Gastropods

378 FOSSIL INVERTEBRATES/Gastropods

exploiting the more specialist boring and cementing habit has continued to grow and there has been a further taxonomic explosion of the shallow burrowers at lower taxonomic levels.

See Also Biological Radiations and Speciation. Fossil Invertebrates: Molluscs Overview; Gastropods; Cephalopods (Other Than Ammonites); Ammonites. Mesozoic: End Cretaceous Extinctions. Palaeoecology.

Further Reading Beesley PL, Ross GJB, and Wells A (1998) Mollusca: the Southern Synthesis, vol. A. Collingwood, Australia: CSIRO Publishing. Harper EM (1998) The fossil record of bivalve molluscs. In: Donovan SK and Paul CRC (eds.) The Adequacy of the Fossil Record, pp. 243–267. Chichester: John Wiley and Sons. Harper EM and Skelton PW (1993) The Mesozoic Marine Revolution and epifaunal bivalves. Scripta Geologica, Special Issue 2: 127–153.

Harper EM, Taylor JD, and Crame JA (2000) The Evolutionary Biology of the Bivalvia. Geological Society of London Special Publication 177. London: Geological Society of London. Johnston PA and Haggart JW (1998) Bivalves: an Eon of Evolution. Paleobiological Studies Honoring Norman D. Newell. Calgary: Calgary University Press. Skelton PW, Crame JA, Morris NJ, and Harper EM (1990) Adaptive divergence and taxonomic radiation in postPalaeozoic bivalves. In: Taylor PD and Larwood GP (eds.) Major Evolutionary Radiations. The Systematics Association Special Volume 42, pp. 91–117. Oxford: Clarendon Press. Stanley SM (1970) Relation of shell form to life habits of the Bivalvia. Geological Society of America Memoir 125: 1–296. Taylor JD (1996) Origin and Evolutionary Radiation of the Mollusca. Oxford: Oxford University Press. Vermeij GJ (1987) Evolution and Escalation. An Ecological History of Life. Princeton, NJ: Princeton University Press. Vermeij GJ (1993) A Natural History of Shells. Princeton, NJ: Princeton University Press.

Gastropods J Fry´da, Czech Geological Survey, Prague, Czech Republic ß 2005, Elsevier Ltd. All Rights Reserved.

Introduction Gastropods are well-known animals which have been associated with humans since the dawn of civilization. Their bodies were gathered for food and their shells were used as tools, ornaments, and later as money. Their widespread occurrence is clear evidence of their successful adaptation to different environments. During a long evolution, they are the only molluscan class to have colonized the majority of marine, freshwater, and terrestrial environments. Marine gastropods occur mostly in shallow-water benthic communities; however, some gastropod species have also lived in the deep sea (e.g., faunas associated with hydrothermal vents), and others, such as holoplanktic animals, have spent their whole lives as freeswimming gastropods. The terrestrial gastropods colonized most land environments, ranging from lowlands to high mountains, and including humid to arid biotopes of tropical to subarctic areas. Such adaptive radiation is quite exceptional amongst all animal phyla and is linked to the extraordinary morphological and functional diversity of their bodies and

shells. The gastropods comprise one of the most diverse groups of living animals (the second after Insecta). All these facts, together with their long and rich fossil record, make gastropods a unique animal group for evolutionary, ecological, and biogeographical investigations. There follows a brief review of gastropod anatomy, shell morphology, classification, and more than 500 million years of evolution.

Definition and General Description The Gastropoda forms one of eight molluscan classes, and is defined by several unique anatomical features which support its interpretation as a molluscan group derived from the same ancestor (i.e., monophyly). The most characteristic feature of gastropods is torsion of their soft bodies during early larval stages, producing a crossing of their nerve connectives, bending of the intestine, and twisting of the mantle cavity (together with associated structures, including the ctenidia, anus, kidney openings, etc.) anteriorly over the gastropod head. Anatomical Features

The exceptional morphological and functional diversity of gastropod bodies is also reflected in their anatomy. Generally, the body consists of a large foot, a

FOSSIL INVERTEBRATES/Gastropods 379

visceral mass, and a head with a mouth, tentacles, and eyes. The visceral mass is mostly enclosed, together with the mantle cavity, in a calcareous shell (Figure 1). Gastropods as soft-bodied animals use the pressure of their blood and muscles for movements of different organs. The circulatory system comprises a contractile heart with one (Caenogastropoda and Heterobranchia) or two (Patellogastropoda, Archaeogastropoda, and Neritimorpha) auricles, and a ventricle, as well as a system of arteries and veins. Gastropod blood transports oxygen using the copperbearing pigment haemocyanin. In most marine gastropods, one or two gills (ctenidia) situated in the mantle cavity are used for respiration. Some marine and freshwater gastropods developed secondary gills after the loss of their ctenidia in previous evolution. In the terrestrial gastropods (e.g., Pulmonata), a highly vascularized internal wall of the mantle cavity (lung) is used for respiration. One or, rarely, two (Patellogastropoda and Vetigastropoda) kidneys serve for excretion through the mantle cavity. The digestive system starts with a mouth containing a tooth-bearing ribbon (radula). The organization of gastropod radulae and stomach, as well as additional parts of the digestive system, reflects their different

