EOCENE EPOCH

major worldwide division of the Tertiary Period that began about 57.8 million years ago and ended about 36.6 million years ago. It follows the Paleocene Epoch and precedes the Oligocene Epoch. The Eocene is often divided into Early (57.8 to 52 million years ago), Middle (52 to 43.6 million years), and Late (43.6 to 36.6 million years) epochs. The name Eocene is derived from the Greek eos (dawn) and refers to the dawn of recent life; during the Eocene, all the major divisions, or orders, of modern mammals appeared. Eocene rocks have a worldwide distribution. In western Europe, several stages and their temporal equivalents (ages) are recognized on the basis of characteristic lithologies and fossil faunas; they are, from earliest to latest, the Ypresian, Lutetian, Bartonian, and Priabonian. In North America, stages are recognized on the basis of marine rocks and fossil assemblages as well as primarily terrestrial or continental deposits and vertebrate faunas; the vertebrate stages, which are well known and of great importance, consist, from oldest to youngest, of the Wasatchian, Bridgerian, Uintan, and Duchesnean stages. Lower Eocene assemblages are poorly represented in England and in the Patagonian region of South America. Later Eocene vertebrate faunas are somewhat better developed in areas outside of North America; the Mokattam Beds in Egypt contain a middle Eocene vertebrate assemblage. Late Eocene faunas occur in Myanmar (Burma) and Mongolia. It is in North America, however, especially the western United States, that the most abundant and extensive Eocene record exists. Eocene rocks were deposited in much the same regions as those of the preceding Paleocene Epoch. During the Eocene, climates were warm and humid. Temperate and subtropical forests were widespread, but grasslands were of limited extent; the Eocene forests of Oregon, for instance, consisted of trees and plants similar to or identical to those now found in Central and South America. The lower Eocene vertebrate faunas of North America and Europe are very similar; many genera occur in both regions, indicating that interchange could occur. Early Eocene faunas were essentially those of the Paleocene with the addition of newer types, but the archaic Paleocene groups gradually became extinct. The start of the Eocene is marked by the appearance of two new groups of animals: the perissodactyls, or odd-toed ungulates; and the artiodactyls, the even-toed ungulates. The perissodactyls include the horses, rhinoceroses, and tapirs; among the artiodactyls are the deer, cattle, and sheep. The early horse ancestor, the dawn horse, known in North America as Eohippus, is among the fossil perissodactyls found in the lower Eocene rocks of North America and Europe. Artiodactyls, rare during the early Eocene, became abundant later in the epoch. The archaic Paleocene primate forms declined during the Eocene; many of their ecological niches were usurped by the more efficient rodents. Middle Eocene vertebrate faunas were not as cosmopolitan as those of the early Eocene; the isolation that resulted allowed different evolutionary trends to occur in the ungulate groups of North America and Europe. Extensive interchanges of faunal elements were possible once again late in the Eocene Epoch. The Eocene Epoch marks the first appearance in the fossil record of the two completely marine mammal groups, the cetaceans (whales, porpoises, and dolphins) and the sirenians (akin to the modern manatees and dugongs). Similarly, the Eocene provides the first elephant-like animals and the early bats. Many essentially modern bird orders appeared during the Eocene.