by Carl Strang
This year’s winter series is a review of the prehistoric life and geologic history of northeast Illinois. Each chapter will summarize current understanding, gleaned from the literature, of what was going on with life on Earth in a particular span of time, what we know about the local landscape, and what we can say about local life. I include some references, particularly to papers published in the journal Science which commonly is available at public libraries. Contact me if you need sources for other items. The Earth is so old that every imaginable environment was here at some point, from ocean depths to mountaintops, from equatorial tropics to tundra, and from wetlands to desert.
Oligocene Epoch (35.4-23.3 million years before present)
The Oligocene Epoch (named in 1854), literally “little recent,” was divided from the Miocene of which it originally was part, based on fossils in Germany.
Life on Earth. The first grassland ecosystems appeared in Asia in the Oligocene, a profound step in the history to follow.
At the beginning of the Oligocene there was a big diversification of North American mammals, with only a quarter of the Eocene genera continuing. Change was slow for the rest of the epoch. This rapid change, preceded by a wave of extinctions, is associated with a time of climatic cooling. Relatively little is known about the Oligocene, because its general lack of change after the beginning has interested few researchers.
The oreodonts were a diverse North American artiodactyl (even-toed ungulate) group in the Oligocene; they declined to extinction at the end of the Miocene. The early Oligocene was the time when the browsing perissodactyl titanotheres (brontotheres) peaked in North America and Asia. Another perissodactyl, the giant rhinoceros Paraceratherium of Asia (also known as Indricotherium or Baluchitherium), is the largest known land mammal ever. It lived in the Oligocene and Miocene, was 18 feet high at the shoulder and could reach twigs and leaves 20 feet off the ground with its 5-foot-long head.
Early in the Oligocene the first beavers appeared (in North America), the first procyonids (raccoon family) in North America and Europe, the first peccaries in North America. The Middle Oligocene brought the Canidae (dog family). Paleosciurus tree squirrels first appeared in the early Oligocene in Europe (Science 299:1568).
The hyaenodontids and nimravids became extinct in the late Oligocene, resulting in the “cat gap,” a lengthy period of time in which the North American carnivore fauna was dominated by larger canids and amphicyonids, and smaller mustelids (weasels), but nothing resembling a cat. The first ground sloths evolved in South America in the Oligocene.
Local landscape. A turning point in post-Cretaceous times was the separation of Australia from Antarctica in the late Eocene. The consequent establishment of a permanent cold current around the latter continent in the Oligocene brought a cooling to the global climate that impacts us to this day. Brief but intense glacial episodes, set off by changes in Earth orbital characteristics and made possible by the cold south polar currents, happened both at the beginning and end of the Oligocene (Science 314:1894). The cooling trend continued until our local climate became temperate in the Oligocene. Deciduous dry forests passed from semi-tropical to temperate.
The oak family became more important both east and south of here. In the Plains to the west, hackberry was common, and that region may have been a low-rainfall scrubland (remember, no grasslands were here yet). The closest Oligocene deposits are in south central South Dakota and western Nebraska.
Local life. One or more species of the carnivore-like creodont Hyaenodon probably occurred here in the Oligocene. Oreodonts also were diverse, then, and likely were represented here. There were widespread, hippolike rhinoceroses called amynodonts in North America and Eurasia. Horses in the genus Mesohippus also were likely here.