Prototheria

Prototheria (, PROH-toh-THEER-ee-ə; from Ancient Greek πρώτος prṓtos "first" and θήρ thḗr "wild animal") is an obsolete subclass of mammals which includes the living Monotremata and to which a variety of extinct groups, including Morganucodonta, Docodonta, Triconodonta and Multituberculata, have also been assigned. It is today no longer considered a valid grouping, but rather a paraphyletic evolutionary grade of basal mammals and mammaliaform cynodonts. 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 four species of echidna. The names Prototheria, Metatheria, and Eutheria (loosely meaning "first beasts", "changed beasts", and "true beasts", respectively) refer to the three mammalian groupings of which we have living representatives. Each of the three may be defined as a total clade containing a living crown-group (respectively the Monotremata, Marsupialia and Placentalia) plus any fossil species which are more closely related to that crown-group than to any other living animals. The threefold division of living mammals into monotremes, marsupials and placentals was already well established when Thomas Huxley proposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880. Initially treated as subclasses, Metatheria and Eutheria are by convention now grouped as infraclasses of the subclass Theria, and in more recent proposals have been demoted further (to cohorts or even magnorders), as cladistic reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank. Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis which defined the group by two supposed synapomorphies: formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoid in therians;...