Metatheria

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.Clemens W.A. 1968. Origin and early evolution of marsupials. Evolution 22 1, 1-18.

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 together united as infraclasses in the subclass Theria. The Theria contains all living mammals except monotremes.O'Leary, Maureen A. et al 2013. The placental mammal ancestor and the post–K-Pg radiation of placentals. Science 339 (6120): 662–667. 

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").

Evolutionary history
Metatherians first appeared in the Cretaceous period.  Some stem group metatherians persisted well into the Neogene period before becoming extinct. Crown group marsupials, the one branch of Metatheria that survives today, diversified close to the time of extinction at the end of the Cretaceous.

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