Provincialism and migration in Lower and Middle Devonian pelecypods

119

Palaeogeography, Palaeoclimatology, Palaeoecology, 23( 1978 ): 119--130 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

P R O V I N C I A L I S M A N D M I G R A T I O N IN L O W E R A N D M I D D L E DEVONIAN PELECYPODS

J. BOWMAN BAILEY

Department of Geology, Western Illinois University, Macomb, Ill. 61455 (U.S.A.) (Revised version accepted January 5, 1977)

ABSTRACT Bailey, J. B., 1978. Provincialism and migration in Lower and Middle Devonian pelecypods. Palaeogeogr., Palaeoclimatol., Palaeoecol., 23:119--130. Comparisons of early Devonian pelecypods of the Rhineland with mid-Devonian taxa from the Appalachian United States reveal pronounced similarities at the specific level. Evidence suggests: (1) either the Lower Devonian Emsian stage of Germany and France is biostratigraphically equivalent to the Middle Devonian Cazenovian stage of New York; or (2) the distinctions between early Devonian European and North American faunal provinces deteriorated as a result of invasion of European endemic species into Appalachian seas (probably via northwest Africa) occurring at the end of the early Devonian; (3) diastrophic processes associated with the Caledonian and Acadian orogenies may have had an important bearing on initial endemism, speciation, and later migration; (4) the distinctiveness of Michigan Basin pelecypods suggests that this region was a separate province or sub-province during the Devonian due to geographic barriers and salinity differences; (5) endemic molluscan taxa of possible Asian affinities in western North America and Devonian transcontinental emergence support biogeographic separation of this region from the Appalachian and Michigan basins; (6) preliminary evidence suggests endemism associated with Gondwana pelecypods. INTRODUCTION Current s t u d y (Bailey, 1975a, b, 1 9 7 6 ) o f the systematics o f Middle Devonian p e l e c y p o d s o f New York reveals m a n y striking similarities b e t w e e n E u r o p e a n and A p p a l a c h i a n Devonian p e l e c y p o d taxa o f t e n previously regarded as specifically distinct (Hall, 1884, 1 8 8 5 ; Frech, 1 8 9 1 ; Beushausen, 1895). Because these E u r o p e a n and A m e r i c a n p e l e c y p o d faunas have n o t been c o m pared as a group, the b r o a d similarities a p p e a r largely to have r e m a i n e d u n n o ticed a l t h o u g h a few past a u t h o r s such as Verneuil ( 1 8 4 7 ) have m e n t i o n e d the similarities o f one species or a n o t h e r t o a s u p p o s e d l y distinct species f r o m the o p p o s i t e side of the Atlantic. General lack o f r e c o g n i t i o n o f the p r o n o u n c e d affinities is due, on the one h a n d , to c h r o n i c oversplitting o f t a x a o n the part of m a n y early a u t h o r s (McAlester, 1 9 6 2 ) and, on the o t h e r h a n d , t o the belief t h a t even closely similar taxa m u s t be regarded as specifically distinct because of an assumed Devonian separation b e t w e e n E u r o p e and N o r t h A m e r i c a

120

envisaged as necessary prior to the advent of plate-tectonic theory (Frech, 1891). RHEINISH AND APPALACHIAN A F F I N I T I E S