feeding habits (herbivory, detritivory, carnivory, or parasitism). The anus opens into the mantle cavity. The gastropod nerve system includes paired ganglia which are linked with different sensory receptors by connectives and commissures. The morphology of the reproductive organs and the reproductive strategies are highly diverse. Generally, more ancient gastropod groups are gonochoristic with a simple reproductive system and external (Patellogastropoda and Archaeogastropoda) or internal (Neritimorpha) fertilization. Caenogastropoda also use internal fertilization with complex reproductive morphology, and some may be simultaneous hermaphrodites. The Heterobranchia have the most complex and variable reproductive system and are hermaphroditic. Ontogeny

Gastropods, like all Mollusca, have a biphasic life cycle (i.e., larval and post-metamorphosis stages), and this feature is shared with closely related animal phyla (Kamptozoa, Sipunculida, Polychaeta, etc.). Like other molluscan groups, the embryonic development is characterized by a spiral cleavage, which differs slightly in the main gastropod groups. The subsequent larval stage is called the trochophore

Figure 1 Some variations in shell form of living gastropods. (A) High-spired: Mitra mitra (Muricoidea). (B) Strombiform: Lambis chiragra (Stromboidea). (C) Turbiniform: Liguus vittatus (Orthalicoidea). (D) Convolute: Cyprea tigris (Cypraeoidea). (E) Spinose fusiform: Chicoreus ramosus (Muricoidea). (F) Fusiform: Pleuroploca trapezium (Muricoidea). (G) Conoidal: Conus litteratus (Conoidea). (H) Discoidal: Architectonica perspectiva (Architectonicoidea). (I) Turriculate: Terebra sp. (Conoidea). (J) Ovoid: Olivancillaria gibbosa (Olivoidea). (K) Involute: Cypraecassis rufa (Tonnoidea). (L) Irregularly coiled: Siliquaria ponderosa (Cerithioidea). (M) limpet: Megathura crenulata (Fissurelloidea). (A, E–G, I, J) Neogastropoda (Caenogastropoda); (B, D, K) Littorinimorpha (Caenogastropoda); (C, H) Heterobranchia; (l) Sorbeoconcha (Caenogastropoda); (M) Vetigastropoda (Archaeogastropoda).


larva, and a similar larval type is developed in all molluscan groups. The trochophore larvae may be free swimming, as in the ancient gastropod groups (Patellogastropoda and Archaeogastropoda), or may occur in egg capsules, as in more advanced gastropods. The last larval stage is termed veliger, which typically bears two ciliate paddles (velum), sometimes subdivided into several lobes. If free-swimming gastropod larvae use planktic organisms for their nutrition, their development is termed planktotrophic. Marine gastropods with such development have small eggs, but numbering over half a million. Planktotrophic larvae may stay planktic for several months and thus can be carried for long distances by oceanic currents. The gastropods, however, developed another ontogenetic strategy in which their larvae were not dependent on an external food source, but on the yolk of their eggs. Gastropods with such a nonplanktotrophic development (lecithotrophic) typically produce fewer eggs, which are relatively large. The larval stages end with a metamorphosis that involves anatomical and physiological reorganization of the larval body into the juvenile, post-larval body. Terrestrial and freshwater gastropods have simplified their development, and their embryonic and larval stages are fixed on egg capsules or the female body (direct development). Such ontogenetic changes considerably decreased their dispersal potential.

The Gastropod Shell Gastropods are not only one of the most diverse animal groups, but the morphology of their shells is extremely varied (Figures 1 and 2). During more than 500 million years of evolution, they developed shells with various shapes and ornament, ranging in size from about 1 mm up to more than 1 m (Eocene Campaniloidea, Caenogastropoda). The shell and its ornament may be broadly linked to the mode of gastropod life (e.g., origin of limpet-shaped shells in unrelated gastropod groups). Generally, the most ornate shells occur in tropical marine environments, but freshwater and terrestrial gastropods are often less ornate. Protoconch and Teleoconch

