placentals

Placentals, or placental mammals, form a large clade within the mammals characterized by the presence of a chorioallantoic placenta that allows prolonged gestation inside the female uterus. They exclude the monotremes (egg-laying) and marsupials, and include most living mammal species.

In placentals, the placenta serves as the interface for nutrient, gas, and waste exchange between mother and developing fetus, connected to the embryo by the umbilical cord. This extended gestation supports the development of relatively large and complex offspring. Live birth is common, followed by lactation.

Placentals are extremely diverse and widespread, comprising the majority of mammal species. They include bats, rodents, primates, carnivores, ungulates, whales, and many others, occupying terrestrial, aerial, and aquatic habitats.

Classification within placentals is organized into four major early-diverging clades or superorders: Afrotheria, Xenarthra, Euarchontoglires, and Laurasiatheria. Molecular data have clarified relationships among these groups, though some uncertainties remain.

The origin of placentals traces to the therian mammals in the Late Mesozoic, with the earliest definite placentals appearing in the late Cretaceous. The evolution of the placenta allowed longer development inside the uterus and facilitated diversification in a variety of ecological niches.