Table I lists selected North American (mostly Hamilton) and European (largely Rheinish Magnafacies sensu Erben, 1964) taxa which are at least morphologically very closely allied if not conspecific. It is strikingly apparent from the tables that although a few of the paired species are of similar stratigraphic age in both the Old World and the New World, in most instances the European species (mostly Emsian in age) occur earlier than their North American middle Devonian (Cazenovian) counterparts. However, in the earliest Devonian the reverse is true in at least three instances [i.e. Nuculites oblongatus (Conrad), Palaeoneilo constricta (Conrad), and Paracyclas proavia (Goldfuss)]. Moreover, J. M. Clarke (1907) reported pelecypods and brachiopods in the lowermost Devonian (Gedinnian sensu Cooper et al., 1942) Dalhousie Shale of New Brunswick and the Chapman Sandstone of northeastern Maine having marked affinities to later Lower Devonian (Emsian) taxa of Europe. The evidence therefore suggests two episodes of pelecypod migration: A lesser incursion of endemic North American species into Europe during the earliest Devonian and a major post-Emsian invasion of the Rheinish taxa into the Appalachian seaways. So similar are the middle Devonian (Cazenovian) pelecypod faunas of New York, Maryland, Virginia, Pennsylvania, Ohio, southern Indiana and Kentucky and the early Devonian Emsian pelecypod faunas of Germany and France, that it is tempting to suggest a readjustment of biostratigraphic correlation to more nearly equate the Emsian and the Cazenovian (Palmer, personal communication, 1975) especially in view of past problems in Lower and lowermost Middle Devonian correlation. However, recent studies (Erben and Zagora, 1968; Oliver et al., 1968) have summarized the extensive biozonation of the Devonian of Europe and North America utilizing mostly conodonts and brachiopods, and these works do not appear to suggest any such readjustment. The similarities, alternatively, appear best explained by migration following breakdown of paleogeographic or paleoecologic barriers especially at the end of the Emsian. Oliver (1973, 1976) indicated a striking breakdown in the early Devonian endemism of fossil corals in post-Emsian times. Chronologically similar partial endemic collapse has also been reported in brachipods (Boucot et al., 1968), although cosmopolitanism was apparently not dominant until the late Devonian Frasnian (Boucot and Johnson, 1973). Goniatites show a somewhat similar pattern although cosmopolitanism, again, does not characterize their distribution until the Frasnian (House, 1973a, b). It has been postulated that some deterioration of early Devonian provincialism may have involved migrations around the northern perimeter of the Old Red Continent into the Cordilleran region (=Trans-Artic Link sensu House, 1973a, b). Burton and Eldredge (1974), however, pointed out the need to

121

TABLE I

North American Devonian pelecypod species and conspecific or closely related European counterparts t! U R O P E A N T A X A

NORTH AMERICAN EQUIVALENTS

SUKCLASS PALAEOTAXOIX)NTA

Korobkov

Nuculoidea fornicata (Goldfuss) "~ (=NL~cula daleidensis Steinin~er) Upper Emsian Eifelian Nucul~dea mu~hlsont {Goldfus$) • Eifelian Givetian Nuculoidea cf. Iodanensls (Beuthausen) upper Emsian Nucu/oidea sanbergari (Beushausen) Givetian (one specimen, dOubtleSSly a variant of N formcata) Nuculoidea aouisgranen~s (Beushausen) - Glvetia~ Nuculites cf triquetrus Conrad, B ~ t h a u s e n ~ lower-upper Emsian Nuculitesposthumus (Beushausen) Eifelian Nueuhres truncatus (Steiningar} $iegenian, lower-upper Emsian Nucu/itese/lipticus (Maurer) ( Oitchia mira S~ndb, Cucullela beug~au~ni Fuchs) lower Emsian Palaeonedo dernigrans (Beushausen) ) lower-upper Emsian Pa/~eonei/o planlformis {Beuthausen) lower Emsian Palaeonei/o da/eidenus (Beu~hausen) upper Ems~an Palaeonei/o maureri (Bedsh)(in part) lower Emsian Palaeonedo pr~maeva (Steiningar } "~ "Coblenz" and "upper Emsian Palaeoneilo unioniform~s (Sandb.){in part) Lower Emsian Palaeoneilo bertkaui (Beuthausefl) lower Emsian Pal~neilo oehlertl (Beushauseo) lower Emsian

Nucu/oidea ~ i m a (Hall) Cazenovi~

Nuculoldea c~'buliformi$ (Hall} C~enovian Chem~ng~an

Nucu/ites triquerrus Conrad Offeso/,Jethaw~ (Gaspe), Cazeno~ian NuculJte=ob/ongatu= Conrad Cazenovian (>Helderber~an of Maine)

Pa/aeoneilo constr~ta (Coorad) Helderpergq~ {Ma~ne) Cazenovian, Chemun~an Palaeoneilo cot~ug~ Pohl upper Middle Devonian(Wise.)

