Earth's Oligocene Epoch

The Oligocene Epoch followed the Eocene extinction and is the final epoch of the Palaeogene Period. It lasted from around 34 million years ago until 23 million years ago. Although it is a period in which very few new mammals appeared as compared to the Eocene, it is the period in which mammals reached their largest size ever.

The continents continued to drift toward their present positions in the Oligocene. Antarctica, which was once a lush, green continent continued to drift to the South Pole and became more isolated from warm-water currents. Consequently the continent attained tundra-like conditions and began to develop a permanent icecap. South America broke away from Antarctica completely and began to migrate north to North America where it would eventually fuse and create a land bridge for the diffusion of flora and fauna. Although marked glaciation appeared during the Oligocene Epoch, the climate remained relatively warm. The vast Eocene forests had receded and grasslands perfect for large grazing animals were replacing them.

Earth's oceans took a severe evolutionary punch in the stomach following the Eocene extinction. The cooling of the planet and glaciation at the poles caused much of the marine flora and fauna to migrate to more temperate waters at the equator. The cooler oceans also reduced the diversity of marine plankton, which form the basis for most modern ocean ecosystems. Many of the large toothed whales of the Eocene had died out as a result of the broken food chains and many other marine mammals were fighting to recover after the disastrous end of the Eocene. By the mid-Oligocene, the overall fauna in the oceans were declining, but new forms were also emerging. The first baleen whales that filter feed on microfauna were making their first appearance and the first pinnipeds (present-day seals and sea lions) appeared as a descendent of a bear or otter-like ancestor. Sharks continued to exert fierce competition on mammals for dominance at the top of the food chains.

Grasses began to dominate the flora of these new open spaces and scrub forests began to replace rainforests in the more mild areas of the world. New grazing groups such as elephants began to join the ranks with swifter grazers like horses. In the isolated continents of South America and Africa, new and different mammals like sloths and armadillos began to make their first steps. The largest land mammal of the Oligocene and indeed of any period in Earth's history was the giant Indricotherium. This titanic relative of rhinos was as tall as a giraffe and could weigh up to 18 tons. These giants rivaled even the large sauropods of the Mesozoic in size and once they reached a certain size had no natural predators. Our ancestors were making strides as well in Africa, where the first apes were appearing in the forests.