Vertebrate fossils from the Late Eocene of Seymour Island, Antarctica.
[sciencythoughts.blogspot.com]
The Southern Hemisphere biota has been profoundly influenced by Mesozoic-Cainozoic continental breakup and climatic change. Before its fragmentation, the supercontinent Gondwana facilitated dispersal of terrestrial organisms between now-separated southern landmasses. Early discoveries suggest that Antarctica was central to this pattern of terrestrial movement, acting as a bridge between what is now South America and Australia. This widespread dispersal ended with the final breakup of Gondwana. Through the last part of this breakup, the Antarctic climate shifted from being warm and seasonally wet to increased periods of ice cover by the early to middle Eocene; by the earliest Oligocene (about 33.9 million years ago) Antarctica experienced complete glaciation. Insights into how the Antarctic biota was shaped by tectonic and climatic shifts have come from the upper Eocene La Meseta and Submeseta formations on Seymour Island, the best-studied fossil Vertebrate fauna from Antarctica. This assemblage has been proposed to most closely resemble contemporaneous faunas from Patagonia which were separated from what is now the Antarctic Peninsula by the flooding of the Weddellian Isthmus at the end of the Palaeocene. The fossil record of the La Meseta and Submeseta formations is famously dominated by stem Penguins, including some of the tallest Penguins that ever lived. The non-Penguin Vertebrate fossil record mostly comprises isolated teeth and bones representing an array of Marsupial, Gondwanathere, Ungulate, Cetacean, and other Eutherian Mammals as well as a possible non-Therian Dryolestoid as well as non-Penguin Birds.