Palaeoneilo fecunda Hall (=P ter~lstriata Haft) C~en~ian

SUBCLASS H ETEROLX)NTA Neumayr P~acycla~ proawa (Goldfuss) Ei felian-Givetia~ Paracyclas rugosa (Goldfu~) lower-upper Emsian-F ramian Par~Kcla~ antigua (Goldfuss) Eifelian Givetian P~acyclas marg~nata Maurer - ~o~r upper Emsian

Par~yclas ellip~ca Hill Ooe~uethlwan, Clzec=ovian Paracycla= litat~ (Conrad) Cazenovian Paracyclas ohioentJ~ (Meek) Cazenovia~ P~r#cycl~ tenut$ Hall Caze~ovian

SUBCLASS ANOMALODESMATA r~ll Grammysta a n o ~ l a (Gold fuss) tower upper Emslan Grammvs/a ovata Sandpergar (in Dart) lower Emsian Grammysia/ohannis Beush~se~ (in part) lower.upper Emsia~ Grammysla nodocostata Hall vat. eifeliensis ~eushausen lower Emsian A/lerisma mosellanum Beushau~en upper Emsian

-~ Grammysla bisu/caca (Conrad) Caze~oviat~

Grammy~a nodoco~t~ta H i l l Cazeno~ia~ --

Grammysioidea a~cuata (Cop.tad) Cazeno~ian

S U B C L A S S P T E R I O M O R P H I A Bcurlen PWchopteria fascfru/ata (Goldfuss) upper Emsian Gosseletia a/ta Follmann upper Emsian Gosse/etia d~stincta Follmann Eifelian Ptychodesma sp B~sh~sen upper Emsian

PWchopreria {Corr~lli~) flabella (Conrad}, Cazenovian "~

/

Gosse/etia triouetra {Co~rad) Cazen~ian Ptych~ knappianum Hall & WhiH. Cazenovian

SUBCLASS PALAEOH ETERODONTA Newell Modiomorpha antlqua (Goldf.)(in part) I o ~ r Emsian Nyassa dorsata Gold fuss Eifelian-Givetian Carydium c f sociale 8~sh., M a i l l i e u x , - - Emsian

Modiomorpha co~¢entricB (Co~r ad } Cazenovian ar~, r# H i l l C~enovian C4~/dium bel/i~riaBim (Conrad) ° [=C v#tlcosum [Hill}} Cazenovian of. C p ~ w ' i u m 8eush. Oriskany. Ny~

--

* Formerly regarded as a species of Nucu/a Lamarck or Nucu/c~d~e Williams and Breger, Howeve¢. one of the otlly known specimens prese~ing the details of the hinge plate (N.Y $.M. 2~o6) indicates that past generic assigl~menthas been i~orrect

122 invoke early and early middle Devonian migrations to explain the distribution patterns of the trilobites Phacops Emmrich and Greenops Delo and provided evidence of a possible route from Europe to North America via northwest Africa. House (1973a) also has demonstrated stronger European affinities in Devonian goniatites southward down the Appalachians in support of a similar southern Europe/Afro-Appalachian Link and reported pathological similarities (i.e. abnormal growths) between European and West African goniatites (House, 1960, 1973a). Close Devonian paleobiogeographic relations between the Armorican Massif (France) and the Cantabrian Mountains and Aragon (Spain) have been demonstrated in the Strophochonetinae (Brachiopoda) by GarciaAlcalde and Racheboeuf (1975). Further evidence for such a connecting seaway was provided by the paleogeographic reconstructions of Erben (1964), Sutton (1968), and Oliver (1976). Although direct evidence is scanty insofar as pelecypods are concerned, a similar migratory route is suggested for the Rheinish pelecypod invasion of North America by a relatively rare and distinctive ambonychiid genus Gosseletia Barrois in the Spanish Devonian (Asturias). Gosseletia devonica Barrois (1882), although unique in itself, closely resembles both German and North American Gosseletia species (Hall, 1885; Beushausen, 1895). Furthermore, LeMaitre (1939) reported the German Devonian nuculid pelecypod "Nucula" fornicata Beushausen from the Eifelian of northwest Africa. As indicated in Table I this species is closely allied and probably conspecific with Nuculoidea opima (Hall) from the Cazenovian of the Appalachians. Arden and Rehrig (1964) reported two common widespread Devonian pelecypod genera, "Cornel. lites" and Grammysia, in the Middle Devonian of northwest Africa. They also noted that the overall Devonian fauna of this region shows pronounced affinities with American, Spanish and Rheinish faunas. American affinities were reported in the Tindouf Basin of the Spanish Sahara while a short distance eastward across the Ougarta chain, Rheinish affinities were noted in the Saoura Basin. The Rheinish pelecypod fauna appears considerably more diverse than the Appalachian although abundances are much lower (Palmer, personal communication, 1975). The greater diversity may be, at least in part, related to the paleoenvironmental complexity of the early Devonian proto-Tethys separating the Old Red Continent from Gondwanaland. Sutton (1968) stated that a complex series of submarine troughs, ridges, and islands occupied the region. The bulk of the German pelecypod fauna having American affinities is associated with shallow marine, arenaceous sediments deposited on the shallower submarine ridges and island flanks of the proto-Tethys and the Old Red and Gondwana continental margins (=Rheinish Magnafacies sensu Erben, 1964). These species are largely absent in the trough-associated argillaceous sediments (Hercynian Magnafacies) although other types of pelecypods (presumably non-byssate, deeper water mud-burrowers) occur here (Erben and Zagora, 1968). Many nuculoids inhabited both environments. The isolated subenvironments afforded by the structure of the proto-Tethys might have

