The diphyletic origin of marsupials

There is ample evidence that one group of marsupials, the ameridelphians, arose during the late Early Cretaceous in the Northern Hemisphere and subsequently dispersed to South America. It has long been accepted that marsupials subsequently crossed Antarctica to reach Australia, becoming a distinct group, the australidelphians. The Australian fossil evidence pertaining to this view was notoriously poor until recently. In 1997, the first known Mesozoic Australian tribosphenic mammal was discovered in late Early Cretaceous deposits. Subsequently two more tribosphenic mammalian genera and species were collected at the same locality. After appearing, did these tribosphenic mammals simply go extinct or did they give rise to the marsupials subsequently found there, the australidelphians? Morphological analysis of these tribosphenic mammals suggests that one of the three species in particular might be annectant to marsupials. This coupled with the geographic distribution of the australidelphians and ameridelphians, which are distinguished from one another by a consistent, distinctly different calcanea is suggestive of a separate origin of the two groups.