IV.

History

Mammals probably appeared on the Earth during the early Mesozoic era (245 million to 145 million years ago). Most zoologists believe that they evolved from a group of extinct mammal-like reptiles, Theriodontia, which existed during the Triassic period (245 million to 208 million years ago). The earliest animal fossils that have definitely been identified as mammals were found in rocks from the Jurassic period (208 million to 145 million years ago).

During the Jurassic period, five distinct orders of mammals existed. One order was made up of small, rodent-like mammals, having gnawing front teeth and grinding teeth with several cusps (molars and premolars), that became extinct in the Eocene epoch (56 million to 35 million years ago). A second order consisted of small, carnivorous mammals, having molar teeth equipped with three simple, conical cusps, that became extinct before the end of the Eocene epoch. A third group of small insectivorous mammals are the probable ancestors of present-day mammals.

Of the mammalian subclasses that still exist, the monotremes are unrepresented by fossil remains; the earliest marsupial and placental fossils were found in rocks of the Cretaceous period (145 million to 65 million years ago). The marsupials were apparently unsuccessful in competition with the placentals, and by the beginning of the Eocene epoch were restricted to the opossum family in North America, several families (now mostly extinct) in South America, and several families in Australia. The earliest fossil remains of placentals discovered thus far have been found in western North America and western Europe; the placentals originated in the late Cretaceous period, and fossil records indicate that they spread rapidly throughout the Cenozoic era (65 million to 10,000 years ago) to form the dominant mammalian group throughout the world except in Australia. The insectivores, considered the oldest order of placental mammals, strongly resemble early fossil placentals.