Why are no eutherians indigenous to australia?

Question

Ivy Acosta · Accepted Answer

Continental drift led to early geographic isolation of Australia almost 100 million years ago: when only prototherian and metatherian mammals roamed the earth. No eutheria ever evolved in the continent.

Evolutionary history of marsupial mammals in the form of fossils is not consistent, but fossils of such early mammals from mesozoic era are excavated from most parts of the world.
Australia separated from erstwhile southern landmass of Gondwana with very early types of prototherian and metatherian (=marsupial) mammals.

In the isolated island continent of Australia, primitive mammalian stock never took a turn to evolve towards eutherian possibility but marsupials evolved, diverged in several lines and eventually occupied all the niche. In the rest of the world marsupials could not thrive during coenozoic era in competition with fast evolving eutherians.
In fact, outside Australia, marsupials completely disappeared except a few in South American continent. So Australia (as well as NewZealand) remained as the only natural home of prototherian and marsupial mammals.

Only two eutherian mammals could eventually reach Australian continent later by migrating from nearby land masses: these were the rats and bats!

Oliver Antoine · Answer

undefined No eutherians are indigenous to Australia because the continent was isolated from other landmasses for millions of years, preventing the natural migration or introduction of eutherian mammals, which evolved elsewhere. As a result, Australia's native mammals are mostly marsupials and monotremes.