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Australia possesses a unique assemblage of mammal species, of which over 80% are endemic.
This high level of endemism is a result of Australia's long period of isolation from other continents, since its separation from Gondwana about 40 million years ago. From that time until about 15 - 20 million years ago, when Australia moved closer to South-East Asia, Australian mammals were evolving independently from those in the rest of the world.
The mammal groups that show the highest level of endemism are the monotremes and marsupials, and both were present in Australia when it was still part of Gondwana. Today, monotremes are only found in Australia and New Guinea while marsupials are found in Australia, New Guinea, Indonesia, South America and North America. New Guinea and Australia have relatively similar mammal faunas because they were once one land mass until 15 million years ago when the land bridge between them subsided.
The levels of endemism among Australia's native eutherian mammals, bats and most rodents, are lower than those found in the marsupials and monotremes. It is most likely that rodents entered Australia as recently as 1 - 4 million years ago during the Pleistocene and Pliocene Epochs when Australia was closer to South-East Asia. The origins of Australia's bats are still being unravelled, although it appears to involve migration from Asia and other parts of Gondwana.