Abstract: Early and Middle Devonian gastropods show biogeographic patterns remarkably similar to those of better studied faunal groups of this same interval, ...
Early and Middle Devonian gastropod biogeography R. B. B L O D G E T - F 1, D. M. R O H R 2 & A . J . B O U C O T 3
US Geological Survey, Branch of Paleontology & Stratigraphy, National Center~MS 970, Reston, Virginia 22092, USA 2 Department o f Geology, Sul Ross State University, Alpine, Texas 79832, USA 3 Department of Zoology, Oregon State University, Corvallis, Oregon 97331, USA Abstract: Early and Middle Devonian gastropods show biogeographic patterns remarkably similar to those of better studied faunal groups of this same interval, notably articulate brachiopods, rugose corals, and trilobites. Three biogeographic realms are recognized: Old World, Eastern Americas, and Malvinokaffric Realms. Gastropod diversity is highest in the Old World Realm and lowest in the Malvinokaffric Realm. The diversity patterns and the high degree of shell ornamentation suggest that the Old World Realm was generally warmer than the Eastern Americas Realm, and that both were considerably warmer than the cool temperate to cold polar waters of the Malvinokaffric Realm. The utility of gastropods for fine-scale delineation of biogeographic units is illustrated for the Eifelian of western North America. At least two subprovincial units (the Alaska-Yukon and Nevada subprovinces) can be recognized. Eifelian gastropods from interior and southeastern Alaska belong to a single unit (the Alaska-Yukon Subprovince), and are most closely related to coeval faunas of northwestern Canada, suggesting little displacement of most of Alaska's so-called 'suspect' terranes. Plotting the data on the Devonian palaeogeographic maps of Scotese results in several suggested emendations: (1) North America should be moved south by 10-20~ (2) Australia is too far south on the Emsian and Givetian reconstructions, it should be in a more palaeotropical position; and (3) Siberia is too far north, it too should also be placed in a palaeotropical position.
The study of Devonian marine biogeography has sparked considerable interest during the past two decades. It has taken on even more significance in its role and implications in supporting various proposed 'mobilist' palaeogeographic reconstructions for this interval. The ~positioning of former Devonian continental land masses must make sense in terms of their contained fossil fauna and flora, allowing for their development under appropriate climatic conditions (i.e. fossil reef buildups should not be expected in polar regions) as well as allowing for reproductive communication between biogeographically similar marine faunas. Brachiopods have been the subject of the greatest amount of attention (Boucot 1974, 1975; Boucot et al. 1969; Johnson 1970, 1971; Johnson & Boucot 1973; Savage et al. 1979; and Wang Yu et al. 1984), followed secondly by rugose corals (Oliver 1973, 1976, 1977; Oliver & Pedder 1979, 1984), and thirdly by trilobites (Ormiston 1972, 1975; Kobayashi & Hamada 1975; Eldredge & Ormiston 1979). Other Devonian faunal and floral groups have only received very scant attention to date. The general picture emerging from these studies indicates the existence of three first order level biogeographic units which existed for much of the Devonian: the Old World Realm, the Eastern Americas Realm, and the Malvinokaffric Realm. The early Early Devonian (Lochkovian) is characterized by a moderate level of provincialism, followed by an acme of provincialism in middle to late Early Devonian (Pragian-Emsian) time. The Middle Devonian evinces progressively waning provincialism (the Eifelian remaining still strongly provincial, and subsequent Givetian being only moderately provincial), with the Late Devonian being nearly cosmopolitan, at least at the generic level. The cosmopolitan distribution in the Late Devonian may be exaggerated due to the absence or poor representation shallow marine benthic faunas in areas earlier occupied by Malvinokaffric Realm faunas.
