The Oligocene epoch of the Paleogene period and Cenozoic era saw the beginning of a cooling trend, with glaciers forming over Antarctica and forests contracting closer to the equator. Sea levels were higher, and grasses began to dominate over forests, leading to the evolution of four-chambered stomachs in animals. The period saw the evolution of many modern mammalian faunas, including deer and canids, while unusual animals like mesonychids and phorusracids began to disappear. Primates were mostly primitive, but apes evolved in Africa at the end of the period.
The Oligocene is an epoch of the Paleogene period and the Cenozoic era, spanning from 33.9 to 23.0 million years ago. The Oligocene marked the beginning of the cooling trend that would continue through the remainder of the Cenozoic, up until today. Continental glaciers began to form over Antarctica, and tropical hardwood forests that had extended 45 degrees from the poles began to contract closer to the equator, while coniferous forests, which had previously extended to the poles, also began to shrink. retreat. The Oligocene was preceded by the Eocene and followed by the Miocene.
Oligocene sea levels were higher than today, due to less freshwater frozen in glaciers, leading much of central Eurasia, the southeastern United States, and southeast Asia to be flooded. North and South America were separated by the Strait of Panama. Although higher sea levels suggest that North America and Eurasia would have been separated, there must have been a land bridge between them during the period, as the faunas of these two continents were so similar.
Grasses, which began their career in the Late Cretaceous (~85 million years ago) as plants that grew on the edge of water, were finally starting to dominate the formerly planetary forests, growing in open areas and challenging animals to evolve from arboreal to grassland. adaptations. Among the most important of these was the evolution of the four-chambered stomach and the practice of rumination (ruminating), which in effect turned the stomach into a fermentation vat for extracting nutrients from tough grass.
Many of the “normal-looking” mammalian faunas we know so well today began to evolve during the Oligocene. The deer first evolved in the early Oligocene. Carnivores during the period were mainly canids – bears and dogs – as felids did not evolve until the end of the Oligocene, about 20 million years ago. The first fully recognizable cetaceans (whales, dolphins, etc.) evolved a few million years before the start of the Oligocene and were progressively increasing in size.
“Unusual” animals, including predators, still existed in the Oligocene, but many of them began to disappear. These include the mesonychids, large ungulate predators with oversized skulls that ruled the Asian steppes and North American plains, but were in decline in the Oligocene and became extinct early in the period. Phorusracids – “terror birds” – were the dominant predators in South America, which they had been since the beginning of the Cenozoic and continued to be until South America connected with North America and competitive mammalian predators started conquering their space. Primates during the Oligocene were mostly primitive, similar to modern apes, but apes evolved in Africa right at the end of the period.
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