AskDefine | Define placental

Dictionary Definition

placental adj : of or having or occurring by means of a placenta; "all mammals except monotremes and marsupials are placental mammals" [ant: aplacental] n : mammals having a placenta; all mammals except monotremes and marsupials [syn: placental mammal, eutherian, eutherian mammal]

User Contributed Dictionary

English

Adjective

  1. Of or pertaining to the placenta, or to the Placentalia
  2. Having a placenta

Noun

  1. Any animal that is a member of the Placentalia

Extensive Definition

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.

See also

Compare and contrast

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.
placental in Bulgarian: Плацентни
placental in Catalan: Euteri
placental in Czech: Placentálové
placental in German: Höhere Säugetiere
placental in Esperanto: Placentuloj
placental in Spanish: Placentalia
placental in French: Placentalia
placental in Croatian: Viši sisavci
placental in Hungarian: Méhlepényesek
placental in Indonesian: Eutheria
placental in Italian: Eutheria
placental in Japanese: 真獣下綱
placental in Korean: 진수하강
placental in Luxembourgish: Héijer Mamendéieren
placental in Ligurian: Placentalia
placental in Dutch: Placentadieren
placental in Norwegian: Placentale pattedyr
placental in Occitan (post 1500): Eutheria
placental in Polish: Łożyskowce
placental in Portuguese: Placentários
placental in Romanian: Eutheria
placental in Russian: Плацентарные
placental in Slovenian: Višji sesalci
placental in Swedish: Högre däggdjur
placental in Thai: ยูเธอเรีย
placental in Chinese: 真獸下綱
placental in Contenese: 真獸亞綱
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