Cranial Anatomy of the Earliest Marsupials and the Origin of Opossums

BACKGROUND: The early evolution of living marsupials is poorly understood in part because the early offshoots of this group are known almost exclusively from jaws and teeth. Filling this gap is essential for a better understanding of the phylogenetic relationships among living marsupials, the biogeographic pathways that led to their current distribution as well as the successive evolutionary steps that led to their current diversity, habits and various specializations that distinguish them from placental mammals. METHODOLOGY/PRINCIPAL FINDINGS: Here we report the first skull of a 55 million year old peradectid marsupial from the early Eocene of North America and exceptionally preserved skeletons of an Oligocene herpetotheriid, both representing critical groups to understand early marsupial evolution. A comprehensive phylogenetic cladistic analysis of Marsupialia including the new findings and close relatives of marsupials show that peradectids are the sister group of living opossums and herpetotheriids are the sister group of all living marsupials. CONCLUSIONS/SIGNIFICANCE: The results imply that North America played an important role in early Cenozoic marsupial evolutionary history and may have even been the center of origin of living marsupials and opossums. New data from the herpetotheriid postcranium support the view that the ancestral morphotype of Marsupialia was more terrestrial than opossums are. The resolution of the phylogenetic position of peradectids reveals an older calibration point for molecular estimates of divergence times among living marsupials than those currently used.