The Oligocene, getting cold and growing huge
Today we talk about the Oligocene epoch, when the climate cooled and forest gave way to areas of open land. Grass spread beyond the lake shores and river sides where it had been living, and began to spread across the landscape. Many animal types changed their bodies to become better runners, and we got the largest mammal ever to walk the earth.
Here are a couple of links with some information about the Oligocene.