PALEOBIOGEOGRAPHY OF ANTARCTIC MOLLUSC BIVALVES AND THE ENVIRONMENTAL CHANGES AFTER THE CENOZOIC COOLING (Invited Presentation)

Evolution of modern marine invertebrates in Antarctica is thought to have resulted from cooling and opening of oceanic gateways throughout the Cenozoic. Timing and intensity of environmental events, and changes in ecological structure in local and broad geographic realms have been debated and detailed, especially considering modern representatives in the Southern Ocean. Some of the proposed hypotheses relate extinctions with the long-term Cenozoic glacial history of Antarctica.

In order to understand how Antarctic bivalves responded to environmental changes occurred throughout the Cenozoic, we used two paleobiogeographic approaches to analyze austral bivalve genera, and bivalves from a geological unit related to a glacial phase in Antarctica. We found that the main southern marine biogeographic provinces were defined as early as the beginning of Paleogene, with tectonics as the primary cause. Faunal differences increased throughout the Cenozoic, coupled with the global temperature trend which could be related to climatic cooling. This suggests that one or more environmental factors related to the cooling may have affected the overall mollusc genera.

In order to consider a smaller set of taxa, we selected the bivalve record of the Early Miocene Cape Melville Formation (CMF), a glaciomarine succession that records a maximum glacial event at the base followed by a glacial retreat with distal glaciomarine facies at the top, and correlates to an important glacial event of the global paleotemperature curve. We found that the shelfal bivalves from the CMF were extinct from Antarctic waters after two glacial events in the early Miocene or early Pliocene. Those Cape Melville genera which have extant representatives in Antarctic comprise eurybathic forms that were not affected by the ice-shelf advance during Neogene, with several living species thriving also in the Antarctic periphery. The extinctions would have occurred after the expansion of ice shelves in Antarctica, during the Plio-Pleistocene glaciation event.

Our results suggest that, instead of occurring during the long-term Cenozoic glacial history of Antarctica, selected taxa became extinct in response to local environmental changes related to sharp glacial events. Our results have implications in future scenarios of ice shelf retreat in a warmer world.