User:PresN/mammals

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 (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles (including birds) 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 (hedgehogs, moles, shrews, and others). The next three are the Primates (including humans, apes, monkeys, and others), the Artiodactyla (cetaceans and even-toed ungulates), and the Carnivora (cats, dogs, seals, and others).

Mammals are divided into three main groupings: the order Monotremata, containing egg-laying species; the infraclass Placentalia, containing the vast majority of extant mammals, for which the fetus is carried in the uterus; and the infraclass Marsupialia, containing the marsupial animals wherein the young are carried in a pouch. Placentalia and Marsupialia are subdivided into superorders, which then contain multiple orders of animals. These orders can contain between one and hundreds of species, grouped into genera and then into families. Orders and superorders are also sometimes grouped into named clades.

Monotremata
Monotremata is the smallest of the three main divisions of mammals, containing only five extant species. It is distinguished from the other two groups in that the monotremes are egg-laying rather than bearing live young, but, like all mammals, the female monotremes nurse their young with milk.

Infraclass Placentalia
Placentalia is one of the three main divisions of mammals, and contains the vast majority of extant species with around 5,500 species. It is distinguished from the other two groups in that the placental animals have fetuses that are carried in the uterus. It contains four superorders: Afrotheria, Euarchontoglires, Laurasiatheria, and Xenarthra; Afrotheria and Xenarthra make up the magnorder Atlantogenata, and the other two the magnorder Boreoeutheria.

Infraclass Marsupialia
Marsupialia is one of the three main divisions of mammals, and contains around 300 extant species. It is distinguished from the other two groups in that marsupials give birth to relatively undeveloped young that often reside in a pouch located on their mothers' abdomen for a certain amount of time. It contains two superorders, Ameridelphia and Australidelphia.