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Oligocene

SYNONYMS OR RELATED TERMS: Oligocene Epoch
CATEGORY: chronology
DEFINITION: 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. Many large mountain systems and herbivorous mammals began to develop, however. During this epoch, many of the older types of mammals became extinct and the first apes appeared. 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.

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Eocene

CATEGORY: chronology
DEFINITION: A major geological epoch of the earth's history -- the second division of the Tertiary Period (Cenozoic Era) 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 mya), and Late (43.6 to 36.6 mya) 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."

Miocene

CATEGORY: chronology
DEFINITION: A geological epoch of the Tertiary period in the earth's history, in which many of the great mountain chains were formed and mammals came to dominate animal life. During this epoch, many mammals of modern form, such as dogs, horses, and humanlike apes, evolved. The Miocene occurred after the Oligocene and before the Pliocene and is dated between 25-5 (23.7-5.3) 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.

Paleogene

SYNONYMS OR RELATED TERMS: Palaeogene
CATEGORY: chronology
DEFINITION: The lower division of the Tertiary system including the Palaeocene, Eocene, and Oligocene periods. The older of the two stratigraphic divisions of the Cenozoic era (which began about 66.4 million years ago and extends to the present). The Paleogene, whose beginning coincides with that of the Cenozoic era, lasted about 42.7 million years and was followed by the Neogene period. The Paleogene, which means ancient-born includes the Paleocene (Palaeocene), Eocene, and Oligocene epochs. The term Paleogene was devised in Europe to emphasize the similarity of marine fossils found in rocks of the first three Cenozoic epochs, as opposed to the later fossils of the Neogene period. In North America, the terms Paleogene and Neogene are not widely used, and the Cenozoic is divided only into the Tertiary period (c 66.4 million-1.6 million years ago) and the Quaternary period (c 1.6 million years ago to present). Thus, the Paleogene period may also be considered to be roughly equivalent to the first two-thirds of the Tertiary period.

Tertiary

CATEGORY: chronology
DEFINITION: The geological period following the Mesozoic (Secondary) era, constituting the first of two periods of the Cenozoic Era, the second being the Quaternary. It comprises the epochs Palaeocene, Eocene, Oligocene, Miocene, and Pliocene. It extended from the end of the Cretaceous to the beginning of the Quaternary, from 66.4-1.6 million years ago. The Miocene and Pliocene epochs were important in Hominid Evolution. Some prefer not to use the term Tertiary and instead divide the interval into two periods, the Paleogene Period (66.4-23.7 mya) and the Neogene Period (23.7-1.6 mya). Most of the existing mountain belts and ranges, notably the Andes, the Rockies, the Alps, the Himalayas, and the Atlas Mountains, were formed either partly or wholly during the Tertiary. The emergence and submergence of land bridges between continents, especially between North and South America, Eurasia and Africa, and Asia and North America, critically affected the migration of faunas and floras. The earliest generally accepted hominid fossils, those of Australopithecus, come from rocks of Pliocene age (5.3-1.6 mya) in eastern Africa.