In shell-bearing gastropods, the shell grows during almost the whole of their ontogeny. The part of the shell formed during the embryonic and larval stages is called a protoconch (Figure 3), and that growing after metamorphosis is termed a teleoconch. The main gastropod groups differ in their early development, which is reflected in their protoconch morphology. The more ancient gastropod groups (Patellogastropoda

and Archaeogastropoda) have the simplest shell ontogeny and their protoconchs have only an embryonic shell (protoconch I), which is followed by a teleoconch (Figures 3B and 3F–3H). On the other hand, the protoconchs of more advanced gastropods (Neritimorpha, Caenogastropoda, and Heterobranchia) consist of an embryonic shell (protoconch I) and a subsequent larval shell (protoconch II). In most caenogastropods, the larval shells have different ornament from the teleoconchs (Figures 3K and 3L), and both shells are coiled in the same direction (such a condition is termed homeostrophic; Figure 4). In contrast, in the Heterobranchia with planktotrophic development, the protoconchs are coiled in the opposite direction to the teleoconchs (Figures 3J and 4). Such shells are termed heterostrophic. The Neritimorpha form typical, strongly convolute protoconchs during planktotrophic development, which are homeostrophic (Figures 3A, 3N, and 4). Higher gastropods with non-planktotrophic development (some marine, freshwater, and terrestrial gastropods) have simplified their early ontogeny and thus also the morphology of their protoconchs. The latter strategy is documented from the Devonian (400 Ma). Operculum

The majority of gastropods have a lid-like structure (operculum) to close their aperture. This operculum is present in all living gastropods during their larval stages, but is lost in some adults (e.g., limpets and the majority of terrestrial gastropods). The operculum is mostly horny (corneous) and may be tightly (multispiral) or loosely (paucispiral) coiled or concentric. Some gastropod groups have calcareous opercula, and the oldest operculum known is from the Ordovician (Macluritoidea). Shell Structure

Most gastropod shells are composed of an outer organic layer (periostracum) and an inner, mostly much thicker, calcified layer. The colour pattern typical of many gastropod shells (Figure 1) is formed by different organic pigments which are limited to the periostracum and the uppermost calcified layer. This shell feature, sometimes reflecting the mode of life, has been known since the Palaeozoic (Figure 5). The inner layers of gastropod shells consist of minute calcium carbonate crystals (aragonite or calcite) in an organic matrix. There are over 20 structural types of gastropod shell and, in general, more ancient groups exhibit more diverse shell structures. The Patellogastropoda (Eogastropoda) had the most complex shell structure. On the other hand, the majority of the higher gastropods have developed simple


Figure 2 Some variations in shell form in the main groups of middle Palaeozoic gastropods. (A) High-spired shell: Murchisonia coronata (Murchisonioidea). (B) Trochiform, slit-bearing shell: Devonorhineoderma orbignyana (Eotomarioidea). (C) Bilaterally symmetrical shell with a prominent selenizone: Kolihadiscus tureki (Cyrtolitoidea). (D) Turbiniform shell: Gyronema armata (Gyronematidae). (E) Openly coiled shell: Pragoserpulina tomasi (Pragoserpulinidae). (F) Discoidal shell: Stusakia pulchra. (G) Sinistrally coiled shell: Voskopiella barborae (Onychochilidae). (H) Naticiform shell: Eifelcyrtus blodgetti (Vltavielidae). (I) Fusiform shell: Havlicekiela parvula (Peruneloidea). High-spired shells: (J) Pragozyga costata; (K) Palaeozyga bohemica (Loxonematoidea). (L, M) Bilaterally symmetrical shell: Bellerophon vasulites (Bellerophontoidea). (N) Limpet: Pragoscutula wareni (Pragoscutulidae). (P) Sinistrally coiled shell: Alaskiella medfraensis (Porcellioidea). (P) Discoidal shell: Nodeuomphalus labadyei (Euomphaloidea); (Q) Bilaterally symmetrical shell covered by secondary shell deposits: Branzovodiscus bajae (Bellerophontoidea). (A, B, D, F, O) Archaeogastropoda; (C) Cyrtonellida; (G) Mimospirina; (H) Cyrtoneritimorpha; (L, M, Q) Bellerophontida; (I) Perunelomorpha; (E, J, K, N) Order uncertain; (P) Euomphalomorpha.

aragonitic shells with a crossed lamellar structure (Figures 6A and 6B). Some structural types are restricted to certain groups (e.g., nacre) and this may be used for their identification in fossils (Figure 6C). Nacreous and crossed lamellar structures have been known since the Palaeozoic. Shell Coiling

The majority of the shell-bearing gastropods have right-handed (dextral) shells, but some have

left-handed (sinistral) shells (Figures 1 and 2). Only a few gastropods have bilaterally symmetrical shells which may be uncoiled (limpets) or planispirally coiled (Figures 1M, 2C, 2L, and 2M). The limpetshaped shells were independently developed within all main gastropod groups from the asymmetrically coiled shells of their ancestors. In contrast, planispirally coiled shells are known only in several groups, such as the Palaeozoic Porcellioidea and Bellerophontoidea (Figures 2L and 2M) or the Holocene Planorbioidea. Some gastropods may change the coiling


