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Journal of Paleontology

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PALEOECOLOGY OF GIANT INOCERAMIDAE (PLATYCERAMUS) ON A SANTONIAN (CRETACEOUS) SEAFLOOR IN COLORADO

¹ Department of Geological Sciences, Indiana University, 1001 E. Tenth Street, Bloomington 47405,
² Department of Geology, University of South Florida, Tampa, SCA 528, 4202 East Fowler Avenue, Tampa 33620-5201,
³ Naturhistorische Museum Basel, Augustinergasse 2, 4001-Basel, Switzerland,
⁴ Ecopetrol, Calle 37, No. 8-43 Piso 8, Bogata, Colombia,
⁵ Paleontological Institute Ivana Rakovca, Slovenian Academy of Sciences and Arts, Novi trg 2, 1000 Ljubljana, Slovenia

Giant Middle Coniacian to Lower Campanian Platyceramus Seitz is among the largest Cretaceous bivalves, commonly reaching an axial length of over 1 m, and occasionally over 2–3 m in size. The genus is characterized by its large size, very low convexity, normal inflation limited mostly to the umbonal area, and flattened flanks. It is especially common in moderately deep calcareous shale facies, as well as in chalks and limestones of the Niobrara Formation and equivalents. Preferred facies contain abundant pyrite, elevated total organic carbon (TOC), and very low biotic diversity. The genus maintains its giant size in these facies, and becomes more abundant. It clearly prefers dysoxic facies. As such, it probably is chemosymbiotic; photosymbiosis is almost ruled out because of inferred water depths of 200–350 m. It is also found more sparsely, and of smaller size, in oxygenated facies, including shoreface sandstone. The study area contains over 81 giant-sized Platyceramus platinus (Logan, 1898) on a single bedding plane; there are very few small ones. This allows spacing, orientation, and size analysis to be performed on an adult population.