Austral Portals and the Paleobiogeography of Antarctic Land Mammals

During the past quarter-century, the list of Late Cretaceous/Paleogene terrestrial vertebrates from Antarctica has increased greatly in number but relatively little in breadth. In addition to dinosaurs, birds, and mammals, other groups that might be expected on various biogeographical grounds to have lived on The Bottom, such as squamates and amphibians, have not yet been recovered. This may have much to do with the availability of suitable contexts: the general lack of exposures in the Antarctic, along with the near-absence of recognized terrestrial/freshwater facies, severely limits the prospects. Nevertheless, paleontological exploration remains critical to the effort to ascertain Antarctica's possible role in the dispersal of a number of austral vertebrate groups with enigmatic distributions, ranging from ceratophryine frogs to abelisaurid dinosaurs to microbiotheriid marsupials. Relevant to this search is the role of so-called “portals”, including the “Scotia Portal” that connected southernmost South America and the Antarctic Peninsula via terranes now dispersed around the margins of the Scotia Sea plate (South Georgia, South Orkney, etc.), and the highly controversial “Enderby Portal” which allegedly connected Madagascar/India and East Antarctica by way of the Kergulen Plateau and/or the Gunnerus Ridge/South Madagascar Rise. This paper will concentrate on the significance of the mammal record for interpreting the reality of these portals as candidate landbridges. The published mammalian fauna of Seymour Island currently consists of 12 species (some not yet formally named) representing at least 7 orders; all are probably or certainly rooted in known early Cenozoic South American groups, suggesting that West Antarctica was a sink, not a source. However, other fossils in existing collections from this island that have never been described or assigned to specific groups suggest additional possibilities.