Figure 3 Variety of protoconch shape. Strongly convolute larval shell (protoconch II), Neritimorpha: (A) Holocene Smaragdia sp. (Neritoidea); (N) Triassic Pseudorthonychia alata (Pseudorthonychiidae). Embryonic shell (protoconch I) followed by teleoconch, Archaeogastropoda: (B) Triassic Wortheniella coralliophila (Vetigastropoda); (C) Holocene Anatoma proxima (Vetigastropoda); (F) Devonian Zlichomphalina sp. (Eotomarioidea); (G, H) Devonian Diplozone innocens (Murchisonioidea). (D) Openly coiled larval shell (protoconch II), Cyrtoneritimorpha, Carboniferous Orthonychia parva (Orthonychiidae). (E) Openly coiled early shell, Permian Euomphalus sp. (Euomphaloidea). (I) Openly coiled larval shell (protoconch II) of the Silurian Peruneloidea. (J) Heterostrophic larval shell (protoconch II), Jurassic Mathilda sp. (Architectonicoidea, Heterobranchia). Larval shell (protoconch II), Caenogastropoda: (K) Holocene Hipponix sp. (Vanikoroidea); (L) Devonian Balbiniconcha cerinka (Subulitoidea). (M) Heterostrophic early shell, Devonian Alaskiella medfraensis (Porcellioidea, Archaeogastropoda).

Figure 4 Schematic diagram showing the relationship between the coiling of larval (protoconch II) and post-larval (teleoconch) shells in planktotrophic Caenogastropoda, Neritimorpha, and Heterobranchia. Coiling of both shells in the same direction (Caenogastropoda and Neritimorpha) is termed homeostrophic. If the handedness of the shells is opposite (Heterobranchia), the coiling is termed heterostrophic.

of their shells (Figure 4) from sinistral to dextral (dextral heterostrophy), or vice versa (sinistral heterostrophy), during ontogeny. Such a change may occur at a developmental stage, when gastropods undergo a metamorphosis from larval to post-larval stages (e.g., Heterobranchia; Figure 3J), or later (e.g., Porcellioidea; Figure 3M). Dextrality or sinistrality of the shell is independent of the coiling of the soft body, and the asymmetrical soft body of gastropods may be dextral or sinistral.

Anatomical dextrality or sinistrality may be easily recognized, even in fossil gastropods, if they developed a spiral operculum. The spiral operculum of anatomically dextral gastropods is coiled counterclockwise (viewed externally), and vice versa in sinistral gastropods. Thus, there are four possible relationships between shell coiling and body asymmetry in the shell-bearing gastropods (Figure 7). If anatomically dextral (or sinistral) animals occupy dextrally (or sinistrally) coiled shells, such a condition


is called dextral (or sinistral) orthostrophy. If the handedness of the shell and soft body is different, the term hyperstrophy is used. All four kinds of coiling (Figure 7) have occurred in gastropods, but their frequencies are very different. The great majority of living gastropods are dextrally orthostrophic, and sinistral orthostrophy is uncommon. Dextral or sinistral hyperstrophy is very rare (e.g., Ordovician Macluritoidea or some Holocene Planorbioidea).

Muscle Scars

Gastropod shells are attached to the soft body by muscles, which may leave distinct scars on the inner shell surface. The geometry of muscle scars has frequently been used as a diagnostic feature for distinction between torted (i.e., gastropods) and untorted states in the Palaeozoic molluscs (Monoplacophora, Helcionelloida, Cyrtonellida, etc.). However, new anatomical studies of living gastropods have shown that the larval muscles taking part in torsion and the post-larval muscles are developed quite independently. Thus, the muscle scar pattern sometimes observable in the fossil molluscan shells may be a good ecological indicator, but has no systematic significance.

Classification of the Gastropoda

Figure 5 Apical (A) and lateral (B) views of the Middle Devonian (about 400 Ma) neritimorph, Paffrathopsis subcostata, showing the colour pattern.

Gastropods as an independent group of molluscs were recognized and named by the French naturalist, Georges Cuvier, more than 200 years ago (see Famous Geologists: Cuvier). Since then, scientists have tried to classify them by using different features of their bodies. However, the classification of such a numerous group with extraordinary morphological and anatomical variability of their bodies and shells has encountered many problems. During the nineteenth century, several different classifications of the Gastropoda were published, based on the shape of the shells, position of the mantle cavity, or on the arrangement of various organs (e.g., gills or head). Generally, these classification schemes used only a limited number of distinguishing characters. At the beginning of the twentieth century, the German zoologist, Johannes

Figure 6 Examples of shell structure in fossil gastropods. Aragonitic crossed lamellar structure in the Carboniferous (about 300 Ma) Amphiscapha catilloides (Euomphaloidea): views perpendicular to (A) and parallel to (B) the shell surface. (C) Nacreous structure (columnar nacre) in the Late Cretaceous (about 80 Ma) Sensuitrochus ferreri (Porcellioidea, Archaeogastropoda).


