OLIGOCENE EPOCH

major worldwide division of the Tertiary Period that began about 36.6 million years ago and ended about 23.7 million years ago. It follows the Eocene Epoch and precedes the Miocene Epoch. The term Oligocene is derived from Greek and means the epoch of few recent forms, referring to the sparseness of the number of modern animals that originated during the Oligocene. In western Europe the beginning of the Oligocene was marked by an invasion of the sea that brought with it new molluscan forms characteristic of the epoch. Marine conditions did not exist for long, however, and brackish and freshwater conditions soon prevailed. This cycle of marine transgression, followed by the establishment of brackish and then freshwater environments, was repeated during the Oligocene. The Oligocene is subdivided into two ages and their corresponding rock stagesnamely, the Rupelian and the Chattian. During the Oligocene, sediments on the floor of the ancient Tethyan Sea, which covered part of Eurasia, were deformed early in the formation of the European Alps. Oligocene climates appear to have been temperate, and many regions enjoyed subtropical climatic conditions. Grasslands expanded during the Oligocene, and forested regions dwindled. Tropical vegetation flourished along the borders of the Tethyan Sea. Warm, swampy conditions prevailed over much of what is now Germany, and extensive deposits of lignite were formed. A prominent group of Oligocene marine animals were the foraminiferans, protozoans similar to amoebas but bearing a complex, often calcareous test, or shell. Among the especially prominent foraminiferans were the nummulites. Other marine forms were essentially modern in aspect. Terrestrial invertebrate life was abundant and diverse. Stream and lake deposits on the Isle of Wight contain the remains, often well preserved, of termites and other insects. In the Baltic, many forms of Oligocene insects, including butterflies, bees, ants, and spiders, are preserved in amber. Oligocene terrestrial vertebrate faunas, diverse and abundant, are known from North America, Europe, Africa, and Asia. The vertebrate faunas of the northern continents possess an essentially modern aspect that is a result less of the appearance of new forms than of the extinction of archaic vertebrates at the close of the preceding Eocene Epoch. The similarities between the various early Oligocene vertebrate faunas of the northern continents suggests a relatively free interchange of animals, but later Oligocene faunas evince a greater degree of provincialism. Bats became more widespread in the Oligocene and at least locally abundant; their droppings in caves contributed to the formation of extensive phosphate deposits that are currently economically significant in many areas. Primitive beavers appeared late in the Oligocene. Throughout the epoch, modern groups of carnivores became diverse and abundant. The largest land mammal of all time, Baluchitherium, is known from Asia, and the first mastodons are known from Egypt. In North America, primitive horses were evolving, including three-toed forms such as Mesohippus and Miohippus. Pigs and peccaries first appeared in the early Oligocene of Europe and reached North America late in the epoch. The earliest apelike form, Parapithecus, is known from Oligocene deposits in Egypt, which also have yielded remains of several kinds of Old World monkeys. The earliest New World monkeys are known from late Oligocene deposits in South America. During the Oligocene, South America was isolated from Central and North America, and a unique mammalian fauna developed there. Remarkably, many South American mammals of the Oligocene exhibit extreme parallelism in adaptation to forms that are found elsewhere in the world and are not even closely related.