. The increase in the morphological diversity was to a large extent connected with a rapid evolution of ornamented forms of the family Pericyclidae, and with the appearance oxyconic, narrowly umbilicate forms in the family Intoceratidae. Compared to other families, Pericyclidae were not only the most taxonomically diverse (16 species, 8 genera) but are represented by various morphotypes and life-forms. This indicates a high degree of differentiation of Uralian Pericyclidae, which agrees with the general trend in the evolution of the group, which reached its maximum diversification at the end of the Tournaisian. Visean Ammonoids are found in all Visean genozones. They come from various localities in Novaya Zemlya, PALEONTOLOGICAL JOURNAL

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21% 16 species

1255 29% 22 species

3% 2 species

47% 36 species 1

2

3

4

Fig. 4.8. Proportions of life-forms in Visean ammonoid assemblages in the Urals. Explanations as in Fig. 4.3.

Subpolar, and South Urals. This fauna was studied by Librovitch (1938), Ruzhencev (1949a, 1949b, 1958, 1966; Ruzhencev and Bogoslovskaya, 1971), Bogoslovskaya (1966), Kusina (1974, 1980, 1983), Kusina and Konovalova (2004), Kusina and Yatskov (1990, 1999), Konovalova (2004), etc. Collections include over 20000 specimens, from the Paleontological Institute, Chernyshev TsNIGR Museum, and in the Department of Paleontology at the St. Petersburg State University. The assemblage is considerably richer than the Tournaisian one: 86 species, 38 genera, and 17 families. The morphospace considerably increased compared to the Tournaisian and is delineated by the values of D = 0.01–0.6; W = 1.31–2.8, and S = 0.35–3.1. In general, the main constituents of the communities of Visean ammonoids are pachyconic, discoconic or oxyconic, narrowly umbilicate nektobenthic (47% of species) and benthopelagic forms (about 30% of species) (Fig. 4.8). The quantity of planktonic forms was low during most of the Visean. Only in the terminal Visean their proportion in the communities increased up to 27% of species. The species diversity among planktonic forms gradually increased by the end of the Visean, and at the same time new planktonic morphotypes—species with laticonic and aperticonic shells, of the new superfamily Neoglyphiocerataceae appeared and became widespread. The end of the Tournaisian–beginning of the Visean in the Uralian basin were marked by a large regression, which reached its peak at the end of the Early Visean (Alekseev et al., 1996; Puchkov, 2000; Sobolev, 2005; etc.). The increased regression, which caused a reduction in the outer shelf areas, led to almost complete disappearance of ammonoids. The extinctions primarily affected highly specialized planktonic and benthopelagic forms, while nektobenthic forms decreased later. The ammonoid diversity and structure of their communities changed considerably throughout the Visean (Fig. 4.3).

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16% 3 species

73% 14 species

1

2

3

4

Fig. 4.9. Proportions of life-forms in the Early Visean (Bollandites–Bollandoceras Zone) ammonoid assemblages in terrigenous sections of the Subpolar Urals.

The community of Early Visean ammonoids (Bollandites–Bollandoceras Zone) inherited the structure from the Late Tournaisian. Their localities are mainly known from the Subpolar Urals and Novaya Zemlya. In the Subpolar Urals, ammonoids come from terrigenous rocks formed on the slopes of an inner shelf depression (Sobolev, 2005). The communities (73% of species) were represented by pachyconic and subdiscoconic, narrowly umbilicate nektobenthic species of the genera Dzhaprakoceras, Beyrichoceratoides, Bollandoceras (families Muensteroceratidae and Maxigoniatitidae) and to a lesser extent, discoconic forms of the families Intoceratidae and Girtyoceratidae. Evolute planktonic (families Nomismoceratidae, Pericyclidae) and benthopelagic forms were represented to a lesser

% 100

extent (16 and 11%, respectively) (Fig. 4.9). They are found in deep water deposits only. As the inner depression was filled and shallowed, the taxonomic and morphological diversity of ammonoids decreased. A small assemblage of Early Visean is found in the deep water rocks of the Milinskaya Formation in Novaya Zemlya. Ammonoids are found in the darkgray fine-grained limestone with a few interbeds and nodules of siliceous rock with radiolarians, which were formed on the slope and bottom of the deep water depression (Kusina and Yatskov, 1999; Sobolev and Matveev, 2002). At the Early–Late Visean boundary there was a renovation of the total taxonomic composition of ammonoid assemblages. Beginning in the second half of the Visean a regression in the Uralian Ocean was replaced by a transgression. A general deepening of the basin led to the formation of the areas of the outer shelf, which were distant from the sources of terrigenous material, had carbonate sedimentation at relatively small depths, and were inhabited by ammonoids. In the South Urals, in the northern part of the Uralian Basin and its continuation (Novaya Zemlya) ammonoids evolved in different environments, hence, despite a considerable number of genera and species in common, each of these regions had its own morphological and ecological structure of communities, which changed during the interval studied. The richest localities for Late Visean ammonoids are known from the western slope of the South Urals, in the Orenburg and Aktyubinsk Regions. Ammonoids come from light gray micritic limestones, which Ruzhencev and Bogoslovskaya (1971) recognize as a separate type of “Dombar Limestone.” Along with ammonoids, the rock contained numerous crinoidal Number of species (b) 25

(a)

80

20

60

15

40

10

20

5

0 Subpolar Urals, South Urals Novaya Zemlya 1

2

0

Subpolar Urals, Novaya Zemlya

South Urals

3

Fig. 4.10. Late Visean Beyrichoceras–Goniatites ammonoid assemblages from various regions and various facies: (a) proportions of life-forms and (b) species diversity. Explanations as in Fig. 4.3. PALEONTOLOGICAL JOURNAL

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ossicles and benthic fauna of bivalves, gastropods, trilobites, tetracorals, brachiopods, and also deep-water ostracodes and conodonts. The development of the morphological and taxonomic diversity of ammonoids was gradual. In the Beyrichoceras–Goniatites genophase the assemblages were dominated by nektobenthic forms with pachyconic and discoconic involute and semiinvolute shells (Figs. 4.3, 4.10). Species of the genera Beyrichoceras, Lusitanoceras, Goniatites, Girtyoceras, and Pronorites are typical representatives of these morphogroups. Species with a spheroconic or subspheroconic involute shell, interpreted as benthopelagic life-form were also widespread. Their mass appearance was connected with the genus Goniatites (Goniatites sphaeroides, G. shimanskyi). The evolute compressed shells of the families Prolecanitidae, Nomismoceratidae, interpreted as planktonic were less widespread. This structure of the assemblage apparently reflects the initial stages of restoration of the diversity in the community. In the Hypergoniatites–Ferganoceras Genophase morphological and taxonomic diversity of ammonoids rapidly increased. Vast accumulations of their shells are found together with numerous crinoids, conodonts, and a few gastropods, bivalves, trilobites, etc. (Ruzhencev and Bogoslovskaya, 1971). Shells are represented by various growth stages, are well preserved, not rounded, and many with fine ornamentation. Ammonoids come from crinoid or micritic limestones. Rock lithology, impoverished diversity of typical shallow-water organisms (corals, brachiopods, algae, and foraminifers), suggest that sedimentation took place in the outer deep water part of the shelf, which is supported by the presence of numerous crinoids. For crinoids, parts of the sea floor with stenohaline environment and good circulation (the conditions on the shelf) are the most suited (Krammer and Ausich, 2006). Nevertheless, in this case the ideal model of a carbonate shelf (Ahr, 1973, 1998) was probably distorted and modified by a strong differentiation of the sea floor resulting from a continuing tectonic evolution of the South Urals (Puchkov, 2000). Territories inhabited by crinoids and ammonoids were apparently semi-isolated from the neighboring shallow water zones inhabited by corals and brachiopods. Apparently, these were quiet areas of the marine basin, near or on the margins of the subaqueous uplifts (Nikolaeva, 2006). The assemblages are dominated by ammonoids with subspheroconic and pachyconic, weakly ornamented shells with slowly expanding whorls, not suitable for active swimming. The assemblages, taxonomically and in number of individuals, contain benthopelagic (32% of species) and planktonic forms (27% of species) of the families Goniatitidae, Neoglyphioceratidae, Cravenoceratidae, Delepinoceratidae, Ferganoceratidae, Rhymmoceratidae, etc. (Fig. 4.11). Nektobenthic forms are also diverse (41 % of species), but the majority of spePALEONTOLOGICAL JOURNAL

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22% 9 species

5% 2 species

41% 17 species 1

2

3

4

Fig. 4.11. Proportions of life-forms in the Late Visean (Hypergoniatites–Ferganoceras Genozone) ammonoid assemblages from the carbonate sections of the South Urals. Explanations as in Fig. 4.3.

cies that belonged to this life-form are represented in the assemblages by a small number of individuals. The nektobenthic life-form is represented by species of the genera Kazakhoceras, Arcanoceras, Megapronorites, families Girtyoceratidae, Dimorphoceratidae, Goniatitidae, Agathiceratidae, Prolecanitidae, etc. An increase in the number of benthopelagic forms is mainly related to the rapidly evolving family Goniatitidae and superfamily Neoglyphiocerataceae. The appearance of the latter is one of the most important evolutionary events at the end of the Visean. Its development is also connected with an increase in the morphological diversity of planktonic forms. Representatives of this superfamily (families Cravenoceratidae and Neoglyphioceratidae) form two new planktonic morphotypes—laticones and aperticones, widely umbilicate shells. These morphotypes were the most successful, which was responsible for the morphological appearance of the Late Carboniferous and Early Permian communities. The evolution of ammonoids in the Beyrichoceras– Goniatites and Hypergoniatites–Ferganoceras genophases on the western slope of the South Urals occurred in similar environments. Transformations in ammonoid communities apparently were not determined by large abiotic changes in the environment (change in the sedimentary settings, sea level fluctuations, etc.) and may reflect successive stages in the evolution of the community: (a) early evolution of the morphological diversity; (b) radiation and exploration of new niches. On the eastern slope of the Urals (Verkhnyaya Kardailovka section), the evolution of Visean ammonoids occurred in the deeper water settings, which is indicated by radiolarians, deep water ostracodes, and by the specific lithofacies (Pazukhin and Gorozhanina, 2002; Pazukhin et al., 2002). The ammonoid assemblages in this region are very impoverished being represented by benthopelagic and planktonic forms.

