metatheria

Metatheria is a major clade of mammals that includes marsupials and their extinct relatives. In therian taxonomy, Metatheria is the sister group to Eutheria (placental mammals) within the clade Theria, while the monotremes lie outside Theria. Metatherians are characterized by a combination of skeletal features, such as epipubic bones, and by reproductive traits that distinguish them from placentals.

Most metatherians give birth to relatively underdeveloped young after a short gestation. The young complete their development, often for extended periods, while nursing from a teat. In many species the mother provides a forward-facing pouch in which the young continue to develop, but pouch presence varies and some species have a reduced or absent pouch.

Fossil evidence indicates that metatherians originated in the Late Jurassic to Early Cretaceous, with early forms known from North America. During the Cenozoic, metatherians dispersed and diversified in South America and Australia, with Australia becoming a major center of metatherian evolution after its isolation. The group includes numerous extinct families and a number of living orders, distributed across the Americas and Australasia.

Today, living metatherians comprise seven or more orders: Didelphimorphia (opossums), Paucituberculata (shrew opossums), Microbiotheria (monito del monte), Notoryctemorphia (marsupial moles), Dasyuromorphia (dasyurids and relatives), Peramelemorphia (bandicoots and bilbies), and Diprotodontia (kangaroos, koalas, wombats, possums). The greatest diversity occurs in Australia, while the Americas retain several marsupial lineages such as the opossums.