Previous studies of Devonian gastropod biogeography Gastropods have received little study in terms of Devonian global palaebiogeography, and up to the present decade no papers had been published on this subject. Forney et al. (1981) discussed the global distribution of selected molluscan genera in the Silurian and Lower Devonian. Gastropod genera with Devonian representatives utilized in that study were Boiotremus, Euomphalopterus, Oriostoma, and Poleumita. The distribution of these genera as well as that of the bivalve genus Hercynella are in agreement
with biogeographic patterns previously established by brachiopod studies for the Lower Devonian. Blodgett et al. (1986, 1988) analysed the distribution of Lower Devonian gastropods in the Western Hemisphere and found them to be consistent with previously utilized biogeographic units. Other significant conclusions reached in the latter studies were the recognition that Old World Realm faunas exhibited the greatest taxic diversity, and that these faunas were also much more highly ornamented than the remaining realms. On the basis of diversity patterns, as well as on other biotic and abiotic factors, it was suggested that the Early Devonian equator probably passed through Alaska and the Canadian Arctic Islands, and not obliquely across the North American continent (Nevada to New York) as suggested by earlier palaeomagnetic-based studies. The remaining two realms (Eastern Americas and Malvinokaffric) were also found to yield characteristic gastropod faunas. The former was moderately diverse, and during most of the Early Devonian it was typified by a plexus of spinose platyceratids and other endemic platyceratid genera (Strophostylus and Crossoceras). The latter realm is most notable for its extreme low diversity (containing only four of the seventeen superfamilies then extant); characteristic taxa included Plectonotus (Plectonotus) and large species of Tropidodiscus. The significantly lower faunal diversity of the latter was suggested to be consistent with its presumed position close to the Lower Devonian South Pole. In the Eifelian (early Middle Devonian), both the Old World and Eastern Americas Realms were recognized in North America, each characterized by a relatively high degree of endemism (Blodgett et al. 1987). Typical Old World Realm elements include:
Hypomphalocirrus, Mastigospira, Odontomaria, Buechelia, Platyceras ( Praenatica), P. ( Prosigaretus), Plagiothyra, Strobeus, and a number of other genera. Typical endemic Eastern Americas Realm taxa include: Pleuronotus, Trochonema (Trochonemopsis), Elasmonema, Isonema, Turbonopsis, Palaeotochus, as well as the entire plexus of spinose platyceratids. In addition, two new Eifelian palaeobiogeographic subprovinces were defined for gastropod faunas of the Cordilleran Region of the Old World Realm: the A l a s k a - Y u k o n and Nevada Subprovinces. The former includes faunas from west-central Alaska (Nixon Fork terrane) and eastcentral Alaska (Livengood terrane), while the latter includes faunas presently known only from Nevada. The former is characterized by greater diversity, more highly ornamented shells, and by a rich accompanying calcareous algal flora (absent in Nevada).
From MCKERROW, W. S. 8r SCOTESE, C. R. (eds), 1990, Palaeozoic Palaeogeography and Biogeography, Geological Society Memoir No. 12, pp. 277-2841
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The differences were ascribed to the positions of these two subprovinces in equatorial and warm, subtropical regions, respectively. The subprovince assignment of intervening areas (District of Mackenzie and Canadian Arctic Islands) were not discussed due to the poorly known character of their gastropod faunas.
Method of study and suggestions for future work In this paper we extend our previous assessment of both Lower and Middle Devonian gastropod biogeography to include rocks of the Eastern Hemisphere, with the aim to establish global palaeobiogeographic patterns for this group, time interval by time interval. This study is based in part on analysis of previously published faunas in the older literature. This is complicated by the fact that outside of a few scattered investigators, little serious attention has been paid to Devonian gastropods subsequent to the First World War. This stands in marked contrast to the prominent position gastropods achieved in the studies by the great Nineteenth-Century Devonian investigators such as Goldfuss, D'Archiac and De Verneuil, the Sandberger brothers, Holzapfel and Oehlert. Of course in terms of mere number of Devonian gastropod taxa named, the most outstanding contributor has been Jaroslav Perner, who in his three tomes (1903, 1907 & 1911) proposed the largest number of Devonian gastropod genera and species. Work subsequent to World War One has been restricted to only a small number of investigators, working primarily on American, Czech, German, and Australian faunas. One of the tedious tasks associated with the assessment of the older literature is the utilization of greatly outdated taxic assignments, especially in the literature of the previous century. Reassignment of these faunas must rely on careful examination of line drawings, often of variable quality, and frequently not showing apertural characters which are most necessary in the delineation of gastropod genera and higher taxa. Also invaluable in the study of the European faunas was the help extended by a number of colleagues in allowing the examination of material in various repositories, or in arranging for loan of critical specimens. We are especially thankful to P. R. Racheboeuf, F. Langenstrassen, H. Jahnke, O. Walliser, H. Jaeger E. Schindler, S. Clasen, I. Chlup~i~ and V. Havli~ek. A serious problem hindering a complete assessment of Devonian gastropod biogeographic patterns is the dearth of knowledge concerning such faunas from large areas of the globe like Siberia, China, and New Zealand. Each of these areas has been the subject of considerable discussion concerning the biogeographic affinities of other Devonian biotic elements (most notably brachiopods, rugose and tabulate corals) found therein. Little can be said about the biogeographic affinities of gastropods from these areas until more basic taxonomic studies have been completed. It is our hope that the results presented here will encourage other workers to be watchful in their future collecting, and to collect and submit Devonian gastropods to qualified specialists for further study.
