The last inoceramid bivalves in Antarctica

The last inoceramid bivalves in Antarctica are no younger than mid- to late Campanian in age. They occur within the Herbert Sound and Rabot Point members of the Santa Marta Formation, which is the lowermost of four component formations within the Late Cretaceous-earliest Tertiary Marambio Group, James Ross Basin. These inoceramids comprise an unusual giant form which is assigned herein toAntarcticeramus rabotensisgen. et sp. nov. Moderately inequivalve and gryphaeoid in form,Antarcticeranusis characterized by a larger and more inflated right valve with a rounded-trapeziform to obliquely elongated outline. Almost all right valves have a sharply defined, shelf-like anterior margin. The left valve is less obviously obliquely elongated and only weakly inflated. Ornament appears to have consisted of low, commarginal folds. There are indications of a possible phylogenetic connection betweenAntarcticeramusandSphenoceramusJ. Böhm, 1915. Antarcticeramus rabotensisgen. et sp. nov. is extremely abundant at certain levels within the Rabot Point Member. It can be demonstrated that the majority of these specimens occur in life position and show a preferred orientation with respect to a horizontal substrate. This is taken as evidence of a positive response to prevailing water currents (rheotaxis), perhaps to enhance the process of filter feeding. Associated sparse macrofossils and abundant trace fossils indicate aerobic bottom conditions at mid-shelf depths. It is postulated that the evolution of giant size was primarily an antipredatory device. However, just as large, benthic predators were radiating in the latest Cretaceous, seawater temperatures in the southern high latitudes were beginning to fall. In the end, secretion of such large calcitic shells may have become physiologically impractical.

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