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The Oligocene
 The Oligocene is the third epoch of the Cenozoic; it started approximately 34 million years before the present and lasted a little more than ten million years. Global cooling marked the start of the Oligocene, an environmental shift that led to changes in the flora and fauna. During this epoch archaic species of the early Cenozoic were replaced by modern herbivores, such as horses, camels and deer, and carnivores such as cats and dogs. During the Oligocene global temperature dropped as much as 10° C, shrinking forests and introducing the grassy plains that would come to dominate in the Miocene. Herbivorous species adapted to eating grass and carnivores adapted to hunting out in the open. Both groups evolved better eyesight to see predators and prey as well as elongated limbs to increase speed over open terrain. The earliest South American primates are known from this epoch. A site in Bolivia dates the arrival of primates in South America to approximately 30 million years before the present. Given the fact that South America was an island for much of the Cenozoic, colliding with North America not until the middle Pliocene, it is entirely clear how these species arrived on the continent and where they came from. It is unknown, for example, whether the primates rafted to the island from North America or Africa. Primate species traveling from either continent would have had to cross major bodies of water. Global cooling, which dropped the sea level, would have shortened some of the gap between the continents, but at no point did it create a land bridge. The South American species resemble African primates more than they do the North American ones, which leads many to believe in an African origin for the New World primates. Primate evolution in the Oligocene involved increases in brain and body size, sexual dimorphism, and possibly social complexity. Environment changes caused the near extinction of primates in the Northern Hemisphere. In Asia and Africa, anthropoid primates (the higher primates) seem to have thrived. On the basis of analysis of the fossil remains, these primates are thought to have been arboreal quadropeds that fed on fruit and leaves. These species are the ancestors to the apes of the Miocene. At the end of the epoch woodland- and grassland-living animals had evolved that were similar to species that populate those types of habitats today. Temperatures cooled throughout the Oligocene, and an icecap formed for the first time over Antarctica, which melted at the start of the next epoch, the Miocene. |