Prothero, Donald R. Occidental College, Los Angeles, California.
- Anatomy and physiology
- Reproduction and classification
- Additional Readings
The dominant group (class) of vertebrates. Members of the class Mammalia, that is, mammals, have ruled the planet Earth since the extinction of the dinosaurs approximately 65 million years ago. There are more than 4200 living species of mammals, classified into more than 1000 genera, 140 families, and 18 orders (Fig. 1). However, the number of extinct mammals is at least five times as great. Most living mammals are terrestrial, including such large beasts as elephants, rhinos, hippos, and giraffes, as well as a great diversity of smaller land animals. The largest known land mammal was the extinct 20-ton hornless rhino Paraceratherium. 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. The living blue whale [reaching 30 m (100 ft) in length and 150 tons] is by far the largest animal that has ever lived. 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. The smallest living mammal, the 1.5-g Kitti's hognosed bat, is at the lower limit of body size for mammals, since physiology and anatomy prevent them from thriving in the tiny-body-size niche inhabited by insects and other arthropods.
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