In the middle Eocene, grasslands had not yet spread North America was dominated by forest and forest mammