Metatheria

Metatheria Temporal range: Lower Cretaceous – Holocene, 125 mya to present
Lycopsis longirostris, an extinct Sparassodont, relatives of the marsupials
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Theria
Infraclass:	Metatheria Thomas Henry Huxley, 1880
Orders and infraclasses
†Alphadontia †Deltatheroida †Asiadelphia †Sparassodonta Marsupialia Didelphimorphia Paucituberculata Australidelphia Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia †Yalkaparidontia

Metatheria is a group in the class Mammalia which contains the marsupials and the sparassodonts.

The group was first proposed by Thomas Henry Huxley in 1880. It is nearly synonymous with the earlier taxon Marsupialia, but it also contains the nearest fossil relatives of marsupial mammals.

Metatheria differ from all other mammals in their dental formula. This usually includes five upper and four lower incisors, a canine, three premolars, and four molars.^[1][2]

The earliest known representative, Sinodelphys, is from the Lower Cretaceous of China.^[3]

The closest relatives of the metatheres are the Eutheria (also erected by Huxley in 1880). Both are together united as infraclasses in the subclass Theria. The Theria contains all living mammals except monotremes.^[4]

During development, metatherians produce a yolk sac placenta and give birth to 'larval-like' offspring.

These offspring have underdeveloped rear limbs, and after birth they migrate to the marsupium where they attach to a nipple. The mouth of newly born metatherians forms an "O" shape into which the mother's nipple fits. Then the nipple swells to secure the offspring in place.

The Greek words meta- and theria roughly means the "other beasts", in contrast with Eutheria ("true beasts").

1. ↑ Szalay, Frederick S. (2006). Evolutionary history of the marsupials and an analysis of osteological ... Cambridge University Press. ISBN 9780521025928.
2. ↑ Clemens W.A. 1968. Origin and early evolution of marsupials. Evolution 22 1, 1-18.
3. ↑ Luo, Zhe-Xi; et al. (2003). "An early Cretaceous tribosphenic mammal and metatherian evolution". Science. 302 (5652): 1934–1940. Bibcode:2003Sci...302.1934L. doi:10.1126/science.1090718. PMID 14671295. S2CID 18032860. Retrieved 2010-12-27.
4. ↑ O'Leary, Maureen A. et al 2013. The placental mammal ancestor and the post–K-Pg radiation of placentals. Science 339 (6120): 662–667. [1]