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Mammals (class Mammalia //) are a clade of endothermic amniotes distinguished from reptiles and birds by the possession of hair, three middle ear bones, mammary glands, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart.
The mammals include the largest animals on the planet, the rorquals and other large whales, as well as some of the most intelligent, such as elephants, some primates, including humans, and some of the cetaceans. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs. The largest group of mammals, the placentals, have a placenta, which enables feeding the fetus during gestation. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 33-meter (108 ft) blue whale.
The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,416 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders. In 2008 the IUCN completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period.
In some classifications, the mammals are divided into two subclasses not counting fossils: the Prototheria, that is, the order Monotremata; and the Theria, or the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria, and include all living metatherians as well as many extinct ones; the placentals are the crown group of the Eutheria.
Except for the five species of monotremes (egg-laying mammals), all modern mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers, are first Rodentia: mice, rats, porcupines, beavers, capybaras, and other gnawing mammals; then Chiroptera: bats; and then Soricomorpha: shrews, moles and solenodons. The next three orders, depending on the biological classification scheme used, are the Primates including the humans; the Cetartiodactyla including the whales and the even-toed hoofed mammals; and the Carnivora, that is, cats, dogs, weasels, bears, seals, and their relatives.
While mammal classification at the 'family' level has been relatively stable, several contending classifications regarding the higher levels—subclass, infraclass, and order, especially of the marsupials—appear in contemporaneous literature . Much of the recent change reflects the advances of cladistic analysis and molecular genetics. Findings from molecular genetics, for example, have prompted adopting new groups such as the Afrotheria and abandoning traditional groups such as the Insectivora.
The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals (Therapsida) in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of the non-avian dinosaurs 66 million years ago.
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