Figure 7 Schematic diagram showing the four possible relationships between shell coiling and body asymmetry. Orthostrophy means that anatomically dextral (or sinistral) animals occupy dextrally (or sinistrally) coiled shells. The term hyperstrophy is used when the handedness of the shell and soft body is different (see text for explanation).

Thiele, integrated earlier classifications and divided the gastropods into three subclasses: Prosobranchia, Opisthobranchia, and Pulmonata. In addition, the Prosobranchia were divided into three orders: Archaeogastropoda, Mesogastropoda, and Neogastropoda. Thiele’s system was used by zoologists and palaeontologists for most of the twentieth century. However, during recent decades, numerous new data on the anatomy of various gastropod groups have been accumulated, mainly by the application of new methods (e.g., transmission electron microscopy). At the same time, studies of the deep-sea faunas associated with hydrothermal vents have brought the discovery of new gastropod groups with unusual anatomical features. The evaluation of this newly gathered data in the light of the existing classification revealed a need for its revision. Recent analyses of numerous morphological and developmental characters of living gastropods have resulted in a new classification scheme (Figure 8), which has been independently supported by results from molecular studies. The placement of fossil gastropods into this classification of living gastropods has been difficult because of the lack of necessary anatomical characters. Recent studies have revealed that Patellogastropoda (¼ Docoglossa, Cyclobranchia) represents the sister group to all other living gastropods. Living patellogastropods with limpet-shaped shells are exclusively marine and occur mostly on rocky shores in all continents. The Patellogastropoda and their coiled ancestors have been united into the subclass Eogastropoda. All other living gastropods and their ancestors have been placed in the subclass Orthogastropoda, comprising four main groups of living gastropods:

Neritimorpha, Archaeogastropoda, Caenogastropoda, and Heterobranchia (Figure 8). The Neritimorpha (¼ Neritopsina) is an ancient gastropod group with a long fossil record (Figures 2, 5, and 9), which colonized many different marine (shallow- and deep-water), freshwater, and terrestrial environments. The Palaeozoic Cyrtoneritimorpha, with openly coiled early shells (Figures 2H and 3D), may represent a closely related group. The living Archaeogastropoda unites the Vetigastropoda (Figures 3B and 3C) and several smaller groups, such as the Neomphaloidea, which occur in faunas associated with deep-sea hydrothermal vents. The Archaeogastropoda have colonized almost all marine and estuarine environments. There are also a number of extinct, mainly Palaeozoic groups (Figure 2) with uncertain relationships to living archaeogastropods. The Palaeozoic Euomphaloidea (¼ Euomphalomorpha; Figures 2P and 3E), known mainly from shallowwater, marine environments, may be a sister or basal group of the Archaeogastropoda. The Caenogastropoda and Heterobranchia are sister groups which are united in the taxon Apogastropoda. Both groups are highly diverse and have colonized almost all marine, freshwater, and terrestrial environments. The Palaeozoic Subulitoidea and Peruneloidea (Perunelomorpha) (Figures 2I and 3L) may be ancestral or basal groups of the Caenogastropoda or of all Apogastropoda. The extant Caenogastropoda unites the two orders Architaenioglossa and Sorbeoconcha. Terrestrial Cyclophoroidea and freshwater Ampullarioidea form the Architaenioglossa. On the other hand, the mostly marine Sorbeoconcha represents a highly diverse group uniting


Figure 8 Recent classification scheme of living gastropods, illustrating their phylogenetic relationships and the distribution of freshwater and terrestrial groups. Based on Ponder WF and Lindberg DR (1997) Towards a phylogeny of gastropod molluscs: an analysis using morphological characters. Zoological Journal of the Linnean Society 119: 83–256.

