16. CRETACEOUS AND PLEISTOCENE BIVALVIA, DEEP SEA DRILLING PROJECTHOLES 415, 415A

Erie G. Kauffman, Department of Paleobiology, U.S. National Museum, Smithsonian Institution,Washington, D.C.

INTRODUCTION

One of the great disappointments of the first 50DSDP legs has been the apparent paucity of well-preserved macrofossils in cores, especially those fromthe older Mesozoic levels where we seek the evolu-tionary rootstocks of the modern deep-sea biota. Onlythe calcitic prisms of inoceramid bivalves, singly or asclustered shell fragments, are commonly preserved inJurassic and Cretaceous core samples—nonetheless re-flecting the ubiquity of this group. Ammonites, in-cluding aptychi, compose the second most commonmacrofaunal element. Diverse assemblages of mainlysmall invertebrates such as those described by Kauff-man (1976) from the Manihiki Plateau on Leg 33 arerarely encountered. Numerous such faunas may lie hid-den in DSDP cores, but they have not been obviousfrom surficial examination or in microfossil washings.Only systematic splitting of DSDP cores at closelyspaced intervals will possibly reveal quantities of addi-tional macrofossil material.

Thus, we still know very little about the evolutionaryhistory of macroinvertebrates from sediments over 75per cent of the Earth's surface. The important challengeof documenting the ecological and biogeographic histo-ry of the deep-ocean biota, and its implications to bio-logical theory and plate tectonics, cannot yet be met forlack of a good data base.

The poor record of macrofossils in DSDP cores mag-nifies the importance of thoroughly documenting thefew assemblages that do come to light in initial surveys.Two small bivalve-dominated assemblages were dis-covered in Core 415-1, and in Core415A-12, during Leg50 drilling. The material is wholly of small, largely frag-mented specimens, which are specifically indeterminate.Still, it provides important data for environmental inter-pretations of sediments deposited at the base of the con-tinental slope off northwestern Africa. The ages deter-mined from these assemblages, although not as preciseas those determined on the basis of microfossils in thiscore, are compatible.

The bivalves discussed herein come from small coresamples and washings kindly provided to me by Dr. W.V. Sliter of the U.S. Geological Survey, Menlo Park,California. Doubtless, much additional macrofossil ma-terial is still contained in the core at these and otherlevels but is not now available for study; for this reason,the following is a preliminary report. Work on DSDPmacrobiotas has been partially supported by the Smith-sonian Research Foundation under a grant to study the

evolution of Mesozoic-Cenozoic marine ecological unitsin a variety of settings, including the deep sea. Speci-mens discussed in this report are housed in the U.S. Na-tional Museum type collections, in the Department ofPaleobiology.

BIVALVIA FROM SAMPLE 415A-12-1, 106-110 cmThis thin interval of middle-Cenomanian mudstone is

part of a slumped sequence of turbidites and is richlyfossiliferous; fragments, and rarely whole shells, of bi-valves not exceeding 13 mm in maximum diameter,strongly dominate the macrofossil assemblage. Fishscales and small fish teeth, echinoderm debris, and asingle fragment of an arthropod claw or spine collective-ly compose about 5 per cent of the sample. Planktonicand benthic foraminifers, and a few ostracodes, occurin all samples and are moderately well preserved. Thebivalve fraction is mainly composed of a small speciesof Ostrea (Ostrea) Linné. Most of the ostreid materialconsists of worn, semipolished, and obviously trans-ported shell fragments of small adult(?) specimenswhich originally did not exceed an inch in maximumdiameter. Very thin, fragile, juvenille shells of Ostrea(Ostrea) sp. down to 0.75 mm maximum diameter arealso present in the sample, completely preserved. Thesewere probably carried in suspension in turbidity flowswhich resulted in transport, breakage, and rounding ofthe larger specimens. Ostreid shell material is wellpreserved, without obvious solution or recrystallizationstructures. Dense laminated calcite predominates, butthin prismatic calcite layers are preserved in some speci-mens between laminated layers.

