Placental Mammals

∞ generated and posted on 2016.11.09 ∞

Subset of eutherians that bear live young and lack pouches.

Placental Mammals are mammals that employ well developed placentas during the gestation of their young. The vast majority of mammals in the world today are placental mammals. We are placental mammals.

Among extant mammals neither the monotremes nor the marsupials are placental mammals. The non-monotreme and non-marsupial mammals nonetheless are described as eutherians, which is a mammalian clade that includes the clade Placentalia, i.e., the placental mammals.

The following video, though OK, is not the most exciting in the world, but in two minutes and six seconds it does provide an overview of what placental mammals are all about, at the level of order (that is, one major taxonomic category below that of class Mammalia):

By far and away the majority of mammals on Earth today are placental mammals.

Note the following evolutionary progression is seen among tetrapods going basically from amphibians to mammals:

External fertilization in which eggs are thus released from the mother's body prior to their fertilization (as seen in most amphibians and fish) →
Internal fertilization in which the eggs are not released until following their fertilization and most embryonic development occurs outside of the mother's body within eggs (as seen in the monotremes and amniotes generally) →
Internal fertilization that is followed by live birth of substantially immature young (as seen in the marsupials) →
Internal fertilization that is followed by live births of substantially developed young (placental mammals).