Metatheria

Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives. It is one of two groups placed in the clade Theria alongside Eutheria, which contains the placentals. Remains of metatherians have been found on all of Earth's continents.

Australidelphia is a superorder of marsupials encompassing about three-quarters of all living marsupial species, including all those native to Australasia and one South American species, the monito del monte. Unlike other American marsupials, which belong to the Ameridelphia, Australidelphia's lineage emerged in South America, with genetic evidence (retrotransposon insertion sites) showing the monito del monte as its most ancient branch.

Polydolopimorphia is an extinct order of metatherians, closely related to extant marsupials. Known from the Paleocene-Pliocene of South America and the Eocene of Antarctica, they were a diverse group during the Paleogene, filling many niches, before declining and becoming extinct at the end of the Neogene. It is divided into two suborders, Bonapartheriiformes, and Polydolopiformes Most members are only known from jaw fragments, which have their characteristically generally bunodont teeth. The morphology of their teeth has led to proposals that polydolopimorphians may be crown group marsupials,

Stagodontidae is an extinct family of carnivorous metatherian mammals that inhabited North America and Europe during the late Cretaceous, and possibly to the Eocene in South America.

Hondalagus is an extinct genus of mammals that lived during the Middle Miocene epoch (Laventan) in South America. Their fossils were found in the Honda Group at Quebrada Honda, in southern Bolivia. Hondalagus represents the smallest and most specialized member of the extinct family Argyrolagidae.

Xenocynus is an extinct genus of metatherian mammal that lived in Brazil during the Eocene. It is a monotypic genus that contains a single species, Xenocynus crypticus.