MIOCENE EPOCH

major worldwide division of the Tertiary period that extended from 23.7 to 5.3 million years ago. It is often divided into the Early Miocene epoch (23.7 to 16.6 million years ago), the Middle Miocene epoch (16.6 to 11.2 million years ago), and the Late Miocene epoch (11.2 to 5.3 million years ago). The Miocene may also be divided into six ages and their corresponding rock stages: from oldest to youngest these ages or stages are the Aquitanian, Burdigalian, Langhian, Serravallian, Tortonian, and Messinian. The Miocene epoch precedes the Pliocene epoch and follows the Oligocene epoch. Important Miocene deposits occur in North and South America, southern Europe, India, Mongolia, East Africa, and Pakistan. Marine and terrestrial environments are represented in the Miocene stratigraphic record. The record of terrestrial life is extensive and varied, providing a rather complete view of the development of vertebrates, especially mammals. Miocene mammalian faunas are essentially modern; many archaic groups were extinct by the end of the Oligocene, and fully half of the known modern families of mammals are present in the Miocene record. Some interchange of faunas occurred between the Northern Hemispheres of the Old and New Worlds. Free communication was possible between Africa and Eurasia, but South America and Australia remained isolated. The very fossiliferous Harrison Formation of Nebraska has produced an abundance of mammalian remains, including large, slender camels; strange deerlike creatures; bear dogs; foxes; peccaries; a small, archaic beaver that apparently had the habits of a ground squirrel; and the Miocene horse. During the Miocene, horse evolution occurred mainly in North America; forms such as Parahippus, Miohippus (a form carried over from the preceding Oligocene epoch), Anchitherium, Hypohippus, Pliohippus, and Merychippus are genera that represent great diversification and development. The first dogs and bears appeared; the bear dog Hemicyon is close to the origin of the bears. The first hyenas, springing from primitive civets, appeared in the Miocene, as did the early sabre-toothed cats. Primitive antelope, deer, and the earliest giraffids appeared in the Miocene of Eurasia. The early elephantlike forms, previously known only from the Oligocene of Africa, spread to the Eurasian continent in the Miocene and became more diverse. In Argentina the Santa Cruz Formation of Middle Miocene time provides an excellent record of the unusual Miocene fauna of South America. Marsupial carnivores, aberrant endentates, litopterns, and toxodonts are among the odd groups represented. These forms were able to evolve because of South America's isolation from other regions. The evolution of the South American monkeys, the platyrrhines, was under way during the Miocene. By the end of the Miocene epoch almost all the modern groups of whales had appeared, as had the early seals and walruses. Birds such as herons, rails, ducks, eagles, hawks, crows, sparrows, pheasants, owls, and partridges were present in the Miocene of Europe, where the uplifting of the Alps continued through Miocene time. The Miocene epoch is of great importance in primate evolution. The last primate to occur in the fossil record of North America, a tarsier-like creature, is known in the United States. Elsewhere, the higher primates, especially the apes, underwent a great deal of evolution. The fossil evidence seems to indicate that advanced primates, including apes, were present in southern Europe. An early gibbon, Pliopithecus, as well as the dryopithecines, a group of advanced manlike apes that probably represent the stock from which modern apes and man originated, are found in Miocene rocks of Europe. The dryopithecines also are present in the Miocene of Africa, the region where manlike forms and the direct ancestors of man probably originated. Ramapithecus, a form known from the latest Miocene or, perhaps, the earliest Pliocene, represents an advancement over dryopithecines toward the human condition.