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Eutheria:
Eutheria (Greek words eu- and theria mean "genuine beasts")is a taxon containing the placental mammals. The sister group of Eutheria is Metatheria, which includes marsupials and their extinct relatives. When Eutheria was introduced by Thomas Henry Huxley in 1880, he meant for it to be more broad in definition than its precursor Placentalia. Some use Eutheria as a total group which includes the crown group Placentalia and extinct mammals which are closer to Placentalia than to Marsupialia. The majority of living mammals are placental. The other two extant groups of mammal are Monotremata and Marsupialia; there are other groups of extinct mammals. Eutherians are distinguished from other mammals in that the fetus 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 offspring are carried in the mother until fully developed. Eutherians, like the related metatherians, are viviparous. Members of Eutheria are found on all continents and in all oceans. The earliest known eutherian species is Eomaia scansoria from the Lower Cretaceous of China. It is undoubtedly a member of Eutheria, 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 role in the development of young.
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