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In the North Urals and Novaya Zemlya, the Late Visean ammonoids also existed on the outer margin of the carbonate shelf, but they lived in significantly shallower environment. In the Subpolar Urals they are found in the bioherm structures similar to Walsourtian mounds on the quiet slope (Antoshkina, 2003; Konovalova, 2004; Skompski et al., 2001). Ammonoids are found in carbonate deposits together with abundant benthic fauna brachiopods and bivalves, sometimes forming lens-shaped formations, with gastropods, bryozoans, corals, and trilobites (Konovalova and Sobolev, 2005). Numerous remains of foraminifers and algae are also present. In places, ammonoids form lens-like accumulations (cephalopod shellstones) and are represented by complete shells of varying size. The assemblages are dominated by spheroconic and pachyconic benthopelagic and discoconic nektobenthic forms: Goniatites olysya, Lusitanoceras kusinae, L. nadotense, Kazakhoceras hawkinsi, Girtyoceras kazakhorum, etc. Upper Visean ammonoids of Novaya Zemlya are found in the deposits of the Milinskaya Formation and in the Gorbovsky Reefoid Massif, which formed in a different environments. The Milinskaya Formation is composed of mainly carbonate, fine-grained, micritic, cherty limestones with infrequent interbeds of clayeycherty limestone and dolomites, scanty ammonoids, conodonts, and foraminifers. The accumulation of the Upper Visean beds apparently occurred on the slope and on the bottom of a deep-sea depression (Platonov and Chernyak, 1982; Schecoldin et al., 1994, Sobolev and Matveev, 2002). The ammonoid assemblage includes benthopelagic (56% of species), nektobenthic (31%) and a few planktonic forms. Ammonoids are also found in bioherm limestones of the Gorbovsky Reefoid Massif (Berkh Island). The frame of the reef is composed of algal and coral-algal limestones, occasionally overfilled with fauna. Ammonoids are found in association with typically shallow-water benthic fauna: gastropods, brachiopods, gigantic tetracorals, fusulinids, etc. (Kusina and Yatskov, 1999). The specific composition of the ammonoid assemblage from the bioherm deposits is impoverished, mainly including spheroconic, pachyconic, and oxyconic benthopelagic and nektobenthic forms of the families Goniatitidae, Berkhoceratidae, and Girtyoceratidae. Ophioconic planktonic forms (Nomismoceras) are uncommon and occur in deeper water deposits of the Milinskaya Formation. In general, the taxonomic diversity of the ammonoid assemblages from the Upper Visean of the northern Uralian basin is low. The communities are dominated by species, characterizing the nektobenthic life-form (54% of species). Benthopelagic forms are also relatively diverse (28%), whereas species of the planktonic adaptive type are few (18%) (Fig. 4.10). Two stages can be recognized in the evolution of the Visean ammonoid communities of the Urals. These stages were separated by a crisis caused by a large

regression, which happened across the entire Uralian basin and adjacent regions of the Russian Platform and Peri-Caspian at the Early–Late Visean boundary (Alekseev et al., 1996; etc.). The Early Visean ammonoid communities developed in the same environments as the Late Tournaisian and had a similar morphological and ecological structure (Figs. 4.7, 4.9). They mainly occurred in the northern region of the basin. During the Early Visean the morphological diversity of ammonoids decreased and many taxa became extinct. In the second half of the Visean ammonoid assemblages were completely renewed while their communities were significantly restructured, as they now developed in shallower environments compared to the Tournaisian and Famennian. At that time, poorly adapted to active swimming benthopelagic and planktonic forms increased in diversity and rapidly diversified. Aperticones and laticones appeared and became widespread, which is related to the appearance of the superfamily Neoglyphiocerataceae. At the same time, the diversity among nektobenthic forms, better adapted for swimming, decreased. At that time, large taxa (families or even superfamilies) were characterized by a single morphotype or morphogroup, in contrast to the Tournaisian, when species of several families (Pericyclidae, Gattendorfiidae) were represented by different morphogroups. This feature may suggest that the groups were highly specialized, i.e., communities were more mature. The above analysis of the ammonoid assemblages of the western slope of the South Urals shows two successive stages in the evolution of communities, unrelated to fundamental changes in the environments: (a) early evolution of the diversity, (b) radiation and exploration of new niches. At this stage new taxa appeared, and new morphotypes were developed, and all that determined the structure of the Serpukhovian ammonoid communities. Serpukhovian In the Urals, ammonoids characterize two genozones of the Serpukhovian stage: Uralopronorites– Cravenoceras and Fayettevillea–Delepinoceras. The richest localities of Serpukhovian ammonoids are known from the South Urals (western slope). On the eastern slope of the South Urals ammonoids are recorded from the Chelyabinsk Region (Verkhnyaya Kardailovka section). In addition, ammonoids are found on Berkh Island (Novaya Zemlya). Ammonoid collections contain over 32000 specimens, housed in the Paleontological Institute, Russian Academy of Sciences, Chernyshev TsNIGR Museum, St. Petersburg, and in the Paleontology Department of St. Petersburg State University. The systematic composition of the Serpukhovian ammonoid faunas was studied by Ruzhencev (1947a, 1947b, 1949a, 1949b, 1956a, 1958, 1965, etc.), Librovitch (1941, 1957), Ruzhencev and


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Bogoslovskaya (1971), Kusina and Yatskov (1990, 1999), Nikolaeva and Konovalova in Pazukhin et al. (2002), etc. In the Serpukhovian time, the taxonomic diversity of ammonoids considerably increased compared to the Visean. The Serpukhovian assemblage includes 121 species, 53 genera, and 19 families. The morphological diversity of ammonoids at that time is also relatively high. The morphospace defined by the parameters W, S, and D is delimited by the values of D = 0.01–0.6; W = 1.26–3.27; S = 0.34–1.81. The benthopelagic spheroconic and cadiconic forms with a narrow and moderately narrow umbilicus were represented by 32% of the overall species diversity. Nektobenthic (30% of species) were mainly represented by pachyconic and discoconic shells, whereas oxyconic and platyconic forms with rapidly expanding whorls were less common. Planktonic forms were quite diverse, both taxonomically and morphologically and were represented by 38% of species in the communities. In general, the ecological structure of communities of Serpukhovian ammonoids is similar to those of Late Visean communities (Fig. 4.12). The diversity among planktonic forms increased due to the radiation of the superfamily Neoglyphiocerataceae, primarily of the families Cravenoceratidae and Neoglyphioceratidae. One of the trends in the evolution of the planktonic forms in the communities was connected to an increase in the number of species with a wide platyconic or aperticonic widely umbilicate shell, and also the appearance of dwarf (not more than 20 mm in diameter) involute species. The appearance among the benthopelagic forms of various cadiconic and subcadiconic shells in the second half of the Serpukhovian, connected with the appearance of the families Glaphyritidae and Stenoglaphyritidae, was also important. At the Visean–Serpukhovian boundary no new highrank taxa or new morphotypes appeared. In addition, the Uralopronorites–Cravenoceras Genophase characteristically shows a higher species and generic diversity, in particular among Neoglyphioceratida (nine new genera), which became widespread. The morphological and taxonomic diversity of ammonoids remained high during the entire Serpukhovian, reaching its maximum in the Early Serpukhovian (Uralopronorites–Cravenoceras Genozone). The maximum species diversity coincides with the transgression at the beginning of the Serpukhovian, across the territory of the Urals. At the end of the Serpukhovian the taxonomic diversity decreased somewhat. The latter was, apparently, caused by a decrease in the area of the shelf in the course of a regression, the maximum of which was at the end of the Serpukhovian–beginning of the Bashkirian (Pazukhin et al., 2002). Ammonoid localities can be divided into three groups, differing in taxonomic composition and lithology of the host rock. These are (1) western slope of the PALEONTOLOGICAL JOURNAL

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28% 32 species

32% 36 species


2

3

4

Fig. 4.12. Proportions of life-forms in the Serpukhovian ammonoid assemblages of the Urals. Explanations as in Fig. 4.3.

South Urals, (2) eastern slope of the South Urals, and (3) Novaya Zemlya. On the western slope of the South Urals (Aktyubinsk and Orenburg Regions) communities of Serpukhovian ammonoids existed in the environments of the outer deep shelf. Their finds are known from the same “Dombar limestone,” as the earlier assemblage of the Hypergoniatites–Ferganoceras Genozone. In general, they retained major evolutionary trends that were outlined in the Late Visean. The communities are mainly composed of benthopelagic (29% of species) and planktonic (33% of species) forms (Figs. 4.13a, 4.13b). The diversity of planktonic forms somewhat increased, mainly as a result of the appearance of new genera and species in the families Rhymmoceratidae, Neoglyphioceratidae, and, especially, in the family Cravenoceratidae. The representatives of this family are dominant among planktonic forms both in terms of species diversity and in the number of individuals. In the family Girtyoceratidae, representatives of which mainly belonged to the nektobenthic life-form, a new genus Tumulites had a subspheroconic shell characteristic of benthopelagic forms. The diversity of nektobenthic forms, most of which were represented by species continued from the Visean, continued to slowly decrease. The changes in the Early Serpukhovian ammonoid communities of the western slope of the South Urals were likely to have been caused by internal factors, because no serious lithological changes have been recorded in the section around that time. They can be considered as the next stage in the evolution of the communities—the phase of maximum diversification with the maximum number of species and morphological diversity. Beginning in the second half of the Serpukhovian time, the sea level on the western margin of the Uralian basin dropped, a process caused by the intensified orogeny of the Urals (Puchkov, 2000). A simultaneous cool-

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% 100


90 80

60

70 60

50

50

40

40

30

30

20

20

10

10 0

South Urals, South Urals, western slope eastern slope of the outer of the outer shelf shelf/slope

0

Novaya Zemlya bioherms

South Urals, western slope of the outer shelf

South Urals, eastern slope of the outer shelf/slope


Late Serpukhovian, Fayettevillea–Delepinoceras Genozone % 100

(c)

(d)

90


80

35

70

30

60

25

50

20

40 30

15

20

10

10

5

0

South Urals, South Urals, western slope eastern slope of the outer of the outer shelf shelf/slope 1

2

0

Novaya Zemlya bioherms 3

South Urals, western slope of the outer shelf

South Urals, eastern slope of the outer shelf/slope


4

Fig. 4.13. Serpukhovian ammonoid assemblages in various facial types of sections of the South Urals and Novaya Zemlya: (a, c) proportions of life-forms and (b, d) species diversity: (a, b) Uralopronorites–Cravenoceras Genozone; (c, d) Fayettevillea– Delepinoceras Genozone. Explanations as in Fig. 4.3.