Devonian gastropod palaeoecology The palaeoecological setting and community analysis of Devonian gastropods has received little comment in the literature. In our experience we have recognized gastropods in a number of shelf environments of varying depths, though in general, gastropods are most abundant and common in shallow shelf, nearshore settings (i.e. interidal zone, lagoons or reefs). These are all in the shallower part of the photic zone, a constraint which appears to be due to the dominance of Devonian (and Palaeozoic) gastropod faunas by the archaeogastropods. Modern archaeogastropods are overwhelmingly herbivorous, and it is commonly presumed that their Palaeozoic antecedents were also predominantly so. Hence, shallow photic zone depths would be most favoured by such a group since it would support a large algal
flora which presumably provided a primary food source. Linsley (1979) considered that all archaeogastropods may not have always been algal grazers, since living pleurotomariaceans live below the photic zone. Nevertheless, we that feel this generalization probably holds true for Palaeozoic forms, due to their overwhelming abundance in very shallow-marine settings, an interpretation based both on sedimentological and biotic factors. The Alaskan Middle Devonian gastropod-dominated communities (often with over 40 gastropod species present at a collection horizon) provide especially compelling evidence for the essential correctness of this supposition. These faunas are nearly always accompanied by a rich and diverse calcareous green algal flora (Blodgett 1987; Blodgett et al. 1987; Poncet & Blodgett 1987) containing both dasycladacean and udoteacean elements. Typical genera encountered include Coelotrochium (a dasycladacean), Lancicula (a udoteacean), receptaculitids (Sphaerospongia tessellata), and charophytes (Sycidium). Silurian gastropod faunas are also dominated by archaeogastropods, and Peel (1984) considered that most Silurian gastropods were probably microherivores Or browsers on algae or colonial animals.
Early Devonian gastropod biogeography Early Devonian gastropods evidence biogeographic patterns remarkably similar to those previously established on the basis of articulate brachiopods, rugose corals, and trilobites (Blodgett et al. 1986, 1988; Forney et al. 1981). A detailed biogeographic analysis for Early Devonian gastropods in the Western Hemisphere was presented by Blodgett et al. (1988). Global endemism increased from a moderate level in the Lochkovian, to a very high level in the Pragian-Emsian. As with other faunal groups, three first-order level biogeographic units (realms) can be delineated. Gastropod diversity is highest in the Old World Realm, which is recognized in Eurasia, northern Africa, Australia, and western, Arctic, and northeasternmost North America. This realm, based on its biotic character and associated climatically sensitive lithologic features of its strata (presence of calcareous green algae (Boucot et al. 1988; Poncet & Blodgett 1986), oolites (Boucot et al. 1988), reef buildups, evaporite deposits, etc.) is interpreted to have been in a warm, tropical to subtropical palaeoclimatic setting. This interpretation is supported by the more highly ornamented nature of shells found in the gastropod faunas of this realm (Blodgett & Rohr 1989). Typical Early Devonian taxa restricted to the Old World Realm include: Paleuphemites,
Coelocylcus, Boiotremus, Aspidotheca, Kodymites, Sinistracirsa, Oehlertia, Hesperiella, Stenoloron, Platyceras (Praenatica), P. (Prosigaretus), oriostomatids, Michelia, Coelocaulus, Scalaetrochus, Euomphalopterus, Planotrochus, Mitchellia, Stylonema, and Katoptychia. The Eastern Americas Realm occupied much of the eastern and mid-continent region of North America, as well as northern South America. On the basis of its biotic and lithologic content, this realm is interpreted to have been in tropical to cool temperate palaeolatitudes. Three palaeoclimatic regimes can be recognized for both the Early and Middle Devonian: Hudson Bay Platform-Michigan Basin (tropical to subtropical with abundant evaporites and oolites; gastropod diversity is also higher here than elsewhere in the realm), Appalachian Basin (subtropical to warm temperate), and the Amazon-Colombian area (moderate to cool temperate with limited or no carbonates). Early Devonian endemic taxa include spinose platyceratids (found in all subprovince of the realm), Crossoceras and Strophostylus. Nevadan gastropods confirm the shifting biogeographic boundaries recognized previously in other faunal groups. In Lochkovian time the area was part of the Old World Realm. During Pragian-early Emsian time Nevada contained Appalachian gastropod genera (Nevadan Subprovince of the Eastern Americas Realm). Late in the Emsian (pinyonensis Zone) mixing occurred with elements of both realms co-occurring.
