Metatheria
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 Female Eastern Grey Kangaroo with a joey in her marsupium
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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 representatives are from the uppermost Lower Cretaceous of North America.
The closest living 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").[1]
[edit] See also
[edit] References
- McKenna MC & Bell SK, (1997), Classification of Mammals Above the Species Level. Columbia University Press.
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