After the cool temperatures of the
Oligocene, the earth started to warm up again at the beginning of the
Miocene, reaching a peak near the middle of the epoch, then cooling once
more (Prothero 1998) and eventually causing an ice cap to form at the
south pole (WGBH 2001). The drying trend that started in the Eocene
continued and intensified as the formation of mountain chains in Asia and
North and South America changed air circulation patterns (WGBH 2001). This
meant that forests continued their decline, and the first extensive grasslands
appeared in North America approximately 18 million years
ago (Janis et al. 2002). Landscapes became more open as
forests shrank, and the primitive grasses that appeared provided less food
for herbivores than the vegetation that previously grew in the forested areas
(Macfadden 1994).
