Metatheria

Metatheria Huxley, 1880 is a grouping within the animal class of Mammalia. It's nearly synonymous with the cohort of Marsupialia Illiger, 1811, though it can be seen as 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 Huxley, 1880. This is nearly synonymous with the cohort of Placentalia Owen, 1837. Both taxon are united within the supercohort of Theria[?] Parker & Haswell, 1897.

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