Eutherians

Eutheria is a taxon containing the placental mammals, such as humans. The sister group of Eutheria is Metatheria, which includes marsupials and their extinct relatives.

Origin of the word
The name Eutheria comes from the Greek words eu- "well[-developed]" and ther "beast". When Eutheria was introduced by Thomas Henry Huxley in 1880, he meant it to be broader 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.

Characteristics of eutherians
Nevertheless, all living eutherians are placental mammals. This means that a eutherian fetus is nourished during gestation by a placenta. Eutherians are also viviparous, meaning that the offspring are carried in the mother's uterus until fully developed.

Differences from other mammals
Because of this, eutherians are different from other mammal groups such as monotremes and marsupials which are not placental. Monotremes, for instance, lay eggs which protect developing young until they are fully developed. Marsupials give birth to partially-developed young who then migrate to a special pouch in the mother's body in which the young continue their development. (Some exceptions do exist. Bandicoots for instance, which are marsupials, develop small placenta-like structures during gestation.)

Earliest example
The earliest known eutherian species is the extinct Eomaia scansoria from the Lower Cretaceous of China. It is a member of Eutheria, but the hips of the animal were too narrowly built to have allowed the birth of well-developed young. Thus it is unlikely that a placenta greatly contributed to the development of E. scansoria's young before they were born.

Members of Eutheria are found on all continents and in all oceans.

Compare and contrast

 * Metatheria
 * Prototheria

Notes and references
Detailed cladogram of Eutheria from Proceedings of the National Academy of Sciences, February 4, 2003, vol. 100, no. 3, pp. 1056-1061.

Плацентни Euteri Placentálové Höhere Säugetiere Placentuloj Placentalia Placentalia Viši sisavci Méhlepényesek Eutheria Eutheria 真獣下綱 진수하강 Héijer Mamendéieren Placentalia Placentadieren Placentale pattedyr Eutheria Łożyskowce Placentários Eutheria Плацентарные Višji sesalci Högre däggdjur ยูเธอเรีย 真獸下綱 真獸亞綱