Marsupial fossil tracks reveal Eocene arboreal life in Antarctica

Antarctica is the only continent where land mammals are extinct today. Before the Eocene-Oligocene boundary (>38 million years ago), however, the "white continent" had a much milder climate that sustained a diverse fauna of terrestrial mammals. Several discoveries of fragmentary Eocene fossils suggest that Antarctica acted as a land bridge connecting South America to Australia, most notably marsupials. However, fossil mammals are scarce and restricted to a single location-Seymour Island. Here we present two well-preserved fossil tracks which are the second Antarctic fossil mammal outside Seymour Island and the oldest fossil tracks assigned to an opossum-like animal in the Cenozoic. They were found in the Fossil Hill Formation on King George Island (lower-middle Eocene) and exhibit possible opposable thumbs, suggesting an arboreal behavior observable in microbiotheriid and some extant didelphid marsupials living in South America today. They further coincide in time with the rise of scansorial lifestyle in several mammalian lineages. This ichnologic record offers a more comprehensive look into the dispersion and ecology of mammals through the Early Cenozoic of Antarctica, evidencing that small arboreal marsupials inhabited the South Shetland Islands during the Eocene. In addition, the new finding indicates that Antarctica was not merely a corridor, but a rich habitat of dense and productive forest canopies during the middle Eocene.