Metatheria

Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier cohort 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.

Reference:

• McKenna MC & Bell SK, (1997), Classification of Mammals Above the Species Level. Columbia University Press.