Therians

Therians, or Theria, are a major subclass of living mammals that includes the two infraclasses Metatheria (marsupials) and Eutheria (placental mammals). They are distinguished from the monotremes, a separate lineage that includes the platypus and echidnas and lays eggs. Therian mammals give birth to live young and typically nourish them with milk produced by mammary glands.

The therian lineage diverged from monotremes after their split in the mammal family tree, with Metatheria and Eutheria separating during the Late Jurassic to Early Cretaceous period. The fossil record shows early therians giving rise to the two major lineages, which then diversified into the broad array of species seen today. This diversification has led to the vast ecological range occupied by therians, from terrestrial to aquatic environments.

In terms of reproduction and development, therians are primarily viviparous, meaning they give birth to relatively developed offspring. Placental therians (Eutheria) typically have a complex placenta and longer gestation, while marsupials (Metatheria) generally have shorter gestations and extended postnatal development outside the womb, often in a pouch. Across therian groups, mothers nurse their young with milk, though lactation strategies vary among species.

Therians are the dominant group of mammals in most parts of the world, with Placentalia accounting for the majority of mammalian diversity and global distribution. Marsupials remain especially prominent in Australia and the Americas, where several distinctive lineages have persisted. Together, therians encompass a wide range of forms, from small insectivores to large whales and primates.