DEVONIAN GASTROPODS
In Eifelian time the gastropod faunas are almost wholly again of Old World Realm character (Nevada Subprovince of the Cordilleran Region of the Old World Realm). The Malvinokaffric Realm of southern and central South America, Falkland Islands, South Africa, and Antarctica contains a strongly depauperate fauna with only four superfamilies represented from the 17 extant in the Early Devonian (18 in the Middle Devonian). This realm appears to have been situated in cool temperate to even colder polar waters of the Devonian Southern Hemisphere, based on its total lack of carbonates, complete absence of biohermal buildups, greatly diminished taxonomic diversity of nearly all invertebrate phyla, as well as the complete absence of warm-water biotic elements such as stromatoporoids, conodonts, gypidulinid and'atrypid brachiopods.
Lochkovian
The biogeographic distribution of well documented Lochkovian gastropod faunas (Fig. 1) is in close agreement with that of other studied faunal groups. Although their reported occurrences are somewhat patchy, Old World Realm gastropods are known from western and Arctic North America, as well as from Nova Scotia (see Blodgett et al. 1988 for detailed listing of Lower Devonian gastropods from the Western Hemisphere). They are also present in France (Massif Armoricain), Czechoslovia (Perner 1903, 1907, 1911; Horn3~ 1963), Belgium (Asselberghs 1930), Germany, Podolia (Siemiradzki 1906), the Urals, Nol~.h Africa (Termier & Termier 1950; Sougy 1964), Kazakhstan (Rohr et al. 1979), and Australia. Eastern Americas Realm Lochkovian gastropod faunas are known from the Appalachians (Gasp6 , Maine, New York, Maryland, Virginia) and the central United State (Tennessee,
Early Devonian (Lochkovian)
279
Missouri); no occurrences have been noted as yet of Lochkovian age marine strata in northern South America (later the site of Eastern Americas Realm faunas). No Lochkovian age gastropods have as yet been recognized from rocks of the Malvinokaffric Realm.
Pragian - Emsian
This interval, including the two uppermost stages of the Lower Devonian, has been lumped together for convenience. Biogeographically distinctive gastropod faunas of this age are reported from a number of places globally (Fig. 2). Old World Realm gastropod faunas are recognized in western (only as far south as British Columbia) and Arctic North America, throughout Europe (France (Oehlert 1877, 1888), Belgium (Maillieux 1932), Germany (Dahmer 1917, 1921, 1926), the Carnic Alps (Frech 1894; Spitz 1907; Jhaveri 1969)), the Urals, North Africa (Termier & Termier 1950; Sougy 1964), Saudi Arabia (Boucot et al. 1988), Australia (Tassell 1976, 1977, 1978, 1980, 1982) and New Zealand. Eastern Americas Realm gastropod faunas of this interval are recognized from the Appalachians (Gasp6 to Alabama), central US (Tennessee, Missouri, Oklahoma, Texas), and northern South America. The most typical gastropod elements of the realm during this interval are the platyceratids, many belonging to a seemingly endemic plexus of spinose species (which may upon further study be delineated into new subgenera). Nevadan gastropods from this interval are overwhelmingly of Eastern Americas Realm affinities, in contrast to their Old World Realm affinities, both before and after. The taxonomically non-diverse Malvinokaffric gastropod faunas are recognized in central and southern South America, the Falkland Islands, South Africa, Ghana, and Antarctica.