Ostrea (Ostrea)Core 12 oyster specimens are typical of Ostrea

(Ostrea) Linné (Figure 1), as emended by Stenzel (1971,p. N1138-N1139); species identification is impossiblewithout larger and more complete valves and isespecially difficult in such simple forms as these. Thespecies is characterized by small size, low convexity ofboth valves, elongate-ovate shell form with height one-third greater than length, blunt slightly curved beaksand umbos, and nearly smooth shells bearing only faintirregular concentric lamellae, some of which are slightlyraised above the shell surface. Internally, the right valveis characterized by a broad, low, subtriangular resilifer,gently arched in the center, and bearing prominentgrowth lines; the resilifer occupies the entire dorsalmargin; auricles are absent. The posterior-adductor-muscle insertion area is large, reniform or comma-

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Figure 1. Enlarged cameralucida drawings and (A,B) composite sketches ofLeg 50 bivalves showing principal morphologic features of the bestpreserved Cenomanian Sample 415A-12-1,106-110 cm: (A-D), and Pleis-tocene Sample 415-1-2, 12 cm: (E) specimens. A,B—Right valve interiorand left valve exterior (× 6), respectively, o/Ostrea (Ostrea) sp. showing(A) adductor muscle scar, resilfier, anachomata, and (B) external formand ornamentation. USNM 242998. C—Laternula ? sp. (×4), internalmold, right valve, showing features of shape and ornament observedbefore partial breakage, original broken edge of specimen (jagged line),and inferred shell form (dashed line). USNM 243001. D—Schedotrape-zium sp., interior view, composite mold of right valve (× 12.4), showingtypical subtrapezoidal form, strong lateral teeth, possible weak cardinaldentition, and posteroventrally directed, weakly defined sulcus. USNM243000. E—Tindaria sp. aff. T. miniscula Sanders and Allen, interiorview (Y.42.4) drawn before breakage of specimen during mounting,showing shell form, chevron-shaped taxodont teeth and interveningsockets spread widely along thin hinge plate, and small pit below beaks inbroad edentulous area between anterior and posterior rows of teeth.USNM 243002.

shaped, situated just posteroventral to the shell center,and well impessed. Low, rounded chomata are situateda short distance on either side of the resilifer, on thedorsolateral commissural margins of the right valve;thin ridglets mark the trace of the chomata on the lateraledge of the shell. The left valve interior is unknown. At-tachment scars on the left valve are small and flat; at-tachment was to small, smooth or finely costate shellfragments.

Small Ostrea (Ostrea) sp. of this type are commonthroughout the Cretaceous and Cenozoic sequences. Al-though their ecology has not yet been fully documentedin ancient deposits, Kauffman (1967, 1979, p. 238)noted that small ostreids similar to those found in Core12 dominated communities of midshelf depth (100-300feet) in the western interior seaway of North America,and that deeper environments were characterized bysmall oysters of the genus Pycnodonte (Phygraea). Thisobservation is compatible with the idea that sedimentscontaining the oysters are largely turbidity-currentdeposits derived from the continental shelf of NorthAfrica. Epibionts and endobionts are wholly lacking on

the oyster shells recovered from Core 12, supporting amiddle to outer-shelf origin for the shells, rather than ashallow-water origin. The specimens of O. {Ostrea) sp.are catalogued in the U.S. National Museum collections(USNM 242998).

Ostreinae (Costate sp.), gen. and sp. indet.Two fragments of a larger, thicker-shelled oyster

(genus indeterminate) appear to belong to a differentspecies. The shell is subovate to subrounded (on thebasis of juvenile-growth-line trace), and of moderatelylow convexity. Its surface bears moderately strong radi-al costae with a slightly sinuous trace. Costal crests arerounded and irregularly noded in adult growth. Thecostae are close set, about 1.5 mm apart on the largestfragment from Core 12. The shell form, and the spac-ing, subregular development, and rounded crests of thecostae all suggest assignment to a costate species withinthe Ostreidae (Ostreinae) — perhaps some species ofcostate Ostrea (Ostrea), or ancestral Cretaceous speciesof Turkostrea or Cubitostrea (see illustrations inStenzel, 1971, figs. J111-J117) — rather than to the

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more strongly plicate or costate Lophinae. Like otheroyster-shell fragments, these are broken, worn, andsemipolished; they are obviously transported and rede-posited. Costate Cretaceous oysters are typical ofshallow to moderately deep shelf depth communities(Kauffman, 1967, 1969). These specimens were proba-bly carried from the North African shelf zone down thecontinental slope by Cenomanian turbidity currents.Specimens are catalogued as USNM 242999.

