Placentalia
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Afrosoricida |
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.) They reproduce sexually, and the "child" is carried in the mother until fully developed. Placental mammals are viviparous. Some examples of Placental Mammals are simians and leopards. Only females carry a placenta. They live in many areas of the world including the Americas, and parts of Asia.
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.
Categories: Mammals