CHRONOSTRATIGRAPHIC DATA FROM PATAGONIA AND ANTARCTICA ADDS PRECISION TO FAUNAL TIME FRAMES, BUT CREATES UNCERTAINTY AS TO THE ORIGIN OF THE EOCENE ANTARCTIC LAND MAMMAL FAUNA

Hypotheses concerning the origin of the land mammal paleofauna from the La Meseta Formation on Seymour Island, Antarctic Peninsula from paleofaunas in South America have changed through time as greater resolution on both faunal composition and time constraints on South American Land Mammal Ages or SALMAs have been obtained.

New chronostratigraphic data from Paleogene-aged Patagonian formations have a direct bearing on time framework of SALMAs, as these data place paleofaunas more precisely. The Peligran SALMA is now constrained within C28n (63-64 Ma) and the Carodina faunal zone is now constrained to between 61.5- 63.6 Ma, both are now, Early Paleocene in age. A new radiometric datum in the Los Flores Fm. places the Las Flores fauna and the Itaboraian SALMA into Early Eocene (50-53 Ma). An updated chronostratigraphic framework for the Eocene La Meseta Fm. of Antarctica places the La Meseta paleofauna (LMP) at 51-52 Ma. This results in LMP also being of an Itaboraian-age.

The Itaborai paleofauna (Rio de Janeiro, Brazil) and the Las Flores paleofauna (LFP; Comodoro Rivadavia, Patagonia) exhibit a high degree of generic level faunal similarity (70.8%). The La Meseta paleofauna (LMP) of the same age, has a low faunal similarity with the Las Flores paleofauna (LFP; 6.5%). The latitudinal separation between the two South America paleofaunas (Patagonia and Brazil) is 2° greater than the latitudinal separation between Antarctic and Patagonian paleofaunas. The low faunal similarity between LMP and the LFP indicates a significant degree of endemism and isolation of the Antarctic mammals since their dispersal from Patagonia.

The origin of the LMP from Paleogene Patagonian faunas comes into question as the Itaboraian-age paleofaunas from South America are contemporaries and isolated from the Antarctic fauna. We now have to look towards Early Paleocene paleofaunas as the source for the LMP, but the generic similarity between the Early Paleocene Patagonian paleofaunas and the LMP is low (13.8%). This suggests a heretofore, unknown paleofauna in South America within the 9 million year mid-to-late Paleocene time gap which gave rise to the Antarctic mammalian paleofauna.