Mammals are a group of vertebrate animals constituting the class Mammalia, and characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the Carboniferous, over 300 million years ago. Around 6,400 extant species of mammals have been described. The largest orders are the rodents, bats and Eulipotyphla. The next three are the Primates, the Artiodactyla, and the Carnivora.
In terms of cladistics, which reflects evolutionary history, mammals are the only living members of the Synapsida; this clade, together with Sauropsida, constitutes the larger Amniota clade. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that included the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds.