Métathériens

Métathériens is a term used in paleontology and evolutionary biology to refer to a group of extinct mammals that lived during the Mesozoic Era. These animals are considered to be either very early members of the therian lineage or closely related to the common ancestor of therian mammals. Therians are the group that includes all marsupials and placentals, which are the dominant mammal groups alive today. The fossil record for early mammals is sparse, making the precise evolutionary placement of many Mesozoic mammals a subject of ongoing research and debate.

The term "métathérien" is not as commonly used as "therian" or specific clade names like "Australosphenida" or "Monotremata" when discussing early mammal evolution. However, when it is used, it generally encompasses a broader collection of early mammals that exhibit some therian-like characteristics but may not be directly ancestral to modern therians. Key features often considered include the structure of the ear bones, the dentition, and the limb anatomy. Studying these ancient mammals helps scientists understand the diversification of mammals and the evolutionary pathways that led to the diverse forms we see today, including the eventual split between monotremes, marsupials, and placentals. The existence of métathériens, or similar basal therian relatives, highlights the complex and branching nature of mammalian evolution during the age of dinosaurs.