The synapsids split into several diverse groups of non-mammalian synapsids-traditionally and incorrectly referred to as mammal-like reptiles or by the term pelycosaurs, and now known as stem mammals or protomammals-before giving rise to therapsids during the beginning of the Middle Permian period. The early synapsids were sphenacodonts, a group that included the famous Dimetrodon.
Mammals are the only living members of Synapsida this clade, together with Sauropsida (reptiles and birds), constitutes the larger Amniota clade. The next three are the Primates (including humans, monkeys and lemurs), the even-toed ungulates (including pigs, camels, and whales), and the Carnivora (including cats, dogs, and seals). The largest orders of mammals, by number of species, are the rodents, bats, and Eulipotyphla (including hedgehogs, moles and shrews). Around 6,400 extant species of mammals have been described and divided into 29 orders. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. A mammal (from Latin mamma 'breast') is a vertebrate animal of the class Mammalia ( / m ə ˈ m eɪ l i.
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