~]I
Old World Realm Eastern Americas Realm
Fig. 1. Biogeographic distribution of Lochkovian gastropods by realm. Some areas bearing biogeographically distinctive marine faunas are left unshaded on this and the following figures due to lack of knowledge about gastropods from these areas. Base map is that for the Gedinnian (approximately equivalent to the Lochkovian) from Scotese (1986).
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R.B. BLODGETT
:i:.
~ii:.
Old World Realm
Early Devonian (Pragian-Emsian)
Eastern Americas Realm
~
Malvinokaffric Realm
Fig. 2. Biogeographic distribution of Pragian-Emsian gastropods by realm. Some areas bearing marine faunas of this age are left unshaded due to lack of knowledge about gastropods from these areas. The area of the Great Basin of North America is shown as belonging to the Eastern Americas Realm. During the Pragian-early Emsian, gastropods from this area were predominantly of Eastern Americas Realm affinity. Late in the Emsian (pinyonensis Zone) mixing occurred with elements of both realms co-occurring. Base map is that for the Emsian from Scotese (1986).
Only a handful of species can be recognized throughout the realm, the most typical elements being Plectonotus (Plectonotus) and large species of Tropidodiscus,
tween the Appalachian area and northwestern Europe (Old World Realm) is suggested by the rare occurrence of such a typical Appalachian taxon as a spinose Platyceras (Platyceras) in the Eifelian of Germany.
Middle Devonian gastropod biogeography Middle Devonian gastropods are well known from both the Old World and Eastern Americas Realms, and show strong provincialism in the Eifelian, with distinct but somewhat lessened provincialism still present in the Givetian. The general decline of provincialism from the Eifelian into the Givetian is a feature which has also been noted in other better studied faunal groups of this interval. As in the Early Devonian, taxic diversity is greater in Old World Realm faunas than in that of the corresponding Eastern Americas Realm. This again is due, we feel, to a generally warmer, more tropical positioning of areas occupied by the Old World Realm faunas (Blodgett et al. 1988; Boucot et al. 1988). Typical Middle Devonian Old World Realm taxa include: Pedasolia, Hypomphalocirrus, Mastigospira, Odontomaria, Platyschisma, Buechelia, Scalitina, n. gen. aft. Scalitina, Baylea, Catantostoma, n. gen. aft. Porcellia, Platyceras, (Praenatica), P. (Prosigaretus), Oriostoma (oriostomatids declined rapidly and became extinct near the end of the Eifelian), Plagiothyra, n. gen. neritopsinid, nodose Murchisonia (Murchisonia) (abundant in Givetian reefs), Astralites, Scoliostoma, Spanionema, and
Strobeus. Typical Middle Devonian, endemic Eastern Americas Realm gastropod taxa include: Pleuronotus, Trochonema (Trochone-
mopsis), Elasmonema, Isonema, Murchisonia (Hormotomina), Turbonopsis, Palaeotrochus, and the entire plexus of spinose platyceratids. Again endemism is much more strongly marked in this realm during the Eifelian, than in the subsequent Givetian. Limited faunal communication during the Middle Devonian be-
Eifelian Eifelian gastropods show a considerable degree of endemism (Blodgett 1987; Blodgett et al. 1987), and all three realms are recognized as present in this interval (Fig. 3). Old World Realm gastropod faunas are recognized in the Eifelian of western and Arctic North America (Blodgett 1987, 1988; Blodgett & Rohr 1989; Cleland 1911; LaRocque 1949; Linsley 1978; Tolmachoff 1926), Europe (Goldfuss 1844; Spriestersbach 1942; Whidborne 1889-1892), North Africa (Termier & Termier 1950), and the Kuznetsk Basin of Siberia (Butusova 1960). Nevadan Eifelian gastropods are of notable Old World Realm character, in marked contrast to their Eastern Americas Realm affinities in the Pragianearly Emsian, and mixed character in the later Emsian. No Eifelian gastropods are known from Australia, presumably due to the erosion or non-deposition of strata during the Tabberabberan Orogeny. Eastern Americas Realm gastropods are known from Eifelian strata of the eastern North America (Hall 1879; Kindle 1901; Linsley 1968; Meek 1873; Nettelroth 1889; Rollins et al. 1971; Stauffer 1909) and northern South America (Colombia, Venezuela and the Amazon Basin). The area of the Michigan Basin is shown as one of mixed biogeographic realm affinities during the Eifelian (Fig. 3.). In fact, it is predominantly of Eastern Americas Realm affinity for much of the Eifelian, with only a short-lived invasion of strongly Old World Realm forms recognized in late Eifelian gastropods of the Rogers City Limestone of Michigan and Lake Church Formation of Wisconsin (Blodgett
DEVONIAN GASTROPODS
Middle Devonian (Eifelian)
281
[~
Old World Realm
~
Eastern Americas Realm
Malvinokaffric Realm Fig. 3. Biogeographic distribution of Eifeiian gastropods by realm. Note area of realm boundary mixing indicated in the Michigan Basin. Base map is that for the Givetian from Scotese (1986).