more than 25 superfamilies of living gastropods (Figure 1). The Heterobranchia encompasses the gastropod groups placed by Thiele’s classification into the ‘Opisthobranchia’ and ‘Pulmonata’, as well as some ‘prosobranch’ groups, such as the Valvatoidea and Architectonicoidea. The Valvatoidea is an ancient group of freshwater gastropods, but the highly diverse Architectonicoidea represents a marine group (Figure 1H). The majority of the lower Heterobranchia (Opisthobranchia or sea slugs) are also marine gastropods, typically with their shells reduced or absent. They are extraordinarily variable and are divided into about 30 superfamilies of nine orders. The higher Heterobranchia (Pulmonata) form a dominant group of terrestrial gastropods, but also occur in freshwater environments. There are several classifications of the Pulmonata, which may be divided into three orders: Systellommatophora, Basommatophora, and Eupulmonata. The ancient marine Basommatophora have been separated into the Archaeopulmonata and the freshwater Basommatophora into the Brachiopulmonata. The Stylommatophora is a dominant group of terrestrial gastropods and is the most numerous group in the Eupulmonata. The higher classification of extinct gastropods is less stable than that for living groups. The Palaeozoic, exclusively marine Pelagiellida, Bellerophontida (Figures 2L and 2M), Macluritoidea, and Mimospirina are amongst the most discussed extinct groups, and the gastropod nature of the Bellerophontida and Pelagiellida is still a frequently discussed problem.

The phylogenetic relationships of the Macluritoidea and Mimospirina (Figure 2G), with sinistrally coiled shells, are uncertain and both groups may be sister groups to more advanced gastropods.

Evolution of the Gastropoda The more than 500 million years of evolution of the Gastropoda is still poorly known. The main difficulties are that the phylogenetic positions and relationships of extinct gastropods can be inferred only from the limited number of characters observable in their fossilized hard body parts (i.e., shell and operculum). However, the number of extinct gastropod species and genera is much higher than those living. In addition, some belong to extinct higher taxa of family or order levels with unknown anatomy. Another complication is the development of similar shells in unrelated groups (homoplastic similarity) which has been documented in many living gastropods. Origin and Early History of the Gastropoda

Since 1970, many new mollusc-like fossils from the Cambrian have been discovered (e.g., Halkieria, Merismoconcha, etc.). Their interpretation has given rise to different models of evolutionary relationships within the Mollusca. Even though these models are controversial, the Gastropoda has been generally accepted to be the sister group of the classes Cephalopoda (see Fossil Invertebrates: Cephalopods (Other Than Ammonites)) or Tryblidiida (‘Monoplacophora’). The latter have been combined with the


Scaphopoda and Bivalvia (see Fossil Invertebrates: Bivalves) within the group Conchifera, which unites the higher Mollusca. Whether or not the Conchifera is monophyletic is uncertain. Torsion of the soft body has been considered to be one of the main diagnostic characters of the Gastropoda. For this reason, the majority of the models of gastropod origin have been based on different interpretations of this anatomical feature in the extinct gastropod-like molluscs. The Early Palaeozoic Helcionelloida, Bellerophontida (Figures 2L and 2M), and Tryblidiida, with bilaterally symmetrical shells, as well as the Pelagiellida and Macluritida, with asymmetrically coiled shells, are the most frequently discussed groups, and have been variously interpreted as untorted or torted molluscs. However, there is no reliable method of recognizing torsion in extinct fossil molluscs. Thus, the unknown nature of the bodies in the Early Palaeozoic gastropod-like fossils has enabled controversial speculations to be made about the origin of the Gastropoda. Generally, it is accepted that the first undoubted gastropods appeared in the Late Cambrian. Palaeozoic Era

During the Early Ordovician radiation, the diversity of gastropod groups which had appeared in the Late Cambrian (Archaeogastropoda, Euomphaloidea,

Macluritoidea, Mimospirina, Peruneloidea) rapidly increased (Figure 9). The Macluritoidea with large shells, together with different groups of Archaeogastropoda, Euomphaloidea, Bellerophontida, and Mimospirina, were typical elements of gastropod faunas of the tropical regions. In contrast, higher latitude faunas were composed mainly of the Bellerophontida and Archaeogastropoda. This arrangement survived until the early Middle Ordovician, when the diversity of some groups (Macluritoidea and Euomphaloidea) decreased and some new groups appeared (the slit-lacking Archaeogastropoda, Subulitoidea, Platyceratoidea, Loxonematoidea, etc.). During the Middle Ordovician, gastropod diversity rapidly increased and, in the Late Ordovician, reached its maximum. Middle and Late Ordovician faunas consisted of members of all the main groups of Palaeozoic gastropods, except the Heterobranchia (Figure 9). The end of the Ordovician saw a dramatic decrease in gastropod diversity, as well as the extinction of the Macluritoidea. The Silurian was a period of increasing diversity of many gastropod groups (e.g., Archaeogastropoda, Bellerophontida, and Platyceratoidea), when some gastropods in all marine communities continually increased, together with an increase in the morphological variability of their shells. This suggests an increase in their ecological adaptation to specific

