protothérien

Prototherian is a term sometimes used in paleontology and evolutionary biology to refer to early, primitive therian mammals or groups that exhibit a mosaic of therian and non-therian (monotreme-like) characteristics. The term is not as widely established or precisely defined as other taxonomic classifications and is often used in a more descriptive rather than formal systematic sense.

These hypothetical or observed transitional forms would bridge the gap between ancestral mammal groups and the crown groups of therian mammals, which include marsupials and placentals. Prototherians might display features such as a more developed jaw articulation compared to earlier synapsids, but still retain some reptilian-like skeletal elements or reproductive strategies. Their existence is inferred from the fossil record, where intermediate forms are sought to understand the evolutionary pathways of mammalian diversification.

The concept of prototherians helps in visualizing the gradual acquisition of key mammalian traits, such as endothermy, lactation, and a more complex brain. It highlights that evolution is not a series of sudden leaps but a continuum of change, with intermediate stages showing a mix of ancestral and derived characteristics. While not a formal taxonomic rank, the idea of prototherians is valuable for conceptualizing the early stages of therian evolution.