1988). Re-establishment of Eastern Americas Realm affinities are evidenced by the succeeding, latest Eifelian age gastropods from Michigan. Malvinokaffric Realm Eifelian gastropods are shown as being present in western and southern South America (Fig. 3), though their occurrences are often in strata whose correlation could also be with the Emsian. An excellent example of the potential utility of gastropods for the discrimination of detailed biogeographic units, providing the sampling base is sufficient, is demonstrated by Eifelian gastropods in western North America. There, extremely large, predominantly silicified faunas collected from across the breadth of Alaska by one of us (RBB) and equivalent material from Nevada (made available by J. G. Johnson) allows the recognition of least two subprovincial biogeographical units ( A l a s k a - Y u k o n and Nevada Subprovinces) within the Eifelian of the Cordilleran Region of the Old World Realm (Blodgett 1987; Blodgett et al. 1987). In interior and southeastern Alaska Eifelian age gastropod faunas are remarkably similar, and their close affinities suggest they belong to a single biogeographic unit ( A l a s k a - Y u k o n Subprovince). Eifelian gastropods of the Great Basin represent a distinct unit, which was termed the Nevada Subprovince. The presence of a nearly homogeneous Eifelian gastropod fauna across the breadth of interior and southeastern Alaska, and the alliance of this and other accompanying faunal groups with coeval faunas from northwestern Canada, suggest that much of Alaska was more or less in place relative to North America in Devonian time, and not formed by the accretion of numerous disparate tectonostratigraphic terranes during the Mesozoic, as others have previously suggested (Coney et al. 1980; Jones & Silberling 1979; Jones et al. 1981, 1982). Both palaeobiogeographic and stratigraphic evidence from Devonian strata of Alaska have been previously used to support this position by Blodgett (1983) and Blodgett & Clough (1985), as well as by a number of recently published studies of Early and Middle Palaeozoic faunas and
floras in the past few years (Poncet & Blodgett 1987; Potter et al. 1988; Rigby et al. 1988; Rohr & Blodgett 1985). Most of the terrane boundaries in Alaska are nothing more than strike-slip faults, requiring only minor dislocation of terrane blocks northward along such well-known features as the Tintina and Denali Faults, not long-distance trans-oceanic voyages as more fanciful interpretations have suggested. Givetian
The biogeographic distribution of Givetian gastropods (Fig. 4) by realm is very similar to that shown for the Eifelian. One major difference is the absence of Malvinokaffric Realm faunas. No definitive Givetian localities bearing gastropods are known from areas previously assigned to this realm earlier in the Devonian. Old World Realm faunas are known from western and Arctic America (LaRocque 1949; Linsley 1978; Whiteaves 1892), Europe (D'Archiac & DeVerneuil 1842; Goldfuss 1844; Holzapfel 1895; Kirchner 1915; Sandberger & Sandberger 1850-1856; Whidb0rne 1889-1892), North Africa (Sougy 1964) south China (Yunnan), and Australia (Heidecker 1959). The European Givetian faunas are highly distinctive and were prominently figured in many monographs, especially in the nineteenth century, from both reefal and lagoonal settings in England and Germany. Perhaps the most distinctive elements among Givetian Old World Realm gastropods are the diverse and rapidly evolving plexus of nodose members of Murchisonia (Murchisonia). This nodose group appears to be limited wholly to strata of Givetian-Frasnian age. Givetian Old World Realm gastropods are best known from North America in the faunas of the Winnipegosis Formation of Manitoba (Whiteaves 1892; Linsley 1978). It should be noted that the area of the Michigan Basin was one of faunal realm mixing, as in the Eifelian, with strongly provincial faunas of either Old World or Eastern Americas Realm affinity found
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R.B. BLODGETT
Old World Realm
Middle Devonian (Givetian)
[~
Eastern Americas Realm
Fig. 4. Biogeographic distribution of Givetian gastropods by realm. Note area of realm boundary mixing indicated in the Michigan Basin. Base map is that for the Givetian from Scotese (1986).
