Metatherians

Metatherians are a clade of mammals that includes all living marsupials and their extinct relatives. They are one of the three major groups of mammals, alongside monotremes and eutherians (placentals). Metatherians are distinguished by a reproductive system in which the young are born at a relatively early developmental stage and typically continue development in a pouch or attached to a teat. Most possess a single functional middle ear bone (the auditory bulla) formed from the ectotympanic bone, a feature that differentiates them from eutherians.

The fossil record shows that metatherians originated in the Late Jurassic or Early Cretaceous, first appearing in what is now North America and Asia. By the Cretaceous they had diversified into several lineages, some of which crossed into South America, Africa, and Antarctica. After the end‑Cretaceous extinction, metatherians persisted largely in the Southern Hemisphere, with the order Diprotodontia radiating in Australia and the order Didelphimorphia remaining diverse in the Americas. A few surviving groups, such as the opossums of the family Didelphidae, retain a broad geographic range from North to South America.

Metatherians exhibit a variety of ecological adaptations, ranging from arboreal insectivores to large herbivores such as kangaroos. Their dentition typically includes a set of incisors, canines, premolars, and molars that reflect dietary specialization. Conservation status varies: many Australian species are threatened by habitat loss and introduced predators, while some opossum species thrive in urban environments. Ongoing research into metatherian genetics and fossil discoveries continues to refine understanding of their evolutionary relationships and biogeographic history.