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.