Placentalia

Placentalia, or placental mammals, is a clade within the therians that includes all living placental mammals and their extinct relatives, excluding the monotremes and marsupials. Members give birth to relatively well-developed young after a prolonged gestation in the uterus, where embryonic development is sustained by a chorioallantoic placenta. The term reflects this shared reproductive strategy, but exact membership can vary with phylogenetic interpretation.

The clade comprises the majority of living mammal orders, including Primates, Rodentia, Lagomorpha, Carnivora, Cetartiodactyla, Perissodactyla, and Chiroptera, among others. Contemporary phylogenies subdivide Placentalia into major lineages such as Afrotheria, Xenarthra, and Boreoeutheria (which further divides into Euarchontoglires and Laurasiatheria). Relationships among these groups are areas of active research and are refined by molecular data.

Fossil evidence indicates that placental mammals originated in the Mesozoic, with the crown group Placentalia likely arising in the Late Cretaceous to early Paleocene and expanding after the Cretaceous–Paleogene extinction. Their ecological and physiological diversity—ranging from small insectivores to large herbivores and carnivores—contributed to their broad distribution across ecosystems.

Key features of Placentalia include viviparity with placental nourishment and extended gestation, followed by lactation after birth. In modern classifications, Placentalia is sometimes treated as a crown-group term, while other systems emphasize higher-level clades such as Afrotheria, Xenarthra, and Boreoeutheria to reflect current understandings of mammalian phylogeny.