123 provided a spawning ground for the Rheinish pelecypod fauna by furnishing the local ecologic isolation requisite to taxonomic differentiation at the specific level. In the Silurian, open, shallow seaways and cosmopolitanism among many groups of organisms predominated (Ager, 1971). Beginning in the early Devonian, however, conditions were substantially altered. A possible scenario relating to the initiation and later migration is described below. Valentine and Moores (1970) have stated that "judging from the emplacement of ophiolites the pre-Caledonide-Acadian ocean began closing by the mid-Ordovician and was sutured in the late Silurian to late Devonian". The closure of this protoAtlantic ocean resulted in broad continental expansion and high influx of sediments associated with the Rheinish--Hercynian ridge--trough facies complex and initiated widespread faunal isolation facilitating speciation. With the exception of a narrow passage in the region of Connecticut and Massachusetts, the paleogeographic reconstructions of Boucot and Johnson (1968) show an extensive land barrier along the entire eastern perimeter of the present coastal states. This barrier was associated with the deposition of Lower Devonian clastic sediments and effectively separated the Appalachian seaways from those of Europe. Oliver (1973) noted that endemism in corals increased from Pridoli (Silurian--pre-Gedinnian) to Emsian time and that: "To an unknown extent this paralleled the development of the Old Red Continent between what is now the Appalachian belt and Europe." Closing and suturing of the proto-Atlantic may have indirectly assisted, to some extent, later migration by increasing the proximity of North America and Europe. Subsequently, as erosion of the highlands proceeded, the clastic sediments became progressively finer grained and argillaceous in the Eifelian proto-Tethys (Erben, 1964) and presumably less hospitable especially for many of the littoral, byssally attached species. However, as indicated in Table I, certain nuculoid pelecypods continued to persist in deeper, muddier substrates of the midDevonian. This is also true of the lucinacean Paracyclas rugosa (Goldfuss) which even became larger in size (Beushausen, 1895) possibly indicating a preference for the increasingly pelitic conditions. Such erosion of uplifted areas, by reducing former areal extent, altered the paleogeography and may thus have introduced migratory possibilities. At the same time that the coarser clastic substrates were disappearing in the proto-Tethys, throughout the middle and late Devonian the Catskill Delta in the northeastern portion of North America expanded as a result of the Acadian orogenic disturbance filling the Appalachian Basin with extensive shallow marine clastic detritus analogous to the Lower Devonian Rheinish lithofacies. This might explain the persistence of the brachiopod Paraspirifer cultrijugatus (Roemer) well into the middle Devonian of North America after it disappeared in Europe by the end of the early Devonian (Cooper et al., 1942). It is conceivable, therefore, that migration of the Rheinish pelecypod assemblage into North America at least partially resulted from following the tectonically associated shallowwater clastic sediments.

