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Metatheria, first proposed by Huxley in 1880, is a grouping within the animal class of Mammalia. It is nearly synonymous with the cohort of Marsupialia (Illiger, 1811) though it is slightly wider. It also contains the nearest ancestors of marsupial mammals.

The earliest known representatives are known from the uppermost Lower Cretaceous of North America.

The closest living relatives of the metatherids are within Eutheria (also erected by Huxley in 1880). This is nearly synonymous with the cohort of Placentalia (Owen, 1837). Both are united within the supercohort of Theria (Parker and Haswell, 1897), which contains all living mammals bar only the monotremes.