Depending on classification scheme, there are approximately 5,500 species (5,490, according to the IUCN Red List) of mammals, distributed in about 1,200 genera, 153 families, 29 orders The early synapsid mammalian ancestors, a group which included pelycosaurs such as Dimetrodon, diverged from the amniote line that would lead to reptiles at the end of the Carboniferous period. Although they were preceded by many diverse groups of non-mammalian synapsids (sometimes misleadingly referred to as mammal-like reptiles), the first true mammals appeared 220 million years ago in the Triassic period.
Mammals are a class of air-breathing vertebrate animals characterized by the (1) possession of hair, (2) three middle ear bones -''malleus'', ''incus'' and ''stapes'', (3) a neocortex, and (4) mammary glands functional in mothers with young. Mammalian fossils, however, are identified by the presence of the ''incus'' and ''malleus'' bones in the middle ear. Most mammals also possess sweat glands and specialized teeth, and the largest group of mammals, the '''placentals''', have a placenta which feeds the offspring during gestation. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumblebee Bat to the 33-meter (108-foot) Blue Whale. (Source:Wikipedia)