Prototheria (class Mammalia) A subclass that comprises the extinct orders Docodonta, Triconodonta (although their inclusion in the subclass is controversial), and Multituberculata, as well as the order Monotremata, members of which (platypus and echidnas) still survive. Prototherians are distinguished from therians mainly by the skull wall, which in prototherians has a small squamosal and alisphenoid, and by their teeth. The two subclasses began to diverge from a common stock during the late Triassic.

Prototheria A subclass of mammals – the monotremes – that lay large yolky eggs. It contains only the duckbilled platypus and the spiny anteater. After hatching, the young feed on milk from simple mammary glands inside a maternal abdominal pouch. In the anteater the eggs are also incubated in this pouch, while the platypus builds an underground nest. Adult monotremes have no true teeth. Their skeleton resembles that of a reptile, and although they are warm-blooded the body temperature is somewhat variable. They are believed to have originated at least 150 million years ago.