Prototheria (/ˌproʊtɵˈθɪəriə/; from Greek πρώτος, prōtos, first, + θήρ, thēr, wild animal) is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals; see Yinotheria § History of classification.Most of the animals in this group are extinct. The egg-laying monotremes are known from fossils of the Cretaceous and Cenozoic periods; they are represented today by the platypus and several species of echidna.