Overview of the Oligocene Epoch

The Oligocene epoch wasn't an especially innovative period of time with regard to its prehistoric animals, which continued along the evolutionary paths that had been pretty much locked in during the preceding Eocene (and continued on in turn during the ensuing Miocene). The Oligocene was the last major geologic subdivision of the Paleogene period (65-23 million years ago), following the Paleocene (85-56 million years ago) and Eocene (56-34 million years ago) epochs; all of these periods and epochs were themselves part of the Cenozoic Era (65 million years ago to the present).

Climate and geography

While the Oligocene epoch was still fairly temperate by modern standards, this 10-million-year stretch of geologic time saw a decrease in both average global temperatures and sea levels. All of the world's continents were well on their way toward moving into their present positions; the most striking change occurred in Antarctica, which drifted slowly south, became more isolated from South America and Australia, and developed the polar ice cap that it retains today. Giant mountain ranges continued to form, most prominently in western North America and southern Europe.

Terrestrial Life During the Oligocene Epoch

Mammals. There were two major trends in mammalian evolution during the Oligocene epoch. First, the spread of newly evolved grasses across the plains of the northern and southern hemispheres opened a new ecological niche for grazing mammals. Early horses (such as Miohippus), distant rhinoceros ancestors (such as Hyracodon), and proto-camels (such as Poebrotherium) were all common sights on grasslands, often in locations you might not expect (camels, for instance, were especially thick on the ground in Oligocene North America, where they first evolved).

The other trend was mostly confined to South America, which was isolated from North America during the Oligocene epoch (the Central American land bridge would not form for another 20 million years) and hosted a bizarre array of megafauna mammals, including the elephant-like Pyrotherium and the meat-eating marsupial Borhyaena (the marsupials of Oligocene South America were every match for the contemporary Australian variety). Asia, meanwhile, was home to the largest terrestrial mammal that ever lived, the 20-ton Indricotherium, which bore an uncanny resemblance to a sauropod dinosaur!

Birds

As with the preceding Eocene epoch, the most common fossil birds of the Oligocene epoch were predatory South American "terror birds" (such as the unusually pint-sized Psilopterus), which mimicked the behavior of their two-legged dinosaur ancestors, and giant penguins that lived in temperate, rather than polar, climates--Kairuku of New Zealand is a good example. Other types of birds also undoubtedly lived during the Oligocene epoch; we just haven't identified many of their fossils yet!

Reptiles

To judge by the limited fossil remains, the Oligocene epoch wasn't an especially notable time for lizards, snakes, turtles or crocodiles. However, the plenitude of these reptiles both before and after the Oligocene provides at least circumstantial evidence that they must have prospered during this epoch as well; a lack of fossils doesn't always correspond to a lack of wildlife.

Marine Life During the Oligocene Epoch

The Oligocene epoch was a golden age for whales, rich in transitional species like Aetiocetus, Janjucetus, and Mammalodon (which possessed both teeth and plankton-filtering baleen plates). Prehistoric sharks continued to be the apex predators of the high seas; it was toward the end of the Oligocene, 25 million years ago, that the gigantic Megalodon, ten times bigger than the Great White Shark, first appeared on the scene. The latter part of the Oligocene epoch also witnessed the evolution of the first pinnipeds (the family of mammals that includes seals and walruses), the basal Puijila being a good example.

Plant Life During the Oligocene Epoch

As remarked above, the major innovation in plant life during the Oligocene epoch was the worldwide spread of newly evolved grasses, which carpeted the plains of North and South America, Eurasia and Africa--and spurred the evolution of horses, deer, and various ruminants, as well as the meat-eating mammals that preyed on them. The process that had begun during the preceding Eocene epoch, the gradual appearance of deciduous forests in place of jungles over the earth's spreading non-tropical regions, also continued unabated.