Eutheria

Eutheria is a major clade of mammals that includes all living placental mammals. It is distinguished from the egg-laying monotremes and from the marsupials (Metatheria) as the derived Therian group with a more complex placenta. Within Theria, Eutheria is the sister group to Metatheria, and together they form the clade Theria. The term Eutheria, meaning “true beasts,” reflects their placement as the more derived therian mammals with a complete, uterus-derived placenta.

A defining feature of eutherians is a complex chorioallantoic placenta that supports longer gestation and more advanced development before birth, in contrast to the shorter development seen in many marsupials. This extended in utero development contributes to larger newborns and more developed brains at birth in many lineages, though there is substantial variation across the clade.

Diversity and distribution: Eutheria comprises more than 6,000 described species across numerous orders, making it the most diverse group of living mammals. Living representatives include Primates (monkeys, apes, humans), Rodentia (mice, rats), Chiroptera (bats), Carnivora (dogs, cats, bears), Cetartiodactyla (whales, hippos, cattle, deer), and Perissodactyla (horses, rhinos), among others. The clade also contains the major subdivisions Afrotheria and Xenarthra, reflecting deep evolutionary splits within the group. Eutherians are cosmopolitan, occupying virtually every habitat from deserts to tropical forests to marine environments.

Evolution and fossil record: The earliest eutherians appear in the Early Cretaceous, with fossils such as Juramaia sinensis (~160 million years old) and Eomaia scansoria (~125 million years old) among the oldest candidate members. The group subsequently diversified and achieved worldwide distribution, ultimately giving rise to the vast array of modern placental mammals.