Marsupials underwent adaptive radiation in South America, and from here several groups spread across the southern supercontinent Gondwana into Antarctica around 70 to 55 mya. This explains why the only known Antarctic marsupials, of middle Eocene age and found on Seymour Island on the Antarctic Peninsula, resemble South American marsupials of the same age rather than Australian groups.
The at least 55 million year old Tingamarra assemblage in south-eastern Queensland contain the oldest known Australian marsupials. The presence of several didelphimorphian-type marsupials and microbiotheriids in this assemblage (both South American marsupial orders) indicate that some South American marsupials arrived in Australia between 70 and 55 million years ago.
Position of the continents during the Eocene (55.8 to 33.9 myr), when marsupials were able to migrate across the supercontinent Gondwana,
consisting of South America, Antarctica and Australia.
Following the complete break up of Gondwana into South America, Antarctica and Australia, the marsupials diverged into the two distinct groups of marsupials seen today, one in the Americas and the other in Australia.
However the seven extant marsupial orders today evolved before the break up of Gondwana. This explains why the monophyletic clade Australidelphia includes all extant Australian marsupials and the South American microbiotheriids. The distribution of orders today is not to do with where they diverged but reflect the patterns of migration before Gondwana broke up.
The prevailing hypothesis is that Australian marsupials had an ancestor that migrated over from South America, with microbiotheriids a phylogenetic line from this common ancestor that remained in South America.
However a recently published paper by Beck et al. (2008) indicates the debate over the evolutionary history of marsupials, and suggests the whole migration pattern from South America to Australia may be incorrect. In their paper, Beck and colleagues recognise Djarthia, found in the Tingamarra assemblage in Australia, as the oldest unequivocal australidelphian. From here, it is suggested that Australian marsupial radiation may have followed, with the South American microbiotheriids a result of migration from Eastern Gondwana west to South America.
Evolution in Australia.
Evolution in the Americas.