Three additional species of Cenomanian bivalves arerepresented only by internal and external molds inmudstone; the originally aragonitic shell material iscompletely dissolved or preserved as small, highlyetched patches on the molds. All are single valves whichare nearly complete, or very large fragments showingmost of the morphologic features. These molds havebeen subsequently compressed and somewhat de-formed. This fact, and lack of external shell features,makes determination even to genus difficult. The taxato which these fossils seem most similar are all found inboth shelf and deeper water (bathyal) environments.These forms conceivably could have lived near the depo-sitional site at the base of the continental slope, andbeen redeposited as part of a turbidity flow; more likely,being thin light shells, they were carried largely insuspension from shelf zones by turbidity currents.

Schedotrapezium sp.The best preserved non-ostreid bivalve is a single

composite mold of a small, subtrapezoidal right valvewith part of the dentition preserved (USNM 243000)(Figure 1). The specimen is 4.3 mm long, 2.1 mm high,and about 0.5 mm wide, strongly prosocline, and mod-erately prosogyrous. The small, blunt beak projects on-ly slightly above the dorsal margin and is situated one-fifth to one-quarter of the length from the anterior mar-gin. The anterior flank is triangular and strongly pro-jecting in front of the beaks; the ventral margin isbroadly and asymmetrically rounded, the ventroposteri-or margin is moderately rounded, and the dorsoposte-rior margin is truncated in two places. The dorsolateralmargins slope very gently away from the beak on eitherside and are straight to very slightly curved. Very faintgrowth lines constitute the only sculpture. A weak pos-teroventrally directed sulcus divides the posterior flankfrom the disc. A long, narrow, prominent lateral toothextends subparallel to each dorsolateral margin for al-most its entire length; traces of two very small cardinalteeth are possibly represented below the beaks.

This unusually shaped bivalve resembles few com-parable known genera; among them the shell form andprobable dentition most closely match SchedotrapeziumStewart, a genus previously known from the Upper Cre-taceous of North America, West Africa(?), andEurope(?). The African occurrence is herein confirmed.The specimen is probably a juvenile, and is too poorlypreserved for species determination.

Laturnulai sp.Another bivalve taxon is represented by one internal

mold of a small, very thin-shelled right valve (USNM

243001) with the posterior and (during preparation)anterior margin broken (Figure 1). The shell height is8.5 mm; before breakage a length of 11-12 mm wasnoted. Shell morphology suggests close similarity toLaternula Röding. The shell appears to have been elon-gate-ovate, with length one-third greater than height,anterior and posterior margins moderately rounded (ac-cording to growth line trace), and ventral and dorsalmargins slightly curved. Convexity is very low; the sur-face is covered by irregularly developed, faint tomoderately strong concentric growth lines. The beak issuberect to slightly opisthogyrate and projects slightlyabove the dorsal margin. The umbo is moderately in-flated, subtruncated posteriorly, and terminates in ashort but prominent umbonal fold between the dorso-central and dorsoposterior flanks of the shell. No denti-tion or ligamenture is preserved; true Laternula is eden-tulous. Keen and Cox (1969, fig. F23-2) illustrate atypical example of the genus. The generic identity of theCore 12 specimen is questioned because of poor preser-vation, and species determination is impossible. Theknown Cretaceous Laternula are mainly associated withmiddle-shelf to upper-slope communities, but some areknown from inner-shelf deposits. Laternula is knownfrom Cretaceous to recent time.

