Oligocene Epoch

Oligocene Epoch, third division of the Tertiary Sub-Era of the geological timescale, spanning an interval from about 34 to about 24 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 per cent) found in strata of this age.

Movement of the plates of the Earth’s crust (see plate tectonics) continued unabated from Eocene time. In the eastern hemisphere, the Afro-Arabian and Indian remnants of the former supercontinent Gondwana, colliding with Eurasia to the north, closed off 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 (see Orogeny), from the Alps in the west to the Himalaya 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 south-western United States. The climate remained subtropical and moist throughout North America and Europe, but a gradual, long-term cooling trend had begun, culminating in the ice ages of the Pleistocene Epoch.

Mammals were firmly established by the Oligocene as the dominant form of terrestrial life. The horse continued to evolve in North America. 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 Rhinoceros-like 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 a horse-like skull, camel-like body, and long, narrow claws.

Oligocene camels, then the size of sheep, became extinct in North America, but some migrated to South America with peccaries and tapirs. Meanwhile vast herds of oreodons (pig-like cousins of the camel) grazed across the plains of North America, as did enteledonts (even-toed, giant “pigs”) that were also native to that continent; both groups became extinct in the succeeding Miocene. The first elephants—short, semi-aquatic forms lacking tusks and trunk—gave rise, in Africa, to mastodons, which were as yet little more than 1.5 m (5 ft) high. Creodonts had by then diverged to form dogs and cats; the latter comprised two groups, from one of which sabre-toothed cats developed. Rodents were widespread by this time, and primates included tarsiers and lemurs. Finally, Oligocene strata have yielded bones of the first Old World monkeys, as well as a single species of great ape.