Figure 9 Diagram illustrating the evolution of the Gastropoda and the types of early shell ontogeny. The bars show the stratigraphical ranges of each gastropod group. Based on Bandel K (1997) Higher classification and pattern of evolution of the Gastropoda. Courier Forschungsinstitut Senckenberg 201: 57–81, and Fry´da J and Rohr DM (2004) Gastropoda, 184–195 In: Webby BD, Droser ML, Paris F, and Percival IG (eds.) The Great Ordovician Biodiversification Event, pp. 408. New York: Columbia University Press.


environments. In comparison with the Ordovician, during the Silurian some gastropods with high-spired shells (mainly Loxonematoidea, Murchisonoidea and Subulitoidea; Figure 2) increased considerably. The Devonian was a time of distinct changes in marine gastropod communities. Some Ordovician– Silurian groups became extinct (Mimospirina; Figure 2G), new groups appeared (Heterobranchia), and many groups underwent rapid radiation and specialization (Caenogastropoda and Neritimorpha). Thus, the Devonian faunas contained representatives of all extant gastropod orders (Archaeogastropoda, Neritimorpha, Caenogastropoda, and Heterobranchia), as well as many Palaeozoic groups (Figure 9). The Devonian was also the time when the protoconch morphology of several gastropod groups underwent considerable change. Gastropods with openly coiled protoconchs (Perunelomorpha, Cyrtoneritimorpha, and Euomphalomorpha; Figures 2H, 2P, 3D, and 3E) formed a considerable, sometimes even dominant, part of the Ordovician and Silurian gastropod communities. During the Early Devonian, their numbers rapidly decreased and none survived the Permian/Triassic extinction. Carboniferous and Permian faunas had a similar composition of marine gastropod communities. A characteristic feature of Late Palaeozoic gastropod faunas was the fast radiation of different groups of Apogastropoda. The dominance of diverse groups of Caenogastropoda with high-spired shells (mainly Ctenoglossa and Cerithiomorpha) and Heterobranchia (Allogastropoda) was typical of shallow-water, muddy bottom communities. Mesozoic and Cenozoic Eras

The Permian/Triassic crisis affected gastropods as well as all other marine animals. The Euomphalomorpha and Cyrtoneritimorpha (Figures 2 and 3), as well as many groups of Archaeogastropoda, Neritimorpha, and Caenogastropoda, became extinct. During the Triassic, the last members of the Bellerophontida disappeared. The Late Triassic was a time of fast radiation of neritimorphs (Neritopsoidea and Neritoidea), caenogastropods (Ctenoglossa, Cerithimorpha, Architaenioglossa, and Littorinimorpha), and heterobrachs (Allogastropoda and Archaeopulmonata). From Triassic strata, the oldest limpets of the subclass Patellogastropoda are documented. The Patellogastropoda is considered to represent the most ancient gastropod group, but their ancestors (probably bearing coiled shells) have not yet been recognized amongst Palaeozoic gastropods (Figure 9). The composition of the Jurassic and Early Cretaceous marine gastropod faunas was roughly the

same as in the Late Triassic. The characteristic feature of Mesozoic and Cenozoic gastropods was the development of more ornamented shells in most groups, as well as the lesser occurrence of openly coiled shells, by comparison with Palaeozoic gastropods (Figures 1 and 2). Both macro-evolutionary trends have been interpreted as adaptation to increasing predation activities by other animals. During the Cretaceous, more advanced caenogastropod groups (higher Mesogastropoda and Neogastropoda) appeared (Figure 9), which underwent fast radiation and diversification after the Cretaceous/Tertiary faunal crises. Both groups developed the possibility of extending their planktotrophic larval stages and, from the beginning of the Tertiary, they formed one of the dominant groups of marine gastropods. During the Cretaceous, some gastropods (lower Heterobranchia) started to reduce their shells, enabling their adaptation to holoplanktic life (e.g., pteropods). The Early Cenozoic marine gastropod faunas are very similar to extant gastropods in higher taxonomic composition. Evolution of Freshwater and Terrestrial Gastropods

In contrast with marine gastropods, the fossil record for freshwater and terrestrial forms is less complete, limiting our knowledge of their evolution. Successful invasion to freshwater and land habitats has been closely linked with the mode of gastropod reproduction. External fertilization, which occurs in the ancient Patellogastropoda and Archaeogastropoda, limited them to marine environments. The freshwater and terrestrial environments were colonized by gastropods with egg capsules and internal fertilization (Neritimorpha, Caenogastropoda, and Heterobranchia). Even though members of these groups are known from the Early (Neritimorpha and Caenogastropoda) or Middle (Heterobranchia) Palaeozoic (Figure 9), the first freshwater and terrestrial gastropods are recorded from Late Palaeozoic strata (Archaeopulmonata). The first freshwater Basommatophora appeared during Jurassic time and, in the Cretaceous, the Stylommatophora started their invasion of the land and soon became the most diversified group of terrestrial gastropods.

