Oligocene Epoch begins

The Oligocene Epoch, spanning 33.9 to 23 million years ago, is the third and final epoch of the Paleogene Period. It began with a cooling event that triggered Antarctic ice sheet expansion and a sea level drop of approximately 105 meters. Tropical forests contracted as grasslands spread, favoring ground-dwelling mammals. Most modern mammal families appeared during this interval, including early horses, rhinoceroses, camels, and deer, while creodonts declined and were replaced by carnivorans such as early dogs and bears.