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Temporal range: Late Jurassic – Holocene, 160–0 Ma
|Lycopsis longirostris, an extinct sparassodont, a relative of the marsupials|
Thomas Henry Huxley, 1880
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is a slightly more inclusive group than the marsupials; it contains all of the living mammals with abdominal pouches (most female marsupials) as well as all animals more closely related to them than to placental mammals. Some female metatherians, like the shrew opossum, lack a pouch.
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, which have underdeveloped posterior limbs (the pes can be webbed), migrate to the pouch where they attach to a nipple. The mouth of newly-born metatherians is fused laterally, but opens medially; this forms an "O" shaped mouth in which the mother's nipple fits; it then swells to secure the offspring into place for further development and growth.
Though metatherians diverged in the Jurassic period from the ancestors of placentals, the earliest known metatherian fossil, Sinodelphys, is from the Lower Cretaceous of China, about . The marsupials, the metatherian crown group, diversified shortly after the mass extinction at the end of the Cretaceous. Some metatherians that may not have been marsupials persisted well into the Neogene Period before becoming extinct. Examples of these include the borhyaenids and herpetotheriids.
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