Mammalia

Mammalia (mammals; phylum Chordata) Class of homoiothermic animals in which the head is supported by a flexible neck, typically with seven vertebrae, articulating through two occipital condyles. The lower jaw is composed of one bone only, the dentary, which articulates directly with the skull; the middle ear contains three small bones, two of which are derived from bones in the lower jaw of reptiles. Typically, mammalian teeth are of differing forms (heterodont), they are set in sockets (the-codont), and milk teeth are shed and replaced by a second set. A hard palate separates the nasal cavity from the mouth. Except in monotremes, the egg is small and develops in the uterus; and the young are fed milk secreted by mammae (which give the class its name). The skin has at least a few hairs. Many mammalian features were present in therapsids (mammal-like reptiles) during the Triassic. Mammals are believed to have appeared first toward the end of the Triassic and to have diversified rapidly from the end of the Mesozoic, 100 Ma later, following the mass extinction which marks the Mesozoic—Tertiary boundary 65 Ma ago.