Late Jurassic inoceramid bivalves from the Antarctic Peninsula and their stratigraphic use
The majority of inoceramid bivalves occurring in Late Jurassic temperate realm assemblages can probably be assigned to the genus Retroceramus. Although there are a few transitional forms, Retroceramus can generally be distinguished from the better-known genus Inoceramus by features of both the external and internal shell morphology. The replacement of Retroceramus by Inoceramus in the Early Cretaceous may reflect the adoption of an epibyssate mode of life, and probably represents one of a series of endo- to epibyssate life habit transitions that occurred during the evolutionary history of the Inoceramidae.Four common Late Jurassic species of Retroceramus form the basis of an inoceramid zonation that can be used for local correlations within western Antarctica, and regional correlations throughout the Southern Hemisphere. Isolated exposures in the Antarctic Peninsula can be correlated with the Fossil Bluff Formation (Kimmeridgian-Albian) of eastern Alexander Island, and it should eventually be possible to correlate the latter unit with the extensive Jurassic Latady Formation (?Bajocian-Tithonian) of the Lassiter and Orville coasts. Used in conjunction with existing cephalopod biozones, this zonation scheme indicates that the lower levels of the Fossil Bluff Formation correlate with the Lower, Middle, and Upper Spiti Shales of the Himalayas and both the Ohauan and Puaroan stages of the New Zealand Jurassic. Tentative correlations can also be made with areas such as South America, New Caledonia, Indonesia, and Western Australia.The occurrence of genera such as Retroceramus, Anopaea, and Buchia in western Antarctica does not necessarily indicate the existence of a late Jurassic anti-Boreal faunal realm. Available evidence suggests that the distribution of benthic faunas at this time was probably controlled by a series of distinctive sedimentary facies which extended around much of the southern margins of Gondwana, rather than by pronounced climatic zonation.