Marsupials? Monotremes?

Mammals are divided into 2 subclasses based on reproductive techniques:  egg laying mammals (the monotremes), and live birth mammals.  The 2nd subclass is divided into 2 infraclasses:  pouched mammals (the marsupials) and placental mammals.

Australia is home to 2 of the 5 extant species of monotremes (the other 3 are in New Guinea) and the majority of the world's marsupials (the remainder are from New Guinea, eastern Indonesia, and the Americas).

Marsupials evolved in North America, found their way to South America, and then into Australia via Antarctica when the southern continents were joined.

Australia Monotremes:  Platypus, Echidna

Australia Marsupials (there are a lot of them!)

Marsupials in North America:  Virginia Opossum



What's even weirder -- there are almost no placental mammals in Australia.

Australia has indigenous placental mammals from two orders: the bats, order Chiroptera, represented by six families, and the mice and rats, order Rodentia, family Muridae. Bats and rodents are relatively recent arrivals to Australia. Bats probably arrived from Asia, and they are present in the fossil record only from as recently as 15 MYA. Although 7% of the world's bats species live in Australia, there are only two endemic genera of bats. Rodents first arrived in Australia 5–10 MYA and underwent a wide radiation to produce the species collectively known as the "old endemics" rodents. The old endemics are represented by 14 extant genera. About a million years ago, the rat entered Australia from New Guinea and evolved into seven species of Rattus, collectively called the "new endemics."

Since human settlement, many placental mammals have been introduced to Australia and are nowferal. The first was the Dingo; fossil evidence suggests that people from the north brought the Dingo to Australia about 5,000 years ago.^[6] When Europeans settled Australia they intentionally released many species into the wild, including the Red Fox, Brown Hare, and the European Rabbit. Other domestic species have escaped and over time have produced wild populations including the cat, Fallow Deer, Rusa Deer, Chital, Domestic Horse, Donkey, Pig, Domestic Goat, Water Buffalo, and the Dromedary. Only three species of Australia's non-indigenous placental mammals were not deliberately introduced: the House Mouse, Black Rat and the Brown Rat.