124 Although climatic factors also may have been involved in causing migration, it has been pointed out that, on the basis of paleomagnetic data, the North Atlantic region may have been located in or near the tropics during the Devonian (Shirley, 1964; Boucot and Johnson, 1968; Ager, 1971; Oliver, 1976). Assuming the accuracy of this judgment, the Afro-Appalachian seaway would appear to be a more plausible migratory route for large numbers of organisms because it would have primarily involved longitudinal movement within the warm seas in contrast to the Trans-Artic Link which probably would have required passage through higher latitudes. MICHIGAN BASIN PROVINCIALISM Comparison of a well-preserved pelecypod fauna of typically Appalachian affinities from the Solsville Member, Marcellus Formation of New York has been made (Bailey, 1975a, b, 1976) with a number of fairly well known pelecypod assemblages from the Middle Devonian of North America including those from the Sellersburg beds of southern Indiana and northern Kentucky (Nettleroth, 1889; Kindle, 1901; Savage, 1931), the Romney Formation of West Virginia and Maryland (Prosser and Kindle, 1913), the pre-Traverse (Dundee and Rogers City) of Michigan (LaRocque, 1950), the Milwaukee Formation of Wisconsin (Cleland, 1911; Pohl, 1929), the Silica Formation of Ohio (Stewart, 1927; Ehlers et al., 1951; Duluk, 1965), and the Arkona Formation of southwest Ontario (Southworth, 1967). In spite of the differing lithologies the faunas are basically similar with the exception of the preTraverse of Michigan (Table II). Data on the Arkona pelecypods is rather scanty although Ptychopteria fasciculata (Goldfuss), Palaeoneilo fecunda Hall and Carydiurn bellistriatum (Conrad), [= "Nucula" bellistriata Conrad and "Nucula" varicosa Hall], common Appalachian Cazenovian species, have been collected there by the author. Southworth (1967) reported Gramrnysioidea Williams and Breger in the same beds. The faunal affinities of the Arkona •Formation would therefore appear to lie with the normal Appalachian assemblage although these beds do contain an endemic species of nuculid pelecypod (Bailey, in preparation). The data suggest that the Michigan Basin represented a distinct faunal province insofar as Middle Devonian pelecypods are concerned. Further support for this conclusion is provided by the only known Devonian occurrence of an otherwise common Carboniferous--Permian genus, Nuculopsis Girty (sensu stricto) in the Middle Devonian Traverse beds (Potter Farm) near Alpena Michigan (Bailey, in preparation). Moreover, the pre-Traverse of Michigan contains a distinctive, mytiliform, byssate genus Liromytilus LaRocque which is also present in the Middle Devonian of Manitoba (Whiteaves, 1892), but, to my knowledge, is not present in the Appalachian region. Eldredge (1972) has provided additional evidence for Michigan Basin provincialism using phacopid trilobites. Phacops rana (Green) is a common faunal component in the Appalachian province during the mid-Devonian, but a

125 TABLE II North American distribution of pelecypod species encountered in the Solsville Member (Marcellus Formation) N.Y. (Bailey, 1975a, b, 1976) SPECIES

Solsv.

Nuculoidea sp. " A " Nuculites triquetrus Nuculites oblongatus Palaeoneilo fecunda Palaeoneilo constricta Paracyclas rugosa Grammysioidea alveata Grammysioidea elliptica Nyassa dorsata Gosseletia triquetra Ptychopteria fasciculata Modiomorpha concentrica Ptychodesma knappianum

Scllersb.

Romncy

prc-Trav. Milwauk. Silica

C4

0

0

0

0

R

0

1

O0

1

C3 C2

0 00

1 1

O0 007

1 1

R C2

00 1

1 1

00? 0

1 1

C1 R R

1 0 O0

1 1 1

O0 O0 O0

~ ? O0

1?

C3 C3 C2

00 1 1

00 1 1

00" 0 07

00 0 1

1 1

C1

1

O0

O0

O0

1?

N OTAT! ON : R C1

-

rare fairly c o m m o n

C2

-

common

C3 C4

-

abundant very a b u n d a n t

0

-

1

-

genus present species present

00

-

absent

* L a R o c q u e (1950) described and illustrated

"'Gosseletia sp." However, on the basis of Pojeta's (1966) analysis, this specimen, w h i c h lacks an anterodorsal salient, w o u l d more reasonably be placed in Lophonychia Pohl.

closely related species P. iowensis Delo is largely confined to the Michigan Basin throughout its chronologic range (Cazenovian--Taghanican). That some structural and salinity control may have been involved in the distinction of Michigan Basin and Appalachian faunas is suggested by the paleogeographic disposition of the Cincinnati, Kankakee, and FindlayAlgonquin arch complexes. Local uplifts occurred along various portions of these structures beginning as early as Champlainian times (Wilson, 1962; Wilson and Stearns, 1962; Rooney, 1966) and continued through the SiluroDevonian. Extensive emergence along these structures especially characterized the late Silurian and early Devonian, and separation of the highly saline waters of the Michigan Basin from the normal marine waters to the south and east was maintained through the mid-Devonian (Atherton, 1971; Summerson and Swann, 1970; Eardley, 1962).