restricted to seemingly separate horizons. The Givetian gastropods of south China (Yunnan) are known from Mansuy (1912), and by themselves are quite remarkable since many of the species are either conspecific or very close to species described from equivalent strata of Germany. Australian Givetian faunas are still poorly known, all illustrated representatives being from Queensland. However, the presence of several prominent, endemic genera (Burdikinia, Austerum, Labrocuspis), which appear to be locally common in the megafauna, suggests that Australia was still characterized by a greater degree of endemism than other, better known parts of the Old World Realm, which now show little differentiation of endemic gastropod genera. Givetian gastropods of the Eastern Americas Realm (Hall 1879; Fraunfelter 1973, 1974) exhibit a lesser degree of endemism than their Eifelian antecedents. Endemism is still evident, however, as witnessed by the common occurrence of spinose platyceratids.
Conclusions Devonian gastropods show biogeographic patterns similar to those noted in other better studied faunal groups for this time interval, most notably the articulate brachiopods, rugose corals, and trilobites. Taxonomic studies of Devonian gastropods tend to show a very strong geographical bias, with the faunas of some areas being relatively well known during certain intervals (notably central Europe), while those from other areas (i.e. China and Soviet Asia) are barely known at all. These strong regional biases in our knowledge are due to the lack of personnel able to undertake taxonomic studies of what many palaeontologists of late have tended to regard as a relatively 'useless' group for Palaeozoic biostratigraphic studies. We feel this view is in error. Emerging detailed knowledge of Eifelian age gastropod faunas of western North America currently allow delineation of at least
two subprovincial level biogeographic units (Blodgett 1987; Blodgett et al. 1987). The remarkable faunal similarities of Eifelian gastropods from interior and southeastern Alaska suggest that they all belong to the same unit ( A l a s k a - Y u k o n Subprovince). These faunas are also closely allied to coeval faunas of northwestern Canada, indicating little transport of the so-called terrane 'blocks', suggested to comprise most of Alaska. This relatively local derivation, and minor ascribed motion to the terranes is in accord with the emerging pattern from other recently published palaeobiogeographic studies of Early and Middle Palaeozoic faunas of Alaska. Devonian gastropod diversity patterns, degree of shell ornamentation, as well as a number of other biotic and abiotic factors (discussed above) suggest that in general the Old World Realms faunas were warmer than that of Eastern Americas Realm, and that both were considerably warmer than the Malvinokaffric Realm faunas, the latter which are considered to have been situated in cool temperate to even colder polar waters of the southern hemisphere. Plotting of the Devonian gastropod biogeographic data (Figs 1 - 4 ) on the maps utilized for this symposium (Scotese 1986) result in several suggested emendations: (1) North America has been placed too high, it is suggested that it should be moved south by 10-20 ~ in order to place Alaska and the Canadian nearly astride the palaeo-equator during Early Devonian-Eifelian time in accord with evidence provided above; (2) the position of Australia on the Emsian and Givetian maps is too far south of the palaeo-equator, the character of the fossil fauna and flora suggest it was in a more tropical (palaeo-equatorial) position; and (3) although described Devonian gastropods are few from Siberia, the richly, diverse calcareous green algal flora known from the Kuznetsk Basin indicates that Siberia is placed too far north in these reconstructions, and must have been situated in a palaeotropical belt during the Early and Middle Devonian.
D E V O N I A N GASTROPODS
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