Mammals are a group of vertebrate animals constituting the class Mammalia, 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 from which they diverged in the Carboniferous, over 300 million years ago. Around 6,400 extant species of mammals have been described divided into 29 orders. The largest orders, in terms of number of species, 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 synapsids were sphenacodonts, a group that included the famous Dimetrodon.