The young of most Marsupials (Infraclass Metatheria) are born extremely undeveloped compared to other mammals; after birth, they fasten themselves more or less permanently to teats in the mother’s brood pouch. The epipubic bones that support this pouch, along with a specific dentition of four molars and five incisors in the upper jaw, are diagnostic traits of this diverse group. Some of the better-known marsupials are the American opossums, the Australian kangaroos and wallabies, the Thylacine, Tasmanian Devil, bandicoots, koalas, wombats, and possums.
The earliest marsupial fossils date from the Early Cretaceous, 110 million years ago, in North America. From there, they quickly spread to South America in one direction and Europe and Africa in another (where they eventually died out). The South American groups differentiated and radiated into Antarctica, where they diversified further, spreading to Australia by the Early Oligocene, 30 million years ago, when it became isolated from the other southern landmasses. All of the 282 living species are endemic to Australia and New Guinea or South America, with the sole exception of the Virginia opossum (Didelphis virginiana) of North America, a relative newcomer in the Pleistocene.
Marsupials achieved their greatest diversity in Australia, where they faced no competition from eutherian (nonmarsupial) mammals. The largest was the extinct, hippopotamus-sized Diprotodon optatum, which may have lingered
long enough for the earliest Australians to prey on them 18,000–6,000 years ago. Other oddities were the horse-sized, huge-clawed Palorchestes, another Pleistocene survivor; Thylacoleo, the marsupial equivalent of a lion, which had sabre-tooth incisors, huge molars, and clawed thumbs; a giant wombat, Phascolonus; and the sthenurine kangaroos, which may have looked like giant rabbits.
Of the eleven cryptids in this section, only one is South American (the Macas Mammal);
The Phantom Kangaroo is an out-of-place visitor reported with some frequency in North America and Europe; the others are from Australia or New Guinea and may represent survivals of supposedly extinct species or folk memories of them. A marsupial origin for the Australian Big Cat has also been suggested,but this animal looks too much like a real cat to argue otherwise with any confidence.