Phelopteriat sp.Two large fragments of a very thin-shelled, subquad-

rate, smooth, flat pteriacean bivalve were found in theCore 12 Cenomanian sample. The hinge line on onefragment has broad, shallow, widely spaced multivincu-lar ligament pits, suggesting a genus within the Bakevel-liidae, especially Phelopteria Stephenson; this genus isknown from the Cretaceous of North America, WestAfrica, and Europe, and is a common component ofcommunities throughout shelf and upper-slope environ-ments. The large, flat posterior auricle preserved on oneCore 12 fragment closely parallels that described forCretaceous Phelopteria, as do other features of theexternal shell morphology. The specimens are too in-complete for species comparison, but they are about thesize and shape of small upper Cenomanian species (P.minuta, P. quadrate sp. aff. P. minuta) recently de-scribed from the Greenhorn Formation of northwesternOklahoma by Kauffman and Powell (1973, p. 50-52, pi.8, figs. 4, 6-8). The specimens are catalogued in the U.S.National Museum (USNM 243002).

Summary

Five taxa of small bivalves are present in Sample12-1, 106-110 cm: Ostrea {Ostrea) sp.; Ostreinae(costate sp.), gen. and sp. indet.; Schedotrapezium sp.;Laternula?. sp.; and Phelopteriat sp. All were previous-ly known from the Cretaceous and some are restrictedto the Upper Cretaceous, suggesting an age compatiblewith that inferred from microfossils. The material is, ingeneral, poorly preserved and composed of worn shellfragments from larger specimens, and nearly completeshells of small (less than 10 mm) and (or) very thin-shelled specimens. Collectively, the assemblage suggeststhat the fauna lived in middle- to outer-shelf en-

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vironments during the Cretaceous, e.g., comparable todeeper portions of epicontinental seas like those of thewestern interior of North America and western and cen-tral Europe. The assemblage is not similar to Cretaceous(Kauffman, 1976) or modern abyssal or bathyal assem-blages (e.g., Knudsen, 1967, 1970). These factors implysecondary transport of the assemblage from shelf dep-ths, probably by suspension in turbidity currents, to thebase of the continental slope of northwest Africa duringthe Cenomanian; this is compatible with physical-sedi-mentology evidence.

A PLEISTOCENE TINDARIA FROMSAMPLE 415-1-2, 12 cm

A single, very well preserved, small bivalve shell wasobtained from washings at this locality by V. W. Sliter.Fortunately, the specimen was identified, described,and sketched (Figure 1) before it broke into severalpieces during final examination. The specimen is trans-lucent and extremely thin shelled, elongate-ovate in out-line, with the length (1.9 mm) nearly one-third greaterthan the height; inflation is moderate and greatest in theumbonal area. The shell surface is smooth except forvery faint concentric growth lines, subregularly andclosely spaced, over the entire shell. A narrow, lanceo-late, well-defined, moderately excavated escutcheonbears the external ligamental groove. The ligament issituated posterior to the moderately projecting, in-curved beak. Internally, a very thin hinge plate supportswidely spaced, chevron-shaped (taxodont) teeth withdeep sockets in each tooth between the lateral extensionsof each chevron and broad interspaces between theteeth; five posterior teeth, and four anterior teeth, onlythree of which are chevron shaped, occur on the hingeplate. A small semicircular depression lines below thebeak of the right valve on the hinge plate in the center ofa broad edentulous area between anterior and posteriorrows of teeth.

This delicate shell clearly belongs to the genus 77/2-daria Bellardi, 1875, and is only slightly different fromT. miniscula Sanders and Allen, a living species dredgedfrom the Angola Basin, Atlantic II cruise, station 197,at depths between 4565 and 4595 meters. The DSDPspecimen is slightly more elongate, with the beak slight-ly less projecting, and has one less tooth on the posteriorpart of the hinge plate than T. miniscula (see Sandersand Allen, 1977, text-figs. 23, 27). Otherwise the twoare identical in size, inflation, ornamentation, generaloutline, and nature of the dentition. The Pleistocene bi-valve is either a population variant of T. miniscula, orvery closely related and ancestral to it.