ing of the climate is likely to have happened. These factors were apparently unfavorable for ammonoids, the diversity of which halved. Late Serpukhovian ammonoids come from light gray, fine-grained and micritic, bioclastic, crinoid or nodular, breccia-like dark-gray limestones with siliceous nodules. In some localities ammonoids are found in association with bivalves, brachiopods, trilobites, and crinoids (Ruzhencev and Bog-

oslovskaya, 1971). The assemblages are dominated by cadiconic, subspheroconic, and pachyconic benthopelagic (41% of species) forms, represented by species of the newly appeared families Glaphyritidae, Stenoglaphyritidae, Ramositidae, and new genera of the old families Agathiceratidae, Delepinoceratidae, etc. The diversity of planktonic forms somewhat decreased (26% of species). Among those, the taxonomic compo-


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sition was considerably renewed. Laticonic widely umbilicate forms disappeared. The number of nektobenthic forms decreased (32%) (Figs. 4.13c, 4.13d). Despite being relatively diverse in species composition, the communities are usually represented by small number of specimens. On the eastern slope of the Urals (Verkhnyaya Kardailovka) the evolution of ammonoids occurred in the deeper environments of the outer slope of a carbonate platform. In the Early Serpukhovian, this area became somewhat shallower, and a few small bryozoan-serpulid bioherm buildups appeared (Pazukhin et al., 2002). The assemblage of Early Serpukhovian ammonoids is less diverse compared to that of the Dombar Limestone in western Kazakhstan, but is more diverse than that from the Upper Visean in the same section. The communities are mainly composed of planktonic species (54% of species) of the families Cravenoceratidae, Rhymmoceratidae, Neoglyphioceratidae, and Prolecanitidae; whereas the remaining part of the community is represented by benthopelagic and nektobenthic forms (Figs. 4.13a, 4.13b). Despite a high proportion of planktonic forms in the total diversity, the number of their shells in the assemblages is very low, and the communities were dominated by benthopelagic forms. In the second half of the Serpukhovian the basin existed in this territory deepened (Kulagina et al., 2001). The ammonoid assemblage is far more diverse, compared to the Early Serpukhovian (27 species instead of 11, respectively). It was dominated by benthopelagic (40% of species) and planktonic (26% of species) species of the families Agathiceratidae, Delepinoceratidae, Cravenoceratidae, Fayettevilleidae, Glaphyritidae, Stenoglaphyritidae, etc. Nektobenthic forms (families Girtyoceratidae, Pronoritidae, Agathiceratidae, etc.) constituted 34% of species of the diversity (Figs. 4.13c, 4.13d). Novaya Zemlya Ammonoid communities of Novaya Zemlya developed throughout the Serpukhovian in the environment of a shallow-water bioherm that existed from the Late Visean to the Bashkirian. In the Early Serpukhovian the diversity of ammonoids increased considerably. Despite a noticeable renovation of the taxonomic composition and reduction in diversity (23 species and 14, respectively), the ecological structure of ammonoid communities was not significantly changed at the transition from the Early to Late Serpukhovian (Fig. 4.13). The assemblages were equally composed of pachyconic, subspheroconic, and cadiconic narrowly umbilicate benthopelagic and widely umbilicate aperticonic and laticonic planktonic forms (37 and 38% of species, respectively). They are represented by species of the families Cravenoceratidae, Goniatitidae, Glaphyritidae, Fayettevilleidae, Delepinoceratidae, etc. Nektobenthic forms were also relatively diverse (27% of spePALEONTOLOGICAL JOURNAL

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cies diversity) and are represented by oxyconic, discoconic, and, to a lesser extent, pachyconic narrowly umbilicate forms of the families Berkhoceratidae, Dimorphoceratidae, Girtyoceratidae, etc. (Fig. 4.13). The assemblages of Serpukhovian ammonoids from Novaya Zemlya show a high degree of endemism at the species level (32 species of 37, over 80%), impoverished taxonomic (at the species and especially generic level) and morphological diversity, and a high number of taxa representing the nektobenthic life-form. Planktonic forms are dominated by aperticones, whereas ophiocones and laticones are uncommon, whereas planktonic small involute forms, widespread in the communities of the South Urals, are absent. Despite the fact that the evolution of the Serpukhovian ammonoids occurred in the distant parts of the basin, several general patterns may be recognized. (1) The main trend in the evolution of the communities was the increase in the taxonomic and morphological diversity of planktonic and benthopelagic forms, which were dominant in the communities. In the second half of the Serpukhovian the number of benthopelagic species with a cadiconic shell increased, mainly due to the appearance of the new families, Glaphyritidae and Stenoglaphyritidae. Thus, in the Serpukhovian specialization in the planktonic and benthopelagic segments of community increased. At the same time, the proportion of the nektobenthic forms slightly decreased. As in the Late Visean, most families contain a single morphotype or morphogroup; i.e., each family represents a single life-form. (2) The evolution of the ammonoid communities of the Serpukhovian in the Urals retained trends that were rooted in the Late Visean, while the ammonoid habitats did not change much. Neither new high-rank taxa nor new morphotypes appeared at the Visean–Serpukhovian boundary. Changes in their ecological structure in the Early Serpukhovian probably reflected the next stage in the evolution of the communities—the maximum diversification phase with the highest number of species and the highest morphological diversity. (3) Serpukhovian ammonoids inhabited the outer part of the open carbonate shelf. Their maximum diversity is registered in the southwestern regions of the basin, where at that time communities of crinoids and ammonoids developed on the outer, deeper shelf in quiet environments. In the north of the Uralian basin, ammonoids inhabited shallow-water settings near the bioherms. Their communities were less diverse both taxonomically, and morphologically. (4) The ammonoid diversity at the beginning of the Serpukhovian and its reduction in the second half of the Serpukhovian coincides with the maximum highstand and subsequent sea level drop, which led to the reduction in the area of the deeper outer shelf, primarily in the western part of the basin. At the end of the Early Carboniferous a large sea level drop is recorded for the Urals. This regression

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19% 20 species


2

3

4

Fig. 4.14. Proportions of life-forms in the Bashkirian ammonoid assemblages of the Urals. Explanations as in Fig. 4.3.

resulted in the considerable reduction of the deeper outer shelf. At the same time the orogeny increased leading to the appearance in the Bashkirian of a deep trough in the Fore-Urals (western part of the basin). The development of the fore-deep and increased processed of uplifting resulted in considerable changes in the sedimentary settings and sea floor paleogeography (Khvorova, 1961; Puchkov, 2000; Kulagina et al., 2000; etc.) and caused a crisis in ammonoid communities. At the Serpukhovian–Bashkirian boundary over 60% of genera and families become extinct, while the area of ammonoid distribution decreased considerably and fundamental changes in the structure of the communities took place. 4.3.5. Pennsylvanian (Middle and Late Carboniferous) Bashkirian In the Urals, numerous ammonoids are found in the lower Bashkirian (Homoceras–Hudsonoceras, Reticuloceras–Bashkortoceras, and Bilinguites–Cancelloceras genozones). The riches localities of Bashkirian ammonoids are known from the western and eastern slopes of the South Urals, in the Aktyubinsk, Orenburg, and Chelyabinsk regions, and Bashkortostan. The ammonoid fauna was studied by Librovitch (1939a, 1939b, etc.) Ruzhencev (1947a, 1947b, 1955, etc.), Ruzhencev and Bogoslovskaya (1978), Nikolaeva (1999; Nikolaeva in Kulagina et al., 2000, 2001, etc.). A few ammonoids were also described by Kusina and Yatskov (1999) from the Bashkirian of Novaya Zemlya. Collections of Bashkirian ammonoids include over 11600 specimens and are housed at the Paleontological Institute, Russian Academy of Sciences, in Chernyshev TsNIGR Museum, and Department of Paleontology of St. Petersburg State University. The Bashkirian assemblage is somewhat more impoverished than the Serpukhovian—106 species,

39 genera, and 15 families. Because of a decrease in diversity the morphological space of the assemblages is somewhat narrower: D = 0.02–0.55; W = 1.28–3.38; and S = 0.32–2.39. The main trend in the morphological evolution of ammonoids at this stage was an increase in the number of species with involute, rapidly expanding whorls and a narrow or moderately narrow umbilicus. These species, characterizing the nektobenthic life-form, constitute 48% of the total taxonomic diversity. Spheroconic and cadiconic benthopelagic forms were not abundant (19% of the total number of species). Evolute widely umbilicate shells and small narrowly umbilicate spheroconic and pachyconic forms, constituting the planktonic part of the community, show lower diversity compared to that of the Serpukhovian (33% of species instead of 38%), but were still numerous. Planktonic forms were now dominated by laticones and aperticones, whereas the number of ophioconic forms considerably decreased compared to that of the Serpukhovian. The species diversity among involute planktonic forms also decreased (Fig. 4.14). The evolution of Bashkirian ammonoids occurred in considerably different environments compared to that of the Serpukhovian, which was considerably reflected by the structure of communities. At the Serpukhovian– Bashkirian boundary an extensive regression occurred in the South Urals. A large portion of the sedimentation area in the South Urals in the Bashkirian represented a relatively shallow shelf, and only the Upper Bashkirian deposits in the Zilair megasynclinorium are interpreted as deposits accumulated in the submerged margin of the shelf and continental slope (Kulagina et al., 2001). From this territory the earliest Lower Bashkirian ammonoid assemblages are described by Nikolaeva (1999). Localities of Bashkirian ammonoids occur both on the western, and the eastern slope of the Urals. Each of these regions has a particular type of sedimentary settings, reflected in the structure of the communities. Ammonoid localities on the western slope of the Urals belong to the Central-Uralian facial zone. This zone was a fore-mountain trough with carbonate-terrigeneous and terrigenous sediments, which began its development at the beginning of the Bashkirian between the East European platform in the west and the area of the basin presently covered by the Ural Mountains in the east. The deposits of the Lower Bashkirian yielding ammonoids belong to the Kuruil Formation (Khvorova, 1961), which is represented by thick series of carbonate rocks, interbedded with series of thinly and medium-bedded dark limestone-cherty-clayey deposits with rare interbeds of carbonate breccias. The western slope of the South Urals contains the most diverse ammonoid assemblage. Three Lower Bashkirian ammonoid genozones are recognized. In the lowermost Homoceras–Hudsonoceras Genozone, the assemblages mainly contain discoconic and pachy-


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(a)

% 100

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60 50

15

40 10

30 20

5

10 0

0 South Urals western slope 1

South Urals western slope

South Urals, eastern slope 2

3

South Urals, eastern slope

4

Fig. 4.15. Early Bashkirian (Homoceras–Hudsonoceras) Genozone ammonoid assemblages in various regions of the South Urals: (a) proportions of life-forms and (b) species diversity. Explanations as in Fig. 4.3.

