Theria

Theria is a major clade of living mammals that includes the two extant infraclasses Metatheria (marsupials) and Eutheria (placental mammals). They exclude the Monotremata, the monotremes, which lay eggs. Therian mammals constitute the vast majority of living mammals.

In taxonomic terms, Theria is a clade within Mammalia comprising Metatheria and Eutheria. Marsupials include species such as kangaroos, opossums, and quolls, while placentals encompass most other mammals, including primates, rodents, cetaceans, carnivorans, bats, and ungulates.

Reproduction is a defining feature: therians give birth to live young. In placentals (Eutheria), development is typically longer within the uterus, aided by a complex placenta that nourishes the embryo, yielding relatively well-developed neonates. In marsupials (Metatheria), birth occurs after a comparatively short gestation, and the underdeveloped young complete their development in a pouch or attached to a teat, often accompanied by extended lactation.

Evolution and distribution: Theria emerged during the Mesozoic era after the split from monotremes, and their descendants diversified widely. Today therians are cosmopolitan, with marsupials largely confined to Australia and the Americas, and placentals occupying most habitats worldwide.

Diversity: The Therian group includes nearly all living mammals, with notable orders among marsupials such as Diprotodontia, and a broad array of placental orders including Primates, Rodentia, Cetartiodactyla, Carnivora, and Chiroptera.