Eutheria, the Placental Mammals

Placental mammals are a rather diverse group, with nearly 4000 described species, mostly rodents and bats (photos at left). The placental mammals include such diverse forms as whales, elephants, shrews, and armadillos. They are also some of the most familiar organisms to us, including pets such as dogs and cats, as well as many farm and work animals, such as sheep, cattle, and horses. And humans, of course, are also placental mammals.

Placental mammals all bear live young, which are nourished before birth in the mother's uterus through a specialized embryonic organ attached to the uterus wall, the placenta. The placenta is derived from the same membranes that surround the embryos in the amniote eggs of reptiles, birds, and monotreme. mammals. The term "placental mammals" is somewhat of a misnomer because marsupials also have placentae. The difference is that the placenta of marsupials is very short-lived and does not make as much of a contribution to fetal nourishment as it does in eutherians, as "placental mammals" are known scientifically.

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navigate deeper into the Eutherian groups by selecting Systematics.