The Mammals

Abstract

About 1050 contemporary genera of mammals are named by Walker (1964) in his monograph Mammals of the World. Three of these belong to the order Monotremata (egg-laying mammals), 90 to the order Marsupialia (typically undeveloped young kept in pouches), and the remainder are eutherians (placentals). Monotremes are known only from the Australian region and are represented by the echidna and platypus. This order probably represents a separate line of mammalian evolution, beginning in the Triassic period. The marsupials and placentals probably diverged from a common stock early in the Cretaceous period, about 85 million years ago. The Australian (75 genera) and American (15 genera) marsupials subsequently diverged late in the Cretaceous. The placental mammals today contain about 950 genera, of which 360 are rodents (Rodentia) and 180 are bats (Chiroptera). The other generically numerous groups are the Insectivora (hedgehogs, shrews), 66 genera; Primates (man and monkeys) 62; Cetacea (whales) 35; Carnivora 103; Pinnipedia (seals) 20 and Artiodactlya (pigs, camels cattle, sheep) 82. The mammals have a wide geographic and osmotic distribution; they are predominantly terrestrial and are even found in the driest desert regions, though some are aquatic and live in rivers and lakes or in the sea. Only one genus of the monotremes (Ornithorhynchus, the platypus), and one marsupial (the South American water opossum, Chironectes) live in fresh water, and these groups have no marine representatives. Most mammalian orders have representatives that live in desert regions.