Marsupialia

Marsupialia is a traditional taxonomic grouping (often treated as an infraclass or superorder) comprising the marsupials, a lineage of therian mammals known for their distinctive reproductive strategy. In most marsupials, the young are born very early in development and complete growth while nursing in a protective pouch (marsupium) or, in some species, on a belly fold. They possess several distinctive anatomical features, including epipubic bones that support the pouch region and locomotion.

Diversity and distribution: About 330 living species are classified as marsupials. They are distributed primarily in Australia and New Guinea, with a relictual distribution in the Americas. The Americas host opossums of the order Didelphimorphia, while Australasia houses the rest of the marsupial diversity, in orders such as Diprotodontia (kangaroos, koalas, possums, wombats), Peramelemorphia (bandicoots and bilbies), Dasyuromorphia (quolls, dunnarts, Tasmanian devil), Notoryctomorphia (marsupial moles), and Microbiotheria (monito del monte). The group Ameridelphia, historically used for American marsupials, is now viewed as paraphyletic; modern classification emphasizes Australidelphia as the major southern lineage.

Evolutionary history: Marsupials first appeared in the Mesozoic; the isolation of Australia fostered their later diversification there. South American marsupials represent an earlier lineage, with Microbiotheria linking to the Australasian radiation. The fossil record is sparse but indicates a long-standing metatherian presence.

Taxonomy notes: The term Marsupialia is variably used; some authorities prefer Metatheria as the infraclass, with Marsupialia as a clade within it. Living orders include Didelphimorphia, Microbiotheria, Dasyuromorphia, Peramelemorphia, Notoryctemorphia, and Diprotodontia.