THE LATE CRETACEOUS DINOSAURS OF ANTARCTICA: A UNIQUE WINDOW ONTO A POLAR GONDWANAN FAUNA APPROACHING THE CRETACEOUS/PALEOGENE BOUNDARY

In many ways, Antarctica remains the ‘final frontier’ in our understanding of Mesozoic dinosaur paleobiodiversity and paleobiogeography. Due to the continent’s remoteness and near-total ice cover, the fossil record of non-avian dinosaurs and Mesozoic birds from Antarctica is the least characterized of any major landmass. Nevertheless, discoveries within the past several decades have gradually brought aspects of Antarctica’s Mesozoic dinosaur fauna to light. All Cretaceous dinosaur discoveries from the continent have come from sediments deposited within the James Ross Basin (JRB) of the James Ross Island Group of the Antarctic Peninsula. Though often fragmentary, especially in the cases of non-avian dinosaurs, these finds have revealed a diversity of taxa. Thyreophorans are represented by the parankylosaur Antarctopelta from the Campanian–Maastrichtian Snow Hill Island Formation (SHIF) and isolated, generically indeterminate material from the Maastrichtian López de Bertodano Formation (LdBF). Ornithopods are documented by at least two probable elasmarians from the SHIF, Trinisaura and Morrosaurus, plus indeterminate and undescribed material from that unit and indeterminate hadrosauroid and probable elasmarian material from the LdBF. A titanosaurian sauropod is indicated by a caudal vertebra from the SHIF, whereas non-avian theropods are known from a distal tibia from the Coniacian Hidden Lake Formation and the holotypic partial skeleton of the large-bodied paravian (possibly unenlagiine dromaeosaurid) Imperobator from the SHIF. The most complete JRB dinosaur specimens are those of ornithuromorph birds, including the holotypic partial postcranium of Antarcticavis from the SHIF, two incomplete skeletons (one preserving the syrinx) and an isolated skull of the anseriform Vegavis from the LdBF, the Polarornis type specimen from the LdBF, and indeterminate and undescribed material from both units. Most elements of the JRB dinosaur fauna exhibit strong similarities to penecontemporaneous assemblages from southern South America, suggesting a degree of biotic connectivity between that landmass and Antarctica during the Late Cretaceous.