The Oligocene epoch wasn't especially innovative with regard to prehistoric animals, which continued along the evolutionary paths that had been locked in during the preceding Eocene (and continued on 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 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 became more isolated from South America and Australia and developed a polar ice cap. 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 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, often in locations you might not expect (camels, for instance, were especially thick on the ground in Oligocene North America).
The other trend was mostly confined to South America, which was isolated from North America during the Oligocene epoch and hosted a bizarre array of megafauna mammals, including the elephant-like Pyrotherium and the meat-eating marsupial Borhyaena, 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) and giant penguins that lived in temperate, rather than polar, climates--Kairuku of New Zealand being a good example. Other types of birds also undoubtedly lived during the Oligocene epoch; we just haven't found 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 before and after the Oligocene is at least circumstantial evidence that they must have prospered during this epoch as well.
Marine Life During the Oligocene Epoch
The Oligocene was a golden age for whales, rich in transitional species like Aetiocetus, Janjucetus and Mammalodon (which possessed both teeth and plankton-filtering baleen plates). Sharks continued to be the apex predators of the seas; it was toward the end of the Oligocene, 25 million years ago, that the gigantic Megalodon 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. The process that had begun during the preceding Eocene epoch, the gradual appearance of deciduous forests in place of jungles in the earth's spreading non-tropical regions, also continued unabated.
Next: the Miocene Epoch