How did global cooling in the late Eocene change mammal communities?

The global cooling that occurred during the late Eocene Epoch significantly altered terrestrial environments, triggering a major shift in mammal communities. As temperatures dropped, dense rainforests were converted into more open forests and grasslands.1

This change in vegetation forced an evolutionary turnover, leading to the extinction of various archaic groups, including the last of the primitive herbivores such as the brontotheres. Following these extinctions at the close of the Eocene, land mammal communities became dominated by the ancestors of modern groups, including horses, rhinoceroses, antelopes, deer, camels, elephants, felines, and canines. This transition established the essentially modern aspect of northern vertebrate faunas that characterized the subsequent Oligocene Epoch.12