Metatheria

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Metatheria
Temporal range: Late Jurassic[1][2]Holocene, 160–0 Ma
Lycopsis longirostris, an extinct sparassodont, a relative of the marsupials
Scientific classification e
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Supercohort:Theria
Infraclass:Metatheria
Thomas Henry Huxley, 1880
Subgroups
 
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Metatheria
Temporal range: Late Jurassic[1][2]Holocene, 160–0 Ma
Lycopsis longirostris, an extinct sparassodont, a relative of the marsupials
Scientific classification e
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Supercohort:Theria
Infraclass:Metatheria
Thomas Henry Huxley, 1880
Subgroups

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 newborn 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.

The combination of the Greek elements meta- and theria in this usage roughly means the "sort-of-beasts" or " behind-beasts", in contrast with Eutheria ("true-beasts").

Evolutionary history[edit]

Though metatherians diverged in the Jurassic period from the ancestors of placentals,[3][4] the earliest known metatherian fossil, Sinodelphys, is from the Lower Cretaceous of China, about 125 million years ago.[3] The marsupials, the metatherian crown group, diversified shortly after the mass extinction at the end of the Cretaceous.[5] 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.

See also[edit]

References[edit]

  1. ^ Hugall, A.F. et al. (2007) Calibration choice, rate smoothing, and the pattern of tetrapod diversification according to the long nuclear gene RAG-1. Syst Biol. 56(4):543-63.
  2. ^ Graves JA, Renfree MB (2013) Marsupials in the age of genomics. Annu Rev Genomics Hum Genet
  3. ^ a b Luo, Zhe-Xi; Yuan, Chong-Xi; Meng, Qing-Jin; Ji, Qiang (2011). "A Jurassic eutherian mammal and divergence of marsupials and placentals". Nature 476: 442–445. doi:10.1038/nature10291. PMID 21866158. Retrieved 2013-04-29. 
  4. ^ Pyron, R. Alexander (2010). "A likelihood method for assessing molecular divergence time estimates and the placement of fossil calibrations". Systematic Biology 59 (2): 185–194. doi:10.1093/sysbio/syp090. Retrieved April 29, 2013. 
  5. ^ O'Leary, M.A., Bloch, J.I., Flynn, J.J., Gaudin, T.J., Giallombardo, A., Giannini, N.P., Goldberg, S.L., Kraatz, B.P., Luo, Z.-X., Meng, J., Ni, X., Novacek, M.J.,Perini, F.A., Randall, Z.S., Rougier, G.W., Sargis, E.J., Silcox, M.T., Simmons, N.B., Spaulding, M., Velazco, P.M., Weksler, M., Wible, J.R., and Cirranello, A.L. (2013). "The placental mammal ancestor and the post–K-Pg radiation of placentals". Science 339 (6120) 662–667. doi:10.1126/science.1229237 PMID 23393258