Holotheria

Holotheria is a clade of mammals that includes all living placental mammals and marsupials, as well as their extinct relatives. This group represents the majority of modern mammalian diversity. Holotherians are characterized by a number of skeletal features, including a fused clavicle and a double-rooted lower premolar. Their evolutionary history is complex, with evidence suggesting a diversification of holotherian lineages in the Mesozoic Era. The split between the marsupial and placental lineages is thought to have occurred during the Cretaceous period, with both groups surviving the K-Pg extinction event. Fossil evidence, such as the discovery of early marsupial and placental-like mammals, helps scientists to reconstruct the evolutionary relationships within Holotheria. The study of holotherians continues to shed light on the origins and diversification of mammals.