While ferns and conifers evolved in the Paleozoic Era and flowering plants appeared by the middle of the Mesozoic, grass is a relative newcomer to the plant kingdom. Grass is thought to have evolved in the rain shadow of the young Rocky Mountains, fostered by a cooling and drying climate.
As the Eocene gave way to the Oligocene Epoch, about 40 million years ago, a period of glaciation began; within 5 million years, glaciation would occur on Antarctica for the first time. The cause for this climate change is uncertain but it is interesting to note that it corresponds to a period of increased volcanism in the Mountain West; in particular, the volcanic San Juans of southwest Colorado began to develop at this time. Whatever the cause, a cooler climate and a reduction of precipitation across the Great Plains both favored the development of grasslands. Persistent wind and intermittent wildfires also played a role and, by the middle of the Oligocene, extensive grasslands covered much of the region.
These nutritious grasslands enticed ancestral horses, rhinos and camels from the retreating forests and they soon evolved into larger species, better adapted to life on the plains. In fact, the Oligocene is known for its "megafauna," including Baluchitherium, a hornless rhino and the largest mammal to ever walk the earth. Today, the White River Formation, well exposed in Badlands National Park, harbors the fossils of many grassland giants.