Glossary Archaeogastropoda Group of extant gastropods. Bellerophontida Extinct group of Palaeozoic molluscs with bilaterally symmetrical shells. Caenogastropoda Group of extant gastropods. embryonic shell (protoconch I) Gastropod shell formed during embryonic development.


Heterobranchia Group of extant gastropods. heterostrophic Condition of the protoconch when its whorls coil in the opposite direction to those of the teleoconch. homeostrophic Protoconch and teleoconch whorls coil in the same direction. hyperstrophy Condition in which anatomically dextral animals occupy sinistrally coiled shells, and vice versa. larval shell (protoconch II) Gastropod shell formed during larval development in members of the Neritimorpha, Caenogastropoda, and Heterobranchia. lecithotrophic Form of development in which larvae use yolk in egg for their nutrition. Mimospirina Extinct group of Early and Middle Palaeozoic gastropods with sinistrally coiled, homeostrophic shells. Neritimorpha Group of extant gastropods. operculum Lid-like structure used for closing of the aperture in gastropod shells. Opisthobranchia Gastropod subclass of Thiele’s classification. orthostrophy Condition in which anatomically dextral (or sinistral) animals occupy dextrally (or sinistrally) coiled shells. Patellogastropoda Group of extant gastropods with limpet-shaped shells. periostracum Outer organic layer of gastropod shells. planktotrophic Form of development in which freeswimming larvae use planktic organisms for their nutrition. Prosobranchia Gastropod subclass of Thiele’s classification. protoconch Gastropod shell formed during larval and/or embryonic development. Pulmonata Gastropod subclass of Thiele’s classification. teleoconch Post-larval gastropod shell. trochophore larva Gastropod larva formed during early larval development which may be free swimming or occurs in egg capsules. veliger Gastropod larva formed during later larval development before metamorphosis to post-larval stages.

See Also Biological Radiations and Speciation. Evolution. Famous Geologists: Cuvier. Fossil Invertebrates:

Molluscs Overview; Bivalves; Cephalopods (Other Than Ammonites). Palaeoecology. Palaeozoic: End Permian Extinctions.

Further Reading Bandel K (1997) Higher classification and pattern of evolution of the Gastropoda. Courier Forschungsinstitut 201: 57–81. Beesley PL, Ross GJB, and Wells A (eds.) (1998) Mollusca: The Southern Synthesis, Part B, Fauna of Australia, vol. 5, pp. 565–1234. Melbourne: CSIRO Publishing. Bieler R (1992) Gastropod phylogeny and systematics. Annual Review of Ecology and Systematics 23: 311–338. Fretter V and Graham A (1994) British Prosobranch Molluscs. Their Functional Anatomy and Ecology. London: Ray Society. Fry´ da J and Rohr DM (2004) Gastropoda, 184–195. In: Webby BD, Droser ML, Paris F, and Percival IG (eds.) The Great Ordovician Biodiversification Event, p. 408. New York: Columbia University Press. Knight JB, Cox LR, Keen AM, et al. (1960) Systematic descriptions. In: Moore RC (ed.) Treatise on Invertebrate Paleontology, Part I, Mollusca 1, pp. I169–I324. Lawrence, KS: Geological Society of America and University of Kansas Press. Lindberg DR and Ponder WF (2001) The influence of classification on the evolutionary interpretation of structure – a re-evaluation of the evolution of the pallial cavity of gastropod molluscs. Organisms, Diversity and Evolution 1: 273–299. Peel JS (1991) The classes Tergomya and Helcionelloida, and early molluscan evolution. Groenlands Geologiske Undersoegelse, Bulletin 161: 11–65. Ponder WF and Lindberg DR (1997) Towards a phylogeny of gastropod molluscs: an analysis using morphological characters. Zoological Journal of the Linnean Society 119: 83–256. von Salvini-Plawen L (1990) Origin, phylogeny and classification of the phylum Mollusca. Iberus 9: 1–33. von Salvini-Plawen L and Haszprunar G (1987) The Vetigastropoda and the systematics of streptonerous Gastropoda (Mollusca). Journal of Zoology 11: 747–770. Taylor JD (ed.) (1996) Origin and Evolutionary Radiation of the Mollusca. Oxford, New York, Tokyo: Oxford University Press. Waren A and Bouchet P (1993) New records, species, genera, and a new family of gastropods from hydrothermal vents and hydrocarbon seeps. Zoologica Scripta 22(1): 1–90. Wenz W (1938–1944) Gastropoda. In: Schindewolf OH (ed.) Handbuch der Pala¨ozoologie, p. 1639. Berlin: Borntraeger. Zilch A (1959–1960) Gastropoda; Teil 2, Euthyneura. In: Schindewolf OH (ed.) Handbuch der Pala¨ ozoologie, p. 834. Berlin: Zehlendorf.