Mammal

The mammals are the class of vertebrate animals primarily characterized by the presence of mammary glands[?] in the female which produce milk for the nourishment of young; the presence of hair or fur; and which have endothermic or "warm blooded" bodies. The brain regulates endothermic and circulatory systems including a four chamber heart. Humans are mammals. Phylogenetically, the Mammalia are defined as the last common ancestor of monotremes (e.g. echidnas) and therian mammals (e.g. hedgehogs), and all of this last common ancestor's descendants.

While most mammals give birth to live young, there are a few mammals - the monotremes - that lay eggs. Live birth also occurs in a variety of non-mammalian species; thus it is not a diagnostic characteristic for class Mammalia. Endothermy[?] is also present in many non-mammals, primarily birds. While monotremes do not have nipples, they do have mammary glands, meaning that they do meet all conditions for inclusion in the class Mammalia. It should be noted that the current trend in taxonomy is to emphasize common ancestry; the diagnostic characteristics are useful for identifying this ancestry, but if, for example, a cetacean were found that had no hair at all, it would still be classed as a mammal.

Mammals have three bones in each ear and one (the dentary) on each side of the lower jaw; all other vertebrates with ears have one bone (the stapes) in the ear and at least three on each side of the jaw. A group of therapsids called cynodonts had three bones in the jaw, but the main jaw joint was the dentary and the other bones conducted sound.

Mammals belong among the amniotes, and in particular among a group called the synapsids, distinguished by the shape of their skulls. They developed from therapsids, more specifically the eucynodonts[?], 220 million years ago during the Triassic period. During the Meosozic[?] period they diversified into the three main groups found today, i.e. monotremes, marsupials, and placentals. They remained small and shrew-like throughout the era, but radiated rapidly after the Cretaceous-Tertiary extinction event 65 million years ago.

The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata, but this theory has been refuted. However, Eutheria and Metatheria are often used in paleontology, especially with regards to mammals of the Mesozoic.