Paleogene Antarctic land mammals: their biogeographic relationships and the final break-up of Gondwana.

ABELLO, M.A.; POSADAS, P.; ORTIZ-JAUREGUIZAR, E.; GELFO, J.N.; CHORNOGUBSKY, L.; REGUERO, M.A.

Congreso; 4th International Paleontological Congress; 2014

The Paleogene land fauna of Antarctica is exclusively known from outcrops of La Meseta Formation (Eocene to Oligocene) from Marambio/Seymour Island (Antarctic Peninsula). The faunal association includes several mammalian groups as ungulates (Sparnotheriodontidae and Astrapotheria), marsupials (Polydolopidae, Derorhynchidae and Microbiotheria) and non-therian mammals (Sudamericidae and Dryolestoidea).This taxonomic composition is similar to those of some Paleogene land mammal assemblages from the southernmost part of South America (i.e. Patagonia), thereby, several hypotheses were proposed to account for the biogeographic history of the Antarctic taxa, as well as for the timing of the paleogeographic events that could be implied in the shaping of the Antarctic land mammal association. To identify the biogeographic events (dispersion, vicariance) that could explain the observed distribution patterns of Antarctic land mammals, we selected those groups for which a phylogenetic hypothesis was available. Dispersal-Vicariance analyses were performed with RASP software for therian mammals (i.e. ungulates and marsupials). The analysis of Astrapotheria and Sparnotheriodontidae yields a widespread ancestral distribution, including South America and Antarctica, since the lower Palaeocene up to middle-late Palaeocene, when a vicariant event took place. In contrast, Polydolopidae marsupials show a distribution restricted to Patagonia during the lower and middle Palaeocene and a dispersal event from Patagonia to Antarctica followed by a vicariance, both of them during the late Palaeocene. The three analyses resulted in a congruent vicariance during late Palaeocene when the ancestral distributions along Patagonia and West Antarctica were fragmented by the rise of a barrier leading to the geographic isolation and differentiation of Patagonian and Antarctic lineages. In absence of evidence of geographic barriers other than the emerging Drake Passage, the fragmentation of the ancestral range could be explained by the earlier stages of rifting and stretching crustal thinning in the opening of this seaway. From the results obtained we can conclude that the biogeographic histories of sparnotheriodontids and astrapotherians were different from those of polydolopids, but both of them support the hypothesis of an early stage (late Paleocene) of a paleogeogeographic event leading to the development of a shallow epicontinetal sea which eventually led to the opening of the Drake Passage.