Placentalia and Eutheria are terms used to describe major groupings within the animal class of Mammalia. The two terms are almost synonymous, although Eutheria has a slightly wider scope.

The majority of living mammals are placental: the other two extant groups are the Monotremata and the Marsupialia; there are other groups of extinct mammals. The Placentalia are distinguished from other mammals in that the foetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals. (Bandicoots are a conspicuous exception to this rule.) Placental mammals are viviparous.

Eutheria is a taxon introduced by Thomas Henry Huxley in 1880, containing the placental mammals and the nearest ancestors of placental mammals (which are known only from the fossil record). The name itself means 'true beasts'. The closest living relatives of the eutheres are part of the subclass Marsupialia.

The earliest known Placentalia species is Eomaia from the Lower Cretaceous of China. It is undoubtably a member of Placentalia, but the hips of the animal are too narrowly built to have allowed the birth of well-developed young. This strongly suggests that a placenta played little rôle in the development of young.

References:

• McKenna MC & Bell SK, (1997), Classification of Mammals Above the Species Level. Columbia University Press.