Mammalia

Click to Play

Humans Overhunting of the Mammoths

Click to Play

As Temperatures Rise, Species Shrink and Genetic Diversity Wanes

View more Animal Life videos

The class Mammalia has been the dominant group of vertebrates since the extinction of the dinosaurs 65 million years ago. There are over 4200 living species, classified into over 1000 genera, 140 families, and 18 orders. The number of extinct mammals is at least five times that. Most living mammals are terrestrial. However, many groups of mammals moved to the water from land-dwelling ancestors. These included manatees and dugongs (which are distantly related to elephants), otters (which are related to weasels), seals, sea lions, and walruses (which are distantly related to bears), and whales (which are distantly related to even-toed hoofed mammals), as well as numerous extinct groups. Mammals have also taken to the air, with over 920 living species of bats, as well as numerous gliding forms such as the flying squirrels, phalangerid marsupials, and flying lemurs or colugos. Mammals are even more successful at small body sizes, with hundreds of small species of rodents, rabbits, and insectivores.

Mammals are distinguished from all other animals by a number of unique characteristics. These include a body covered with hair or fur (secondarily reduced in some mammals, particularly aquatic forms); mammary glands in the female for nursing the young; a jaw composed of a single bone, the dentary; and three middle ear bones, the incus, malleus, and stapes. All mammals maintain a constant body temperature through metabolic heat. Their four-chambered heart (two ventricles and two atria) keeps the circulation of the lungs separate from that of the rest of the body, resulting in more efficient oxygen transport to the body tissues. They have many other adaptations for their active life-style, including specialized teeth (incisors, canines, molars, and premolars) for biting, tearing, and grinding up food for more efficient digestion. These teeth are replaced only once in the lifetime of the animal (rather than continuous replacement, found in other toothed vertebrates). Mammals have a unique set of muscles that allow the jaw to move in many directions for chewing and for stronger bite force. Their secondary palate encloses the internal nasal passage and allows breathing while they have food in the mouth. Ribs (found only in the thoracic region) are firmly attached to the breastbone (sternum), so that expansion of the lung cavity is accomplished by a muscular wall in the abdominal cavity called the diaphragm. See also Cardiovascular system; Dentition; Ear (vertebrate); Hair; Lactation; Mammary gland; Tooth.

All mammals have large brains relative to their body size. Most mammals have excellent senses, and some have extraordinary senses of sight, smell, and hearing. To accommodate their large brains and more sophisticated development, mammals are born alive (except for the platypus and echidnas, which lay eggs), and may require considerable parental care before they are ready to fend for themselves. Juvenile mammals have separate bony caps (epiphyses) on the long bones, separated from the shaft of the bone by a layer of cartilage. This allows the long bones to grow rapidly while still having a strong, bony articulation at the end. When a mammal reaches maturity, these epiphyses fuse to the shaft, and the mammal stops growing (in contrast to other vertebrates, which grow continuously through their lives). See also Brain; Nervous system (vertebrate); Skeletal system.

The living mammals are divided into three major groups: the monotremes (platypus and echidnas), which still lay eggs, retain a number of reptilian bones in their skeletons, and have other primitive features of their anatomy and physiology; the marsupials (opossums, kangaroos, koalas, wombats, and their relatives), which give birth to an immature embryo that must crawl into the mother's pouch (marsupium), where it finishes development; and the placentals (the rest of the living mammals), which carry the young through a long gestation until they give birth to relatively well-developed progeny. In addition to these three living groups, there were many other major groups, such as the rodentlike multituberculates, now extinct. The most recent classification of the mammals can be summarized as follows:

Class Mammalia

     Subclass Prototheria (monotremes)

     Subclass Theriiformes

          Infraclass Holotheria

               Cohort Marsupialia (marsupials or pouched mammals)

               Cohort Placentalia (placentals)

               Magnorder Xenarthra (sloths, anteaters, armadillos)

               Magnorder Epitheria

                    Grandorder Anagalida (=Glires) (rodents, rabbits, elephant shrews)

                    Grandorder Ferae (carnivores, pangolins, many extinct groups)

                    Grandorder Lipotyphla (hedgehogs, shrews, moles, tenreces, and kin)

                    Grandorder Archonta

                         Order Chiroptera (bats)

                         Order Primates (lemurs, monkeys, apes, humans)

                         Order Scandentia (tree shrews)

                    Grandorder Ungulata (hoofed mammals)

                         Order Tubulidentata (aardvarks)

                         Order Artiodactyla (even-toed hoofed mammals: pigs, hippos, camels, deer, antelopes, cattle, giraffes, pronghorns, and relatives)

                         Order Cete (whales and their extinct land relatives)

                         Order Perissodactyla (odd-toed hoofed mammals: horses, rhinos, tapirs, and extinct relatives)

                         Order Hyracoidea (hyraxes)

                         Order Tethytheria (elephants, manatees, and extinct relatives)

This classification does not list all the extinct groups, which include at least a dozen more ordinal-level taxa. See separate articles on each group. See also Reproductive system.

Mammals evolved from the Synapsida, an early branch of the terrestrial amniotes that has been erroneously called the mammallike reptiles. (This name is inappropriate because synapsids were never related to reptiles.) The first undoubted mammals appeared in the Late Triassic (about 210 million years ago), and were tiny insectivorous forms much like living shrews. Through the rest of the age of dinosaurs, a number of different groups evolved over the next 145 million years of the Jurassic and Cretaceous. Most remained tiny, shrewlike animals, hiding from the dinosaurs in the underbrush and coming out mostly at night. The first two-thirds of mammalian history had passed before the dinosaurs became extinct 65 million years ago, and this allowed mammals to emerge from their shadow. Between 65 and 55 million years ago, a rapid adaptive radiation yielded all the living orders of placental mammals and many extinct forms as well.