conic shells of the genera Isohomoceras, Homoceras (the newly appeared family Homoceratidae), Hudsonoceras (Nomismoceratidae), Glyphiolobus (Dimorphoceratidae), Ramosites (Ramositidae), Subitoceras (Stenoglaphyritidae), etc. They mainly belong to the nektobenthic (54% of species) life-form. The number of planktonic forms was also large (25% of species) and virtually did not change compared to the assemblage existing in this territory in the Late Serpukhovian (Fig. 4.15). Plankton was dominated by small narrowly umbilicate wide shells of the genus Physematites (Stenoglaphyritidae), whereas laticonic shells were less common. It is noteworthy that the planktonic morphospace could also be occupied by young individuals of the genus Homoceras, the shell shape of which changes in ontogeny from aperticone to laticone and discoconic. The number of benthopelagic forms, in contrast, sharply decreased compared to that at the end of the Serpukhovian (21% instead of 41% in the western slope of the South Urals). A reduction of their proportion is primarily connected with almost complete disappearance of the family Cravenoceratidae. At that time, among the benthopelagic forms, the genus Glaphyrites was the most diverse. Thus, it is evident that the crisis mainly affected the benthopelagic part of the assemblage, which, apparently, related to a general sea level drop and reduction in the specific crinoid-ammonoid communities that were widespread in the outer shelf in the Serpukhovian. The forms with a low aperture (microphages, apparently, feeding in the near-bottom waters) became PALEONTOLOGICAL JOURNAL

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extinct. At the same time the number of the nektobenthic species with a high aperture and rapidly expanding whorls increased. Among the planktonic forms, species with a small spheroconic narrowly umbilicate shell or laticonic shell, less commonly aperticones, prevailed. Ammonoids of the second Bashkirian genozone (Reticuloceras–Bashkortoceras) are known only from the western slope of the Urals. At that time, both the morphological and taxonomic diversity of ammonoids increased considerably (74 species instead of 29), which could be partly have been due to the deepening of the basin. Considerable changes in the ecological structure of communities also happened at that time (Figs. 4.3, 4.16). The number of planktonic forms increased considerably (41% instead of 25% of species in the preceding genophase). The family Surenitidae, which emerged at about that time, and a few new genera: Aenigmatoceras, Chumazites, Chartymites, Brevikites, and a considerable number of species from the family Reticuloceratidae belonged to the planktonic life-form. The morphological diversity of ammonoids also increased, the evolute, widely umbilicate forms became dominant. A numbers of pachyconic benthopelagic forms with a moderately narrow umbilicus and slowly expanding whorls, also increased somewhat mainly, due to the appearance of diverse species in the newly appeared families Decoritidae and Reticuloceratidae (genera Decorites, Phillipsoceras, and Tectiretites). The diver-

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radiation and exploration of new niches, and an acme phase. The evolution of ammonoids in this territory was completed in the Bilinguites–Cancelloceras Genophase. At that time the taxonomic and morphological diversity strongly decreased, caused by the general sea-level drop and reduction in the area of the outer shelf as a result of the development of the fore-deep basin. The structure of the communities also considerably changed: the benthopelagic life-form completely disappeared, whereas the nektobenthic form became dominant (about 75% of species), with the planktonic forms being subdominant (Fig. 4.17). In the eastern slope of the Urals, ammonoids were less widespread and evolved in considerably different environments. They are known from a few localities (Chelyabinsk Region, Murchison Hill, Bolshoi Kizil River) and come from bioclastic and detrital limestones often containing diverse benthic fauna of brachiopods, corals, bryozoans, gastropods, etc. The deposition apparently occurred on a relatively shallow shelf with carbonate buildups. Ammonoids are found in two genozones: Homoceras–Hudsonoceras and Bilinguites– Cancelloceras. The ammonoid assemblage of the Homoceras–Hudsonoceras genozone includes eight species, but it is relatively representative in number of specimens (367 specimens). The species diversity of the communities is dominated by nektobenthic forms (72%) (Fig. 4.15). The amount of planktonic and benthopelagic forms is low both in number of species (14 and 14%,

27% 18 species

31% 21 species


2

3

4

Fig. 4.16. Proportions of life-forms in the Early Bashkirian (Reticuloceras–Bashkortoceras) ammonoid assemblages on the western slope of the South Urals. Explanations as in Fig. 4.3.

sity of nektobenthic forms, in contrast, considerably decreased, mainly as a result of the extinction of the species of the family Homoceratidae (Fig. 4.16). Possibly these changes may have been related to the higher sea level resulting from a short-term transgression, which was followed by a regression at the beginning of the next genophase. The proportions of life-forms in the ammonoid assemblage of the Reticuloceras–Bashkortoceras Genozone is similar to those of the Late Visean–Early Serpukhovian and, apparently, they characterize the same phases of the evolution of a community: phases of % 100

(a) Number of species 16 (b)

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8

40

6

30 20

4

10

2

0

South Urals western slope 1

0

South Urals, eastern slope 2

South Urals western slope 3

South Urals, eastern slope

4

Fig. 4.17. Early Bashkirian (Bilinguites–Cancelloceras) ammonoid assemblages in various regions of the South Urals: (a) proportions of life-forms and (b) species diversity. Explanations as in Fig. 4.3. PALEONTOLOGICAL JOURNAL

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CEPHALOPODS IN THE MARINE ECOSYSTEMS OF THE PALEOZOIC % 100

(a) Number of species 80

90 80

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(b)

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60

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50 40

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30

20

20

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0

1265

0

Homoceras– Reticuloceras– Billinguites– Hudsonoceras Bashkortoceras Cancelloceras 1

2

4

3

Fig. 4.18. Changes in the proportions of the life-forms (a) and species diversity (b) in the Early Bashkirian ammonoids from the localities on the western slope of the South Urals. Explanations as in Fig. 4.3.

respectively) and in the number of specimens. In general such ecological structure is characteristic of the crisis phase or of the initial stage of radiation in a community. The evolution of ammonoids of the Bilinguites– Cancelloceras Genozone occurred in a similar environment, which was reflected in the structure of the communities. In general, the proportions of the life-forms repeat the structure of the communities of the Homoceras–Hudsonoceras Genozone. The proportion of planktonic forms somewhat increased (27% of species) (Fig. 4.17). In general, ammonoid communities of the Bashkirian were unstable, rapidly changing, with high rate of evolutionary processes, which can reflect the response of the group to sharp changes in the environment (Figs. 4.3; 4.18). The second half of the Bashkirian was marked by a local (confined to the Urals) extinction of ammonoids, which was caused by sharp changes in the overall structure of the basin, in connection with the development of the fore-deep basin. Moscovian During the Moscovian the ammonoid communities of the Urals entered a deep crisis, apparently related to the total change in the settings in the whole of the Uralian basin following the development of the fore-deep basin. Thick coarse-clastic and flysch formations accumulated in the eastern part of the fore-deep basin, whereas the western part was delimited by the steep slope of the carbonate platform. The Moscovian in the western slope of the South Urals is mainly characterPALEONTOLOGICAL JOURNAL

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ized by foraminifers, which are used in the recent stratigraphic schemes (see Ivanova, 2000, 2002) and by benthic fauna. Ammonoids in the Moscovian Stage are very poorly represented, and are found only in the western slope of the Urals and in Novaya Zemlya. The Moscovian ammonoid assemblages were studied by Ruzhencev (1951a, 1952a, 1955); Librovitch (1957), Kusina and Yatskov (1999), Popov (1975), etc. The collections are housed in the Paleontological Institute, Russian Academy of Sciences. Despite low species diversity, ammonoids are represented by several morphotypes, characterizing the nektobenthic, planktonic, and benthopelagic life-forms. In the South Urals, it is difficult to reliably identify the composition of the ammonoid assemblage of the Moscovian Stage, due to the uncertainty in the stratigraphic positions of the few finds. Various authors indicate the presence of the following species: Diaboloceras uralicum, Syngastrioceras orientale, Stenopronorites karpinskii, and Pseudoparalegoceras tzwetaevi. Tentatively, this assemblage may be supplemented by Wellerites russiensis and Aktubites trifidus. A limestone boulder in the Ziyanchura breccia (Kasimovian) contained Eoasianites kajraklensis (Ruzhencev, 1950); however, the dating of this boulder as the Moscovian is doubtful because other ammonoids from this locality are not referred to the Moscovian. In Novaya Zemlya, Moscovian ammonoids (Diaboloceras–Winslowoceras) Genozone are found in Berkh Island in the bedded carbonate series with a slight admixture of terrigenous material (Krestovsky Horizon). Ammonoids are represented by a few shells of

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the species Pseudopronorites aquilonalis, Pseudobisatoceras gorbovense, Glaphyrites vulgaris, and Diaboloceras sp. To the north of this region in the Russian Haven there was a record of Winslowoceras sp. (Kusina and Yatskov, 1999, identified by M.F. Bogoslovskaya). The deposits containing ammonoids also contain foraminifers and Moscovian conodonts (Sobolev and Matveev, 2002). Despite a very impoverished taxonomic composition of ammonoids, the assemblage contains four different morphotypes, representing two life-forms. Pachyconic or subdiscoconic shells with a very narrow umbilicus and a subdiscoconic shell with a moderately narrow umbilicus and with flattened flanks (Prolecanites-like) are characteristic of the nektobenthic life-form. The planktonic life-form (P-1) also contains two morphotypes: aperticonic widely umbilicate and ophioconic, with triangularly coiled whorls. Kasimovian In the South Urals the Kasimovian is mainly represented by clayey deposits with subdominant interbeds of clayey and sandy limestone. In the east of the region, the sections contain clayey breccias and pebbled limestones. Kasimovian ammonoids of the Urals were studied mostly by Ruzhencev (1950). The material on this stage includes about 200 shells housed in the collection of the Paleontological Institute of the Russian Academy of Sciences (coll. nos. 319, 320). The Kasimovian assemblage is one of the most impoverished (9 species, 9 genera, 9 families). Although the taxonomic diversity is somewhat higher than in the Moscovian, the assemblage had still not recovered from the crisis. During the Kasimovian, a very slow recovery of diversity is observed, mostly due to the nektobenthic pachyconic forms. The total morphospace occupied by Kasimovian ammonoids is small: D = 0.1–0.59; W = 1.5–2.5; S = 0.4–2.4. In addition, the actual coverage of this morphological space is very small. Despite the extremely low taxonomic diversity, Kasimovian ammonoids represent a considerable number of morphotypes because of their scattered distribution in the morphospace. Except the ancient Pronoritidae and Glaphyritidae, all the other families are newly appeared (Neoicoceratidae, Agathiceratidae, Thalassoceratidae, Uddenitidae, Schistoceratidae, Shumarditidae, and Marathonitidae). It is essential that only one widely umbilicate morphotype was present, represented by aperticonic Eoasianites kajraklensis (Neoicoceratidae), which we interpret as a benthopelagic form (its stratigraphic position was discussed above). The planktonic life-form is also represented by only one spheroconic form, Kargalites (Subkargalites) neoparkeri. It is noteworthy that all Kasimovian ammonoids come from a single limestone boulder in the lower part of the Ziyanchura breccia (Lower Gzhelian). This boul-

