Prototheria

Prototheria is a historical term that was used to refer to egg-laying mammals. It traditionally encompassed the monotremes, the platypus and the echidnas, and stood as the opposite of Theria, the group containing marsupials and placental mammals. In current mammalian taxonomy, Prototheria is not recognized as a valid, monophyletic clade; instead, all living egg-laying mammals are placed within the group Monotremata. As a result, Prototheria is largely treated as an outdated or paraphyletic concept.

Monotremes are distinctive among mammals for several primitive features that set them apart from therian mammals. They are oviparous, laying leathery eggs that hatch outside the mother's body. After hatching, the young are fed milk produced by the mother, but monotremes generally lack teats; milk is secreted onto the skin or fur and lapped up by the young. Monotremes also possess a cloaca, a reptile-like reproductive opening, and many characteristics retained from early mammalian ancestors. Their body temperature is comparatively low for mammals, and they retain electroreceptors in their bills or snouts, which aid in locating prey.

The two living groups are the platypus (Ornithorhynchus) in Australia and the echidnas (Tachyglossus and Zaglossus) in Australia and New Guinea. Fossil evidence indicates monotreme lineages diverged from other mammals early in mammalian evolution. Today, monotremes are the rare, surviving representatives of a once more diverse lineage and occupy a unique position in the study of mammalian evolution.