Prothero, Donald R. Occidental College, Los Angeles, California.
Last reviewed:March 2020
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- Anatomy and physiology
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The dominant group (class) of vertebrates. Members of the class Mammalia (subphylum Vertebrata, phylum Chordata), that is, mammals, have ruled the planet Earth since the extinction of the dinosaurs approximately 65 million years ago. There are possibly more than 6400 living species of mammals (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 (for example, many rodents and insectivores). The largest known land mammal was the extinct 18,000-kg (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). The living blue whale [reaching 30 m (100 ft) in length and weighing 136,000 kg (150 tons)] is by far the largest animal that has ever lived. Mammals also have taken to the air, with more than 1000 living species of bats, as well as numerous gliding forms (for example, flying squirrels and phalangerid marsupials). See also: Biodiversity; Chordata; Extinction; Vertebrata; Zoogeography; Zoology
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