METATHERIA
Back to SpecMarsupials are the first of the two great groups of therian (live-bearing) mammals, distinguished by their tendency to expel the embryo from the womb at a very early stage, and then nurse the infant externally while it develops. Marsupials originated in the early Cretaceous, possibly in North America, and then spread around the globe. Today, marsupials are most numerous in Australia, where almost every mammal belong to that order. They also exist in great numbers in South America and some dwell in Asia, but our concern here is the marsupials of North America.
SAMPLE TAXA:DIDELPHIDAE
All of North America's native marsupials belong to the large and varied family Didelphidae, which on Home-Earth is represented by opossums. These marsupials are distinguished by the fact that they give partial birth to an embryo from one womb into another, where it develops for a time before graduating to a pouch. Didelphids are very similar to the marsupial precursors, such as Astrodon, and most have retained the ancient ways of the arboreal insectivore. There are a few exceptions, however.The Pliocene and Pleistocene brought with them massive environmental changes that did much to loosen the dinosaurs' hold on terrestrial niches. The Ice Age replaced the forest and jungle of the dinosaur world with tundra and steppe, driving many clades to extinction and opening opportunities to others.
Many mammalian lineages went through a diversification during the Plio-Pleistocene, and the North American didelphids are no exception, with many new forms evolving that eerily parallel the familiar giant mammals of Home-Earth.
Didelhicaninae
Opossum-hounds are a wide-spread subfamily of the didelphids, and are almost all nocturnal carnivores. These predators are mostly small (half a meter in length), but some, like the Baskerville opossum-hound approach the size of Arel dogs.
- Bat-eared hoek
Thylacoursinae
Thylacoursines, called variously "teddies", "spec-bears", and "non-koalas", are placid, heavily-built didelphids that evolved in North America during the Miocene and invaded South America during the Great Interchange of the Pliocene. Most thylacoursines are omnivorous or herbivorous, but one genus Thylacoursus is actively predatory. The "drop-bear", if it exists, is probably a member of this genus.(Text by Daniel Bensen) (Picture by Matti Aumala)Fig 1: A tentative sketch of the mysterious "drop-bear", possibly a very large species of Thylacoursus.