Mammals 1:Since the extinction of most of the dinosaurs 65 million years ago, mammals (along with birds) have been the dominant vertebrates on land. This class includes terrestrial, aerial, and aquatic forms. Having developed from the therapsids, the first true mammals— small, nocturnal, shrewlike creatures, such as Megazostrodonappeared over 200 million years ago during the Triassic period
(250–200 million years ago). Mammals had several features that differed from those of their ancestors: an efficient four-chambered heart allowed these warm-blooded animals to sustain high levels of activity; a covering of hair helped them maintain a constant body temperature; an improved limb structure gave them more efficient locomotion; and the birth of live young and the immediate supply of food from the mother’s milk aided their rapid growth. Since the end of the Mesozoic era (65 million years ago), the number of major mammal groups and the abundance of species in each
have varied dramatically. For example, the Perissodactyla (the group that includes Coelodonta and modern horses) was a common group during the Early Tertiary period (about 54 million years ago).
Today, the mammalian groups with the most species include the Rodentia (rats and mice), the Chiroptera (bats), the Primates (monkeys and apes), the Carnivora (bears, cats, and dogs), and the Artiodactyla (cattle, deer, and pigs), while the Proboscidea group, which formerly included many genera, such as Phiomia, Moeritherium, Tetralophodon, and Mammuthus, now has only three species of elephant. In Australia and South America, millions of years of continental isolation led to increased diversity of the marsupials, a group of mammals distinct from the placentals that existed elsewhere.
