Theria: Characteristics, Diversity, Types, Examples

Theria is a subclass of mammals, including placental mammals (Eutheria) and marsupials (Metatheria). Placental mammals have a placenta, nourish their young internally, and include diverse groups like primates and rodents. Marsupials give birth to undeveloped young and continue their development in a pouch.

It evolved around 160 million years ago from non-therian mammals. The exact transition is still under study. The earliest therian fossils date back to the Late Jurassic and Early Cretaceous periods. 

Further, it underwent diversification and expansion during the Cretaceous and Paleogene periods and represented the majority of mammalian species.