Although most species of nondomestic mammals in Australia are marsupials, over 100 species -- including seals, bats, and mice -- are not marsupials but placentals
It is clear, however, that these placentals are not native to this island continent: all nonhuman placentals except the dingo, a dog introduced by the first humans that settled Australia, are animals whose ancestors could swim long distances, fly, or float on driftwood
Some marsupials now found in Australia might not be native to that continent, but rather might have been introduced to Australia by some other means.