A single placental tooth found in Australia suggests both placental and marsupial mammals dispersed to Australia, but only marsupials survived. This gives rise to the possibility that marsupials out-competed placentals in Australia, a pattern not observed elsewhere.
Apart from this and the radiation of marsupial forms prior to and following the isolation of Australia, little is known about marsupial evolution between the late Palaeocene (55mya) and the late Oligocene (about 24mya).
Position of the continents during the Miocene (23 to 5.33 myr), showing Australia is now isolated from the rest of the southern continents.
During the early Miocene (23-16mya), arboreal and terrestrial marsupials dominated the Australian rainforest environment, and marsupial radiation continued to flourish. Climate change to cooler, drier conditions 15mya led to the replacement of rainforests with savannah woodlands and eventually widespread grasslands. Such changes were accompanied by a decrease in overall marsupial diversity, but did favour radiation in groups such as grazing kangaroos.
The terminal Pleistocene extinctions of terrestrial mammal megafauna that accompanied increased aridity and predation from human populations strongly affected the marsupials. 15 out of 16 giant marsupial genera became extinct, causing a decline in marsupial diversity leading up to the present day.
Evolution in the Americas.