The most important aspect of the occurrence of 77/2-daria sp. aff. T. miniscula is that it is an in situ lower-bathyal and (or) upper-abyssal mollusk associated withPleistocene sediments in Hole 415 that is similar in mor-phology, preservation, and ecology to modern forms.Its known depth range (4565 m to 4595 m) is somewhatgreater than the present water depth of Site 415 (2794 m)but the environments are essentially compatible. T.miniscula may range higher in the bathyal zone; its

geographic and depth limits are still poorly known, itbeing a rare and only recently discovered form.

CONCLUSION

Fossil mollusks, although rare and small in DSDPcores, provide useful and interesting biostratigraphicand ecological information. Even in the sparse andpoorly preserved samples studied here, both the Ceno-manian (Upper Cretaceous) and Pleistocene molluscanassemblages contained taxa which provided an agedesignation compatible with the designations made onthe basis of the microfossils. Both provided an indepen-dent check on the environment of deposition of the sedi-ments. The Cenomanian assemblage of Ostrea (Ostrea)sp., Ostreinae (costate) sp., Schedotrapezium sp.,Laternulal sp., and Phelopterial sp. is clearly ofmiddle- to outer-shelf origin and obviously has been re-worked by turbidity currents downslope into deeper-water environments. The Pleistocene Tindaria sp. aff.T. miniscula is characteristic of living lower-bathyal toupper-abyssal forms occurring at depths similar to thoseof the drill site; it was not transported and redeposited.

I urge a greater awareness of the value of macrofos-sils in the interpretation of DSDP cores and hope thatthis contribution will encourage others to intensify thesearch in DSDP core material for macrofossils and thusincrease the number of discoveries made. One cannothelp but speculate that a wealth of important biologicaldata bearing on age determination, environmental anal-ysis, and on understanding the evolution of oceanic,especially deep-sea, communities, lies hidden in existingand forthcoming DSDP core material.

REFERENCES

Kauffman, E. G., 1967. Coloradoan macroinvertebrate as-semblages, central western interior, United States. InKauffman, E. G. and Kent, H. C. (Co-convenors), Paleo-environments of the Cretaceous Seaway: a symposium,Colorado School of Mines, Spec. Publ., preprints ofpapers, p. 67-143.

, 1969. Cretaceous marine cycles of the western inte-rior, Mountain Geoi, v. 6, 226-245.

, 1976. Deep-sea Cretaceous macrofossils: Hole317A, Manihiki Plateau. In Schlanger, S. O., Jackson, E.D., et al., Initial Reports of the Deep Sea Drilling Project.,v. 33: Washington (U.S. Government Printing Office), p.503-535.

Kauffman, E. G. and Powell, J. P., 1977. Part 2. Paleontol-ogy. In Kauffman, E. G., Hattin, D. E., and Powell, J. D.(Eds.), Stratigraphic, paleontologic, and paleoenviron-mental analysis of the Upper Cretaceous rocks of Cimma-ron County, northwestern Oklahoma, Geol. Soc. Am.Mem. 148, p. 47-150.

Keen, M. and Cox, L. R., 1969. Family Laternulidae Hedley,1918. In Moore, R. C. (Ed.), Treatise on invertebrate pale-ontology, Part N, v. 2. Mollusca 6, Bivalvia: Geol. Soc.Am. (Univ. Kansas Press), p. N8; 44-N845.

Knudsen, J., 1967. The deep-sea Bivalvia, Brit. Mus. (Nat.Hist.), Sci. Rept. J. Murray Exped., 193 3# 1934, v. 11, p.237-343.

, 1970. The systematics and biology of abyssal andhadal Bivalvia, Galathea Rept., v. 11, p. 7-241.

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Sanders, H. L. and Allen, J. A., 1977. Studies on the deep sea Stenzel, H. B., 1971. Oysters. In Moore, R. C. (Ed.), TreatiseProtobranchia (Bivalvia); The Family Tindariidae and the on invertebrate paleontology: Part N, v. 3, Mollusca 6,genus Pseudotindaria. Harvard Univ., Bull. Mus. Comp. Bivalvia. Geol. Soc. Am. (Univ. Kans. Press), p. iv +Zooi, v. 148, p. 23-59. N953-N1224.

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