126 WESTERN NORTH AMERICAN PROVINCIALISM Few data are available regarding the paleobiogeographic affinities of early and middle Devonian pelecypods in western North America. However, a faunule consisting of two unique veneroid pelecypod genera (Congeriomorpha Stoyanow, and Tusayana Stoyanow) and two singular gastropod genera described from the late Devonian Island Mesa beds (Jerome Formation) in northcentral Arizona (Stoyanow, 1948; Saul, 1976) are suggestive of some degree of endemism in western North America. Similar distinctions in eastern and western North American Devonian rugose corals have been , ~tributed by Oliver (1976) to emergence of the Transcontinental Arch throughout much of the Devonian. Oliver indicated affinities of the western North American corals with those of Asia as part of the "Old World Realm." This conclusion is in harmony with Runnegar's observation (in Saul, 1976, p. 156) that Tusayana of the Island Mesa beds exhibits morphological traits of Tanaodon Kirk from the Middle Devonian of Szechuan, China. GONDWANA PROVINCIALISM Devonian pelecypods from the Southern Hemisphere are poorly known. However, McAlester (1965) made the following assessment of the pelecypod fauna from the Horlick Formation, Ohio Range, Antarctica: "The three identifiable species appear indistinguishable from South American and South African Devonian s p e c i e s . . . " Included among these Southern Hemisphere endemics was the nuculid pelecypod Nuculoidea musculosa (Knod) originally described from the Devonian of Bolivia. Bradshaw (1974) described two new species of nuculid pelecypods from the Devonian near Reefton, New Zealand (Nuculoidea vespa and N. umbra) and pointed out that these species are somewhat similar to N. musculosa. I have examined McAlester's (1965) specimens of the latter species from Antarctica in the National Museum of Natural History (Washington, D.C.) [USNM nos. 145423, 145424, 145427, 145428] and, on the basis of Bradshaw's illustrations it is my opinion that N. musculosa, N. vespa, and N. umbra are so similar to one another and sufficiently dissimilar from Nuculoidea Williams and Breger (sensu stricto) that distinct generic or subgeneric status is warranted for these Gondwana species distinguishing them from Northern Hemisphere Devonian nuculids (Bailey, in preparation). This kinship, as well as McAlester's observations (above) begin to suggest some degree of Gondwana pelecypod provincialism associated with Antarctica, South Africa, South America, and New Zealand. CONCLUSIONS The following major conclusions are reached in this study: (1) A suite of pelecypod species previously considered more or less unique to Middle Devo~fian rocks of the eastern United States has been shown to be

127

virtually identical to a group of species first appearing in Lower Devonian rocks of central Europe. (2) This distribution may be explained either by inadequate international biostratigraphic correlation or, more probably, by migration of European pelecypods into Appalachian seas following a collapse in early Devonian endemism. (3) A European origin is supported not only by earlier occurrences there, but also by greater pelecypod species diversity in early Devonian paleoenvironments associated with the submarine ridge--trough complex of the European proto-Tethys. (4) Occurrences of European--Appalachian Devonian peleeypod taxa in Spain and North Africa support the southern Europe/Afro-Appalachian migratory link suggested by House (1973a). (5) Tectonic and subsequent erosional processes associated with the Caledonian--Acadian suturing of the proto-Atlantie ocean may have had an important bearing on early Devonian endemism and later migration. (6) Faunal evidence suggests three peleeypod provinces in North American middle Devonian times: (a) the Appalachian Basin (composed of European-Appalachian vagrants, in part); (b) the Miehigan Basin (composed of North American endemic species); and (e) western North America (composed of North American endemic species of possible Asian affinities). Segregation of Michigan Basin taxa may have been due to the peripherally disposed geographic barriers and to higher salinities within the basin. Segregation of western North American species may have been the result of emergence of the Transcontinental Arch. (7) Although evidence is scanty, Southern Hemisphere endemic pelecypods suggest some measure of Gondwana provincialism. ACKNOWLEDGMENTS

I wish to extend my gratitude to the following friends for their instructive criticism in the preparation of this study: Dr. Norman D. Newell, Department of Fossil and Living Invertebrates, American Museum of Natural History, New York, N.Y., Drs. D. B. Blake, P. A. Sandberg, R. L. Langenheim, D. S. Wood (Department of Geology), D. C. Sweeny (Department of Evolution, Ecology and Ethology), D. L. Johnson (Department of Geography), University of Illinois at Urbana--Champaign; and Dr. K. W. Ciriacks, Amoco Production Co., Denver, Colorado.

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