I.

Introduction

Oligocene Epoch, third and final division of the Paleogene Period of the Cenozoic Era, spanning an interval from about 34 million to 23 million years ago. Like the Eocene Epoch, which preceded it, and the Miocene, which followed, the Oligocene (Greek, “little life”) was originally defined by the percentage of modern species of shellfish (10-15 percent) found in strata of this age.

II.

Geology

Collisions between the plates of Earth's crust continued unabated from Eocene time (see Plate Tectonics). In the eastern hemisphere, the Afro-Arabian and Indian remnants of the former supercontinent Gondwanaland, colliding with Eurasia to the north, pinched shut the eastern end of the Tethys Sea, leaving in its place a much shrunken remnant—the Mediterranean. Compressional forces generated by the collision helped to push up an extensive system of mountain ranges, from the Alps in the west to the Himalayas in the east. Meanwhile the Australian plate collided with the Indonesian, and the North American plate had begun to override the Pacific. As a result, the seafloor-spreading process originating at the East Pacific Ridge was diverted to a direction perpendicular to the ridge axis. A great transform fault—the earthquake-producing San Andreas Fault of California—developed to accommodate this shift in motion between the plates. Other effects of the collision included the creation of the Basin and Range structure of the southwestern United States, the continued uplift of the Sierra Nevada, and the outpouring of massive basalt flows that built up the Columbia Plateau. The climate remained subtropical and moist throughout North America and Europe, but a gradual, long-term cooling trend had begun, culminating in the Pleistocene ice ages.

III.

Life

Mammals were firmly established in the Oligocene as the dominant form of terrestrial life. The horse, a native of North America, continued to evolve there. Three groups of rhinoceroses inhabited both the Old World and the New World; one, now extinct, included the central Asian Baluchitherium, 5.5 m (18 ft) high and 7.6 m (25 ft) long—the largest land mammal of any age. Another extinct mammalian tribe, the Rhinoceroslike titanotheres, included Brontotherium, North America's largest land animal of that time, which stood 2.4 m (8 ft) high at the shoulder. The extinct chalicotheres group, of North America and Asia, was characterized by horselike skulls, camel-like bodies, and long, narrow claws.

Oligocene camels, which were then the size of sheep, became extinct in North America, but some migrated to South America with the peccaries and tapirs. Meanwhile vast herds of oreodons (piglike cousins of the camel) grazed across the plains of North America, as did the enteledonts (even-toed, giant “pigs”) that were also native to that continent; both groups became extinct in the Miocene. The first elephants—short, semiaquatic forms lacking either tusks or trunk—gave rise, in Africa, to the mastodons, which were as yet only a little more than 1.5 m (5 ft) high. Creodonts had been largely replaced by the ancestors of dogs and cats and by the niravids, a group of predators that resembled saber-toothed cats (see Saber-Toothed Tiger) but that were not closely related to true cats. Rodents were well represented by this time, and primates included the tarsiers and lemurs. Finally, Oligocene strata have yielded bones of the first Old World monkeys, as well as a single species of great ape.