der was dated Kasimovian based on certain lithological and tectonic evidence (Ruzhencev, 1950, p. 16). Thus, if Ruzhencev’s view is accepted it has to be remembered that the distribution of the life-forms for the Kasimovian is based on a single ammonoid association from a single locality. Gzhelian In the Gzhelian, the geodynamic structure of the Uralian Fore-Deep can be considered to be completely formed. This is responsible for the transverse lithological and facial zonation typical of such structures. Three major types of sections corresponding to different sedimentary settings are recognized for the Gzhelian (and for the overlying Permian stages). The sedimentary settings in the Uralian basin were mostly controlled by the influx of clastic material from the eastern and western margins. From the east to the west the following zones are recognized: (1) zone of very thick terrigenous series; (2) zone of considerably thinner clayey-carbonate deposits; and (3) zone of moderately thick carbonate formations. These zones correspond, respectively, to eastern orogenic margin, central part of the depression, and western platform margin. In the South Urals, in the environment of active tectonics accompanying the collision of plates and elevation of the Magnitogorsk meganticlinorium, accumulation of terrigenous sediments considerably exceeded carbonate deposition. On the eastern margin of the southern regions of the strait, bullion and boulder-rich breccias, conglomerates, and gravelites accumulated. The slope facies contain numerous traces of turbidite flows and slides. In the central zone of the basin clayeysandy rocks contain large boulders, brought along the slope from the east. In the west of the strait, carbonates with frequent interbeds of coarse limestone breccias were accumulated. This indicates that the Gzhelian, especially its first half, was marked by a series of earthquakes. The earthquakes were caused by sliding along the contact zone of the plates. The active tectonic movements were accompanied by noticeable volcanic activity. The lower horizons of the Gzhelian contain many ash layers originated from the eastern slope (Khvorova, 1961). Gzhelian ammonoids are relatively numerous in thin layers of marls and sandy limestones in coarsegrained flyschoid series on the eastern slope of the depression and are virtually unknown from the areas of distribution of cherty-carbonate-clayey and carbonate rocks of the central and western zones. The affinity of ammonoids to the regions of flyschoid sedimentation apparently explains the almost complete absence of ammonoids to the north of the Belaya River basin, an area where, at that time, carbonate mud was mostly deposited. On the other hand, the remains of the benthic fauna mostly come from carbonates of the western, platform


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facial zone. Apart from ammonoids, fusulinids are the only fossils that are found in considerable quantities in flyschoid rocks. However, ammonoids and fusulinids do not usually occur together in the same bed. Major finds of ammonoids come from carbonate members of flyschoid cycles, whereas fusulinids come from terrigenous members. This suggests different time and place of their habitats. A similar pattern of the facial affinity of fossil groups was observed to the end of the basin’s existence. Gzhelian ammonites were studied by Karpinsky (1874), Ruzhencev (1950), Bogoslovskaya and Popov (1986a, 1986b), and Borisenkov (2002, 2003, 2004a, 2004b). Material on Gzhelian ammonoids includes over 5000 specimens, housed in the collection of the Paleontological Institute nos. 319, 320; TsNIGR Museum, coll. no. 12257, etc. The Gzhelian assemblage includes 44 species of 24 genera of 16 families and has a complex ecological structure. The total morphospace increased considerably: D = 0.1–0.62; W = 1.4–3,7; S = 0.3–3.29, which is somewhat larger than in the Serpukhovian, although the taxonomic diversity is much lower. However, it can be positively suggested that the ecological structure of the community recovered almost completely. The morphological distribution is more or less bimodal, with peaks separated at approximately D = 0.18. Ammonoids with lesser values of D belonged to the nektobenthic life-form, while the group with high values of D is represented by planktonic and benthopelagic forms. The first morphogroup includes the families Pronoritidae, Medlicottiidae, Uddenitidae (all Prolecanitida), Thalassoceratidae, Neodimorphoceratidae, Agathiceratidae, Pseudohaloritidae, Adrianitidae, Vidrioceratidae, and Marathonitidae. This group comprises narrowly umbilicate morphotypes with moderately and rapidly expanding whorls, which we interpret as nektobenthic forms. The second group comprises species of the families Shumarditidae, Glaphyritidae, Daraelitidae, Schistoceratidae, Somoholitidae, and Neoicoceratidae. These morphotypes can be interpreted as planktonic and benthopelagic adaptive types. Despite the relatively high morphological diversity of Gzhelian ammonoids, it has to be said that the assemblage contains very few compressed morphotypes with a medium-sized or wide umbilicus—platycones and ophiocones. The ecological distribution of the Gzhelian assemblage is unusual. The nektobenthic and benthopelagic groups are represented as follows: 48% of species— nektobenthic and 27%—benthopelagic. The remaining portion of the assemblage is interpreted as planktonic: 18% of species—evolute forms, 7%—involute forms (Fig. 4.19). Ophioconic widely umbilicate forms are represented by a single form only (Eoasianites vodorezovi), with the whorl width considerably exceeding typical PALEONTOLOGICAL JOURNAL

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18% 8 species

27% 12 species

7% 3 species

48% 21 species 1

2

3

4

Fig. 4.19. Proportions of life-forms in the Gzhelian ammonoid assemblage of the Urals. Explanations as in Fig. 4.3.

representatives of this morphotype in the younger assemblages. E. vodorezovi is apparently an ancestor of the genus Svetlanoceras (Borisenkov, 2003), which was initial in the family Paragastrioceratidae, representatives of which belong to the morphotype of widely umbilicate shells with slowly expanding whorls. One of the most ammonoid-rich sections is located near the village of Nikol’skoe on the watershed between the Sakmara and Urals rivers. This section belongs to the clayey type of sequences, in which ammonoids are usually rarely found, and therefore it is of particular interest. Numerous ammonoids are found in brownish, compact clay with layers of marly nodules (Ruzhencev, 1950). The assemblage is represented by 20 species, among which 57% represent nektobenthic, 32%—benthopelagic, and 11%—planktonic ophioconic forms (Fig. 4.20). This distribution of life-forms suggest an unusual nature of this locality. A considerable proportion of nektobenthic and benthopelagic species is usually characteristic of communities inhabiting relatively shallow waters. This is contradicted by the deep-water nature of this sequence largely composed of clayey rocks. The absence of the involute planktonic forms (such as Vidrioceras borissiaki, Marathonites uralensis, and Emilites plummeri) also remains unexplained. Flyschoid sequences on Aidaralash Creek are rich in ammonoids. A large ammonoid material was collected from several levels of sandy limestones and marl nodules. Of 22 species described from this section (Ruzhencev, 1950) 45% of species belong to the nektobenthic life-form and 32% of species to the benthopelagic life-form (Fig. 4.20). Planktonic forms are more numerous, compared to assemblages from clayey sequences, and constitute 23% of species diversity. They are represented by both evolute (18%) and involute (5%) morphotypes. In general, the ecological structure is similar to that of the entire Gzhelian ammonoid assemblage.

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Fig. 4.20. Gzhelian ammonoid assemblages in various facial types of sections of the South Urals: (a) proportions of life-forms and (b) species diversity. Explanations as in Fig. 4.3.

In the deep-water sections represented by thin siliceous-carbonate series, ammonoids are almost universally absent (like other faunal remains, except radiolarians and siliceous sponges, which formed siliceous varieties of these beds. As mentioned above, no ammonoids were found in massive carbonate series. Thus, a considerable reduction of the proportion of planktonic forms in the ammonoid assemblage and increase in the proportion of benthopelagic forms is recorded for the Gzhelian compared to the Bashkirian. In analyzing the dynamics if changed in the planktonic life-form during the interval considered, it appears that the planktonic life-form was the most specialized. Planktonic organisms are the most sensitive indicators of environmental changes and slowly recover after crises. 4.3.6. Early Permian Asselian The Asselian deposits contain considerably more terrigenous material than the underlying Gzhelian series, which is related to an increased intensity of tectonic movements on the eastern strait. All the above lithofacial zones remained. Along the strike of the Asselian deposits in the northward direction the number of occurrences and diversity of assemblages decrease. It is noteworthy that almost everywhere Asselian rocks unconformably overlie the Upper Carboniferous (Khvorova, 1961).

Asselian ammonoids are known from relatively few localities in the South Urals, including Bashkortostan, Orenburg, and the Aktyubinsk Region of Kazakhstan. The material on Asselian ammonoids of the Urals includes over a thousand shells (Paleontological Institute, Russian Academy of Sciences, coll. nos. 318, 323, 472; Chernyshev TsNIGR Museum, coll. no. 12257; etc). These ammonoids were mainly studied by Gerasimov (1937), Maksimova (1948), Ruzhencev (1936, 1937, 1950, 1951b), and Bogoslovskaya (1986). The assemblage contains 26 species of 14 genera of 11 families. Morphological diversity remained high (D = 0.01–0.62; W = 1.25–2.95; S = 0.35–3). The general field is distinctly bimodal (delineated approximately at D = 0.3). This delineation is more distinct than it was in the Gzhelian. One of the modal areas (D = 0.38–0.62; W = 1.2–2.0) contains planktonic and benthopelagic forms, whereas another (D = 0.01–0.3; W = 1.25–2.95) mainly contains nektobenthic forms. The most of the assemblage is represented by nektobenthic morphotypes (Pronoritidae, Thalassoceratidae, Agathiceratidae, Bisatoceratidae, Medlicottiidae, Popanoceratidae)—48% of species. This is followed by benthopelagic forms (Neoicoceratidae, Paragastrioceratidae, Metalegoceratidae, Vidrioceratidae)—24%. Planktonic forms are dominated by forms with a wide umbilicus and low whorls—Neoicoceratidae and Paragastrioceratidae (Fig. 4.21).


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20% 5 species


2

3

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Fig. 4.21. Proportions of life-forms in Asselian ammonoid assemblages of the Urals. Explanations as in Fig. 4.3.

Most significant changes are registered in the planktonic portion of the community. Its proportion increased compared with the Gzhelian (from 23 and 28%). While planktonic forms with an evolute ophioconic shell reach 24%, involute planktonic forms include only one species, Somoholites artius. The proportion of benthopelagic species slightly decreased. An increase in the proportion of planktonic species (mainly evolute forms) in the ammonoid assemblages may indicate a deepening of the basin, which is supported by the lithological and facial studies (Khvorova, 1961). Taking into account that at the end of the Asselian–beginning of the Sakmarian there was a peak of the Late Paleozoic South glaciation (Chumakov and Zharkov, 2002), the deepen-

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ing of the basin should be related not only to an increase of the oceanic level, but also to local tectonic events in the South Urals. Asselian ammonoids are found in sections of all three types (flyschoid, basinal, carbonate); however, the number of occurrences and the taxonomic and ecological diversity of assemblages may be different. One of the most important localities in the flyschoid sequences in the northern South Urals is the occurrence in the section in the upper reaches of the Yuryuzan River from where a relatively rich assemblage, represented by mass material, has been recorded. S.V. Maksimova, and later B.I. Chuvashov made large collections in the thin beds of clayey limestone in the shalesiltstone series. Of several hundred specimens in this assemblage, Agathiceras uralicum and Svetlanoceras serpentinum constitute about 80% of all specimens, approximately 40% each. Both species are very typical representatives of the nektobenthic and planktonic lifeforms, respectively. In terms of the proportion of species, the assemblage mainly includes nektobenthic species—64%, while 18% of the assemblage are represented by benthopelagic forms (Eoasianites trapezoidalis and Juresanites primitivus) and both types of planktonic forms 9% each (S. serpentinum and Somoholites artius (Fig. 4.22). Another, very important flyschoid sequence is located in the extreme south of the region under consideration on Aidaralash Creek, Aktyubinsk Region, Kazakhstan. Ruzhencev (1951b) collected a representative assemblage of 10 species in the nodules of compact sandstone from the sandy series with large quan-

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tity of fusulinids. Bogoslovskaya and Popov (1986a, 1986b) supplemented this record with another eight species. Unfortunately, Bogoslovskaya and Popov did not indicate the actual number of shells. The analysis of Ruzhencev’s data gives the following distributions: specimens of nektobenthic taxa ≈ 60% (Boesites, Neopronorites, Artinskia, Agathiceras, Prothalassoceras, Aristoceras), benthopelagic ≈ 25% (Prostascheoceras, Glaphyrites), planktonic ≈ 15% (Eoasianites). It is important that the proportions of the lifeforms based on quantity of species gives similar results: nektobenthic, 61%; benthopelagic, 28%; and planktonic, 11% (Fig. 4.22). The sections discussed represent a typical example of flyschoid series and indicate relatively deep-water environments at the foot of the eastern slope of the strait. In the clayey-carbonate basinal sections typical of the central regions of the strait (basins of the Sim, Assel, and Usolka rivers), ammonoids are uncommon, and their diversity is minimal (usually two or three species are represented by a few specimens). All known samples are to some extent accidental. Apparently ammonoids did not live permanently in the mainstream of the strait, where strong meridional currents could have existed, but entered this zone only occasionally. In the reefoid sections, ammonoids are known only from Sterlitamak Shikhans (large fossil reefs). In general, it is known that ammonoids are rarely found in the extremely shallow-water facies, although sometimes a few specimens can be found in reef deposits. Ruzhencev (1951b) recorded an assemblage of 10 species from the Tra-Tau Shikhan. Except Neopronorites rotundus (≈25%) and Agathiceras uralicum (≈50%) other species are represented by a few specimens. The life-forms are distributed as follows: nektobenthic— 70%, benthopelagic—10%, and planktonic—20%. It is noteworthy that Agathiceras uralicum almost always when found in a sample noticeably prevail over other species in the number of specimens, sometimes constituting over 50%. This applies to almost all stages and all localities. It is possible that specific adaptations of this species allow it to inhabit various environments, retaining considerable abundance everywhere. Asselian assemblage is almost half of the Gzhelian and displays a somewhat different distribution of lifeforms. Nektobenthic ammonoids dominate, representing about half of the total diversity, and at the same time the diversity of planktonic forms increases. The plankton shows a predominance of ophioconic and similar forms, whereas morphogroup 1A' (involute shells) is represented by a single species. The appearance of the ophioconic morphotype in the Gzhelian and its further evolution in the Asselian is exclusively connected to the superfamily Neoicocerataceae. Furthermore, in the subsequent epochs this life-form will be represented solely by neoicoceratids.

Sakmarian In the Sakmarian, flyschoid series continued accumulating in the Uralian Strait. By the end of the Sakmarian, the thickness and abundance of the terrigenous series considerably increased. In the eastern coastal zone, thick underwater fans were formed. These fans continued the underwater flow systems, which existed from the Asselian. This suggests the existence of longexisting water flows running across the Paleouralian Mountain Range. The underwater fans formed a good substrate for coral reefs, being populated by abundant benthos. Ammonoids preferred inhabiting deeper parts of the basin with flyschoid sedimentary settings. General trends in the evolution of Sakmarian ammonoids continued from the Asselian. No new families appeared at this time. Evolution was restricted to the generic and species levels, while Carboniferous taxa continued to become extinct. As in the Asselian (and even to a greater extent), Sakmarian ammonoids were more abundant and diverse in the south of the region. Localities for Sakmarian ammonoids are not known north of the Ufa River basin. Material on Sakmarian ammonoids includes several thousands shells, occurring mainly from the sections in Bashkortostan, Chelyabinsk Region, and the Orenburg and Aktyubinsk regions of Kazakhstan (collection of the Paleontological Institute of the Russian Academy of Sciences, nos. 318, 323, 472, 590, etc.). Sakmarian ammonoids of the Urals were mainly studied by Ruzhencev (1938, 1951b, 1952b) and Maksimova (1935, 1938). The assemblage includes 34 species of 16 genera of 10 families, i.e., taxonomically somewhat richer than the Asselian ones. The morphospace of the Sakmarian ammonoids: D = 0.01–0.62; W = 1.22–3, S = 0.3–2.8. Although the total number of species is higher than in the Asselian, the boundaries of the morphospace remained the same. The distinctness of the groups increased. The group with D ≥ 0.24 (morphotypes with a wide and medium-sized umbilicus) is almost exclusively Neoicocerataceae—species of the genera Svetlanoceras, Paragastrioceras, Uraloceras (Paragastrioceratidae), and Metalegoceras (Metalegoceratidae). These assemblages are dominated by nektobenthic and planktonic forms. The proportion of benthopelagic species changed considerably. A large portion of the assemblage is represented by nektobenthic morphotypes (Pronoritidae, Medlicottiidae, Daraelitidae, Thalassoceratidae, Agathiceratidae, Popanoceratidae, some Paragastrioceratidae)—46%. The Sakmarian communities show more even distribution of planktonic lifeforms, plankton-1 (evolute forms with low whorls) constitutes 18% (Paragastrioceratidae), plankton-2 (involute forms with wide whorls)—15% (Adrianitidae) (Fig. 4.23). We also refer to this life-form the species of the genus Sakmarites, because they are always represented by small shells. The smallest proportion is rep-


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resented by benthopelagic forms—21% (Metalegoceratidae, Popanoceratidae, Vidrioceratidae). The proportion of nektobenthic and benthopelagic forms is similar to that of the Asselian assemblage. At the same time, despite the Sakmarian regression, the proportion of planktonic forms increased, mainly due to the involute forms. The shallowing of the basin was apparently not sufficient to seriously affect planktonic forms. Ammonoid occurrences are mainly found in thin carbonate interbeds in flyschoid series, carbonate nodules in clayey series and limestones and sandy limestones, frequently in limestone lenses. Ammonoids are rarely found in association with other fossils. For instance, the interbeds of compact micritic limestone with ammonoid shells contain layers and lenses of detrital limestone with fusulinids and bryozoans. The latter possess traces of transportation. In other cases ammonoids are found in black marl nodules in shale with fusulinids. Judging from the facial preferences of ammonoids, at that time they mostly inhabited relatively deep zones with carbonate-clayey substrates at the foot of the underwater fans and on the local uplifts with clayeysandy bottom in the central zone of the strait. One of the richest Sakmarian ammonoid assemblages is described by Ruzhencev (1951b) (the sample contains almost 500 specimens of 12 species) from sandy limestones in the Ultugan-Sai Gully, in the basin of the Aktasty River, Aktyubinsk Region of Kazakhstan. The sample is dominated by specimens of Agathiceras uralicum, Neopronorites tenuis (36 and 26%, in total 62%), i.e., nektobenthic forms prevail in the taphocoenosis. At the same time, planktonic species are represented by evolute paragastrioceratids, constituting only slightly more than 2% of all specimens. When the abundances of life-forms are compared by the number species they include, nektobenthic forms prevail (50% species), whereas the proportion of planktonic forms (35% species) is considerably different from the proportion based on the number of specimens (Fig. 4.24). The number of benthopelagic forms is almost half of that of planktonic, and a third of that of nektobenthic. Apparently, in this case, the conditions in a relatively shallow marine basin with considerable carbonate precipitation and moderate influx of clastics were favorable for a nektobenthic ammonoid assemblage. The sections from the Sakmarian type region—right bank of the Sakmara River near the village of Kondurovka (Mt. Kurmaya). Ammonoids are found in beds of gray micritic, in places cherty limestone in the Upper Sakmarian limestone-clayey series (Ruzhencev, 1951b). The assemblage, composed of 16 species, contains 37% of nektobenthic forms (less that average for the stage, 46%), 19% of benthopelagic forms (close to average), 19% of planktonic subspherical forms, and 25% of planktonic evolute forms (Fig. 4.24). In this assemblage the proportion of all planktonic species PALEONTOLOGICAL JOURNAL

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18% 6 species

21% 7 species

15% 5 species

46% 16 species 1

2

3

4

Fig. 4.23. Proportions of life-forms in the Sakmarian ammonoid assemblage of the Urals. Explanations as in Fig. 4.3.

(44%) is higher than the average for the stage (33%). Apparently, the ammonoid-bearing beds accumulated in the more basinal parts of the basin. The terminal part of the Sakmarian contains numerous ammonoids in the Verkhnii Ozernyi Section 10– 15 km to the south of the previously described locality. In the topmost part of the stage, the series of sandstones interbedding with platy limestones contains bullions near the basal part of the bed, overfilled by ammonoid shells. The assemblage contains eight species (Leven et al., 2002). Apparently, the same bed was the source of the sample described by Ruzhencev (1951b), which supplements this list with three more species names. In the assemblage, the proportion of benthopelagic species (Metalegoceras, Andrianovia) is higher than the average for the stage—27% instead of 21%, and in terms of the number of specimens, benthopelagic forms also constitute about 30%. At the same time, the proportion of involute planktonic forms (plankton-2) was reduced to a total of 9%. The proportion of evolute planktonic forms (Paragastrioceras, Uraloceras) is 18% and the proportion of nektobenthic (46%) corresponds to the average percentage (Fig. 4.24). This assemblage is the closest to the assemblage average for the whole of the Sakmarian. Apparently, the Verkhnii Ozernyi Section can be considered to represent a typical ammonoid habitat for this time. The section consists of a series of thick terrigenous members (sandstones and siltstones—tens of meters) and thin carbonates (rarely more than 0.5 m). It is noteworthy that in the stratotype region, the upper and the lower parts of the Sakmarian are facially different. The Lower Sakmarian is represented by carbonate-clayey beds interlayered by calcareous breccia. A similar type of sedimentation is more likely characteristic of the foot of the western margin. The upper substage is represented by flyschoids dominated by

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fine-grained sandstones and siltstones, which completely correspond to the interpretation of the topography of the eastern margin of the strait. Thus, a change in the facies in the section indicates a change in the sedimentary settings most likely related to the tectonic movements and an eastward shift in the axis of the strait. In the basinal sections, for instance, in the Sim Depression, ammonoid localities are considerably more impoverished. Clayey shales of the right bank of the Sim River contained Neopronorites tenius, Sakmarites postcarbonarius tetragonus, S. postcarbonarius latus, Agathiceras uralicum, Uraloceras simense, Uraloceras sp., Propopanoceras postsimense, and Propopanoceras sp. (Ruzhencev, 1951b). Morphotypes with wide whorls, interpreted as benthopelagic lifeforms are completely absent. From the sections of the reef type containing ammonoids, only the shikhan Shakh-Tau can be named in this context. It contained Sakmarites postcarbonarius tetragonus (3 specimens), Agathiceras uralicum (5 specimens), Somoholites shikhanensis (1 specimen) (Ruzhencev, 1951b). The assemblage is too impoverished to reach any conclusions about its ecological structure. Similar to most previously discussed assemblages, the Sakmarian ammonoid assemblage is dominated by nektobenthic forms, whereas other life-forms are represented by reasonable numbers of taxa. It is difficult to say what proportion of life-forms should be considered optimal. However, it is possible to suggest that the greater and more even the diversity, and the closer the community is to filling all available niches, the closer it is to the phase of maturity and balance. Apparently in

the Sakmarian, the topography of the strait, its zones, and hydrology were favorable for ammonoid communities. Artinskian The Artinskian is the final stage of the existence of the South Urals basin. By the end of the Artinskian, the southern margin of the Uralian basin was almost completely closed. The Artinskian flyschoids in this region are everywhere overlain by gypsum-bearing gray Kungurian sandstones. The short-term invasions of the sea at the very beginning of the Kungurian reached relatively far to the south, approximately to the latitude of the city of Aqtöbe. However, the Kungurian Stage is considered to be the beginning of another phase of the geological history of the South Urals. As the sea became shallower, the hydrology became more settled, while the influx of clastics decreased. Artinskian flyschoids contain sandy material from shallower fractions, while arenaceous beds in cyclites are less abundant. Carbonate and clayey-silty beds are dominant. The top layers of the stage often contain lenses of dolomitized algal limestone, which apparently represent the relics of small biostromes that formed in small hypersaline bays. On the other hand, the topmost beds of the stage often contain trails of medium-grained conglomerates and gravelites that suggest a series of drying events at the end of the epoch. The Artinskian beds are extremely facially variable, on the same bedding plane and across the region of distribution. The underwater fans formed in the previous epochs were transformed into large accumulative structures extending deep into the sea and cutting the eastern mar-


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Fig. 4.25. Proportions of life-forms in the Artinskian (Aktastynian) ammonoid assemblages: (a) of the South Urals; (b) Aktasty section. Explanations as in Fig. 4.3.

gin into a series of bays. These bays were places of deposition of carbonate mud with ammonoid shells. Limestones overfilled with ammonoid shells form frequent narrow lenses in sandy-clayey series. It may be suggested that semi-liquid mud containing ammonoid shells was sliding down the underwater slopes and filled in the depressions on the sea floor next to the feet of the fans. The absence of the remains of benthic organisms in these ammonoid limestones suggests that initially ammonoids inhabited the bays with their soft mud grounds and avoided unstable coastal environments with high turbulence of water and sediment. In some cases, another sedimentary model explaining the accumulation of ammonoid shellstones may be proposed, which interprets it as concentration of shells in the tidal waters of semi-isolated lagoons. In contrast to the beginning of the Permian, Artinskian ammonoids are known almost throughout the Urals, from the Mugodzhary Hills in the south to PaiKhoy and Vaigach Island in the north (Bogoslovskaya, 1997). The further north, the more impoverished are the assemblages. While on the extreme south of the region, the communities are extremely rich, richer than all other synchronous associations in the world, in central Bashkortostan the assemblages are noticeably more impoverished, not more than two to four genera. In the South Urals, the Artinskian Stage has a twofold division (Ruzhencev, 1956b). The existing model of subdivision of the Artinskian Stage into four horizons (Rauzer-Chenousova, 1949) cannot be used in the extreme south of the regions, from where the main ammonoid occurrences are recorded. Attempts to use a scheme based mainly on the sections of the Ufa Plateau to stratigraphic subdivision of the southern section have not been successful. In addition, in recent decades, this scheme is updated (the upper horizon is assigned to the Kungurian). The controversy of this approach has been PALEONTOLOGICAL JOURNAL

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repeatedly discussed (see Chuvashov et al., 2002). In our analysis we accept the twofold subdivision of the stage. The numerous and diverse Artinskian ammonoids of the Urals were studied by Karpinsky (1890), Krotov (1885), Tchernov (1907), Maksimova (1935, 1945), Voinova (1934), Ruzhencev (1936, 1956b), and Bogoslovskaya (1962). Material on Artinskian ammonoids includes over 20 thousand shells collected in the sections all over the western Fore-Urals. Of these, the collections made by Ruzhencev, coll. PIN, no. 317, represent three-quarters. The Early Artinskian Aktasty assemblage includes 28 species of 17 genera of 12 families, i.e., somewhat less diverse than that of Sakmarian. The total morphospace did not change considerably D = 0.1–0.53; W = 1.3–2.8; S = 0.35–3.25. The bimodal distribution remained, while the peaks are separated by an interval with D between 0.2 and 0.25. The morphospace with the lower values of D contains narrowly umbilicate morphotypes of the nektobenthic and benthopelagic types. The group of the higher values of D includes representatives of all three life-forms. The nektobenthic life-form (Pronoritidae, Medlicottiidae, Daraelitidae, Thalassoceratidae, Agathiceratidae, Popanoceratidae) constitutes 57%. The proportion of the types of planktonic life-forms is as follows: plankton-1, six species (21%) (Paragastrioceratidae, Eothinitidae) and plankton-2, three species (11%) (Adrianitidae, Marathonitidae). Benthopelagic forms (Metalegoceratidae, Somoholitidae) constitute 11% (Fig. 4.25a). The proportion of the benthopelagic species noticeably decreased due to an increase in the proportion of the nektobenthic life-form. The proportion of planktonic life-forms remained virtually the same as that in the Sakmarian, but the distribution of its subtypes was shifted towards a decrease in the proportion of the involute species.

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13% 7 species

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21% 11 species

19% 9 species

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Fig. 4.26. Proportions of life-forms in the Artinskian (Baigendzhinian Substage) ammonoid assemblages: (a) of the South Urals; (b) from the Zhil-Tau section. Explanations as in Fig. 4.3.

In the Aktasty time, the South Uralian basin became shallower (Khvorova, 1961). The role of shallowing in the reduction of the diversity of benthopelagic ammonoids is relatively easy to explain. Throughout their history the ammonoid communities in the basin show an increase in the proportion of the nektobenthos as the basin became shallower. Among the sections of the Aktasty Substage, the section Aktasty in the Aktyubinsk Region of Kazakhstan is one of the most important. The dolomitized limestones of the upper part of the substage contain an assemblage of 26 species. In a sample of this assemblage over half the specimens are Kargalites typicus, whereas Agathiceras uralicum, which normally dominates other samples, is here represented by only a few specimens. The nektobenthic life-form constitutes 52%. The proportion of the subtype of the planktonic life-forms is as follows: 6 species (24%) evolute and 3 species (12%) involute (Fig. 4.25b). Benthopelagic forms constitute 12%. This is different from the total assemblage in the absence of three nektobenthic species, whereas all other species are present. This is reasonable because almost the entire Aktasty assemblage comes from a locality on the Aktasty River. In addition, there are a few more localities, with much poorer content. The Late Artinskian (Baigendzhinian) assemblage shows an increased taxonomic diversity, which was the highest since the Late Carboniferous (Fig. 4.3). The assemblage includes 52 species of 19 genera of 13 families. The total morphospace is delineated in the following way: D = 0.02–0.6; W = 1.4–2.8; S = 0.3–3.4. Nektobenthic species constitute 39% of the total diversity. Plankton-1 constitutes 21%, and plankton-2, 27%. Benthopelagic species constitute 13% (Fig. 4.26a). In the Baigendzhinian, the community experienced explosive speciation, primarily among representatives of the superfamilies Neoicocerataceae (Paragastrioceras, Uraloceras, Metalegoceras, and Eothinites).

Almost all new species of this group are, according to our interpretation, planktonic or benthopelagic. They are responsible for the very rapid growth of the planktonic portion of the assemblages. Within the planktonic group, the number of species with a subspherical shell increased, increasing the proportion of the involute portion of this community. Thus, the total proportion of plankton reached 48%. In this case, the reduction in the proportion of nektobenthic species is not related to a decrease in their absolute numbers (they became 5 species more) but to a sharp growth of species, which belonged to other life-forms. The section of Mt. Zhil-Tau in the Aktyubinsk Region contained a uniquely rich ammonoid assemblage of Baigendzhinian age. In the lower part of the substage the basal layers of the member of dolomitized limestone contain a lensshaped bed of cephalopod shellstones. Various workers collected more than 2000 shells of 41 species from the lenses (Ruzhencev, 1956b). Nektobenthic species constitute 40% of the total diversity. Plankton constitutes 41%, of which 14% are involute. Benthopelagic species constitute 19% (Fig. 4.26b). Although the total number of specimens of various species may fluctuate from several specimens to three thousand specimens, while the distribution of the shells across the life-forms was approximately the same. In addition, no single species can be identified as dominant in this sample, as the proportion of the most numerous one does not exceed 12%. The composition of this assemblage differs from that of the assemblages of the entire stage (substage) in the reduced proportion of involute planktonic forms. Compared to the above-mentioned sections, this assemblage is most similar in the proportion of the life-forms to that of the Sakmarian assemblage of Mt. Kurmaya. Apparently the rootstocks of this similarity are quite different in different sections. Ammonoids from the section of Mt. Kurmaya come from several levels, and


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hence reflect a sequence of communities. All ammonoids from Zhil-Tau were collected from the same thin layer. It seems that this was a result of accumulation of floating shells from a large territory, which explains average and similar proportions of various life-forms. It is supported by the virtually complete absence of benthos. Thus, the Baigendzhinian at the same time shows the closure of the Uralian Strait and a peak of diversity of the ammonoid community. This suggests that this basin ceased to exist very quickly when the decline began. There are not any features of degradation of the ammonoid assemblage, whereas the richest collections come from the topmost beds of the stage. In addition, Kungurian ammonoids, which are known from the sections of the Middle Urals are taxonomically impoverished, i.e., the continuity of the assemblages is very small. Hence, the drying event in the south part of the basin had a catastrophic effect on ammonoid communities, which could not adapt to new conditions. Changes in the ecological structure of Paleozoic ammonoids communities of the Uralian Paleobasin are to a large extent connected with general changes in the abiotic environment, including the depth of the basin, sedimentary settings, and spatial distribution of adaptive zones. Several stages during which the ecological structure remained relatively stable can be recognized. (1) Early Devonian (Emsian)—ammonoids are represented by exclusively planktonic forms, evolute or loosely coiled whorls, which indicate the period of early evolution of ammonoids, whereas their occurrences in the open shelf deposits suggest that this was the adaptive zone of their origin. The species diversity is low. (2) Eifelian–Frasnian. The ammonoid community became considerably more diverse ecologically, although the species diversity remains the same. Nektobenthic involute forms dominate (over 80%), plankton is represented by the evolute morphotype; the benthopelagic life-form is represented by only one species. They are found in various types of deposits, which suggests that ammonoids inhabited various marine environments. (3) Famennian. In the Famennian, the ammonoid communities changed fundamentally: the taxonomic and morphological diversity sharply increased, the proportion of planktonic form increased (the second type of planktonic morphotype represented by involute shells with slowly expanding whorls appeared) and the proportion of benthopelagic forms became larger. The maximum diversity of ammonoids is recorded from the most basinal sections with condensed sedimentation. The catastrophic reduction of the biological diversity at the end of the Famennian primarily affected benthopelagic and evolute planktonic forms, which can be connected with shallowing and reduction in the area of the outer (deep) shelf. Only involute nektobenthic and planktonic forms continued into the Carboniferous. (4) Early Carboniferous. In the Early Carboniferous, the Uralian basin can be interpreted as a large epiPALEONTOLOGICAL JOURNAL

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continental marine basin, which is reflected in the ecological structure of ammonoid communities. In the Tournaisian and Early Visean–beginning of the Late Visean the assemblages are dominated by nektobenthic life-forms and contained benthopelagic life-forms, while the proportion of planktonic life-forms was minimal. At the end of the Visean and in the Serpukhovian, a specific ammonoid community was formed, the main feature of which was the prevalence of planktonic and benthopelagic forms, which reflected the extension of the adaptive zone of the relatively deep outer shelf. (5) Early Bashkirian. New ammonoid communities in the Early Bashkirian after a large-scale regression at the end of the Serpukhovian in its structure is similar to the Serpukhovian, but at the end of the Early Bashkirian the diversity decreased as the number of planktonic and benthopelagic life-forms became lower. Very few or no Late Bashkirian and Moscovian ammonoids are found in the Urals. (6) Kasimovian–Baigendzhinian (Permian). The structure of the communities in this long period was a subject of fluctuations, but in general it differed considerably from the Early Carboniferous and Early Bashkirian structures. At this stage the assemblages were dominated by nektobenthic forms with gradual increase in the proportion of planktonic forms and decrease in the proportion of benthopelagic forms. Interestingly, at the end of this stage, in the Baigendzhinian, the ecological structure of ammonoids does not show traces of sharp changes indicating its degradation and shows features of a mature community with proportional quantities of lifeforms. This apparently suggests that the Uralian Basin closed very quickly, and fast change in the abiotic conditions did not lead to changes in the community. CONCLUSIONS The main feature of the cephalopod archetype is the development of the phragmocone, a new functional buoyancy device, allowing cephalopods to colonize a new adaptive zone (pelagic zone), which was at that time outside the reach of other mollusks. The development of the main body plans within the archetype, which correspond to the higher taxonomic divisions, can functionally be explained by different approaches to buoyancy and orientation control and facilitation of active swimming. In the course of discussion of fundamentally different approaches to these problems, 23 orders of Paleozoic cephalopods are considered and substantiated. Seven ammonoid orders represented a functionally and phylogenetically connected group, which can be considered as a single taxon of a higher rank. The combination of other cephalopod orders, most of which did not survive beyond the Paleozoic in supraordinal taxa, cannot be justified from the functional point of view. They are considered as a group of nonammonoid cephalopods with uncertain taxonomic status.

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A discussion of differences in the hydrostatic and hydrodynamic features of various shell morphotypes allows the evaluation of their potential colonization of various adaptive zones in the sea. Based on this, in each of 23 genera, the following life-forms are recognized: benthic, benthopelagic, nektobenthic, and planktonic, and changes in the ecological structure of each of the cephalopod orders are tracked throughout their evolutionary history. The predominant ecological specialization of cephalopod orders is defined by the shell shape initial for the body plan of each order: cyrtoconic, orthoconic, or spirally coiled. Ellesmerocerids, discosorids, and oncocerids initially with a cyrtoconic shell were the most ecologically flexible and capable of living in all adaptive zones. The orders with an initially orthoconic shell and well-developed mechanisms of orientation and stability control in the form of cameral and/or endosiphuncular deposits (endocerids, actinocerids, orthocerids, and pseudorthocerids) are mainly represented by the nektobenthic life-form only. Bactritids, which did not have specific mechanisms of orientation control are considered as planktonic organisms. Orders with an initially planispiral shell (nautilids, ammonoids) were the most adapted to life in the pelagic zone and gave a broad spectrum of planktonic forms. The recognition of the life-forms in cephalopod orders and tracking their ecogenesis created a basis for consideration of the ecological evolution of the entire taxocoenosis of cephalopods throughout the Paleozoic. The benthopelagic life-form was initial in the evolution of cephalopods, and it was the life-form of the earliest Late Cambrian genus Plectronoceras. From the beginning of the Early Ordovician, the ecological structure of the cephalopod community included all known lifeforms. A relatively constant proportion of the life-forms remained throughout the entire Ordovician, despite the changes in the taxonomic composition of the community in the mid-Ordovician. This could be because the newly appearing orders relatively evenly filled adaptive zones and niches, and due to a relatively stable structure of the adaptive sea zone throughout the Ordovician. The first significant change in the ecological structure of the cephalopod community occurred in the Silurian, when the number of bottom-dwelling (benthic and benthopelagic) life-forms increased, while the number of planktonic life-forms decreased. In the Devonian, the ecological structure becomes more balanced, although the relative number of pelagic forms is higher than in the Ordovician. Similar to the Ordovician, the appearance of new groups (nautilids and ammonoids) in the Devonian did not change the proportions of life-forms in the community. This change, the largest in the Paleozoic history of cephalopods, was related to the global events at the end of the Frasnian. The number of benthic and benthopelagic forms halved, but the number of planktonic forms increased three times (mainly because of the appearance of clymeniids). The reduction in the number of benthic forms quite logically follows the dis-

tribution of anoxic conditions, which led to the reduction of bottom habitats. A catastrophic decrease in the taxonomic diversity of cephalopods at the Devonian–Carboniferous boundary was restored relatively quickly due to the radiation of a few nektobenthic ammonoids that survived the catastrophic event. From the Carboniferous onward, no benthic forms appeared in the cephalopod community. The Carboniferous–Middle Permian ammonoid community is characterized by a relatively stable ecological structure, taxonomically represented by the orders of ammonoids and nautilids with a planispiral shell. The taxonomic structure changed at the end of the Middle Permian, and like in the previous periods of instability, was related to an increase in the number of pelagic taxa. While the changes in the ecological structure in the Silurian and at the end of the Frasnian were related to the global extinction events, in this case, changes in the ecological structure actually preceded the Permian–Triassic extinction event. Apparently, the ecological change in the cephalopod community resulted from the appearance in the mid-Permian of the order Ceratitida, an initially pelagic group, which survived the crisis and became the rootstock of the Mesozoic radiation of ammonoids. Thus, the analysis of the changes in the ecological structure of the cephalopod community and changes in the taxonomic composition of life-forms in the Paleozoic epochs shows that the appearance of new higher taxa in the cephalopod community has no certain correlation with abiotic events and is not caused by these events. The appearance of new groups usually did not change the ecological structure of the cephalopod community. The major trend in the evolution of the ecological structure of cephalopod community in the Paleozoic was the reduction in the benthic life-forms and increase in abundance and morphological diversity of pelagic life-forms, while the number of higher taxa decreased. Periods of “anomalous” ecological structure of the community always resulted from changes in the environment. This is observed not only when the entire cephalopod community is analyzed from stage to stage, but also in the analysis of changes in ecological structure in ammonoid communities based on the example of the Uralian Paleobasin. The ecological structure of the ammonoid community in this paleobasin is analyzed for stages and substages from the Early Devonian to Early Permian. Several periods are recognized when the structure of the communities was relatively stable: Emsian, Eifelian–Frasnian, Famennian, Early Carboniferous (Tournaisian–Visean), Early Bashkirian, and Kasimovian–Early Permian. The disruptions in the ecological structure partly reflect changes in the global cephalopod taxocoenosis and partly result from the changes in the geology and oceanography in the Uralian Paleobasin.


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