Metatheria Huxley, 1880
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia (Illiger, 1811) though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.
The earliest known representative, Sinodelphys, is from the Lower Cretaceous of China.
The closest relatives of the metatheres are the Eutheria (also erected by Huxley in 1880). Both are conventionally united as infraclasses within the subclass Theria (Parker and Haswell, 1897), which contains all living mammals except monotremes.
During development, metatherians produce a yolk sac placenta and give birth to "larval like" offspring. These offspring have under developed posterior limbs (the pes can be webbed), and following birth they migrate to the marsupium where they attach to a nipple. The mouth of newly born metatherians is fused laterally, but open medially; this forms an "O" shaped mouth in which the mothers nipple fits, then swells to secure the offspring into place for further development and growth.
The Greek words meta- and theria roughly means the "other beasts", in contrast with Eutheria ("true beasts").
Metatherians first appeared in the Cretaceous Period, with such forms as Deltatheridium and Asiatherium. Some stem metatherians persisted well into the Neogene Period before becoming extinct. Examples of these include the borhyaenids and herpetotheriids. Crown marsupials, the one branch of Metatheria that survives today, diversified close to the time of extinction at the end of the Cretaceous.
McKenna MC & Bell SK, (1997), Classification of Mammals Above the Species Level. Columbia University Press.
Source: Wikipedia, Wikispecies: All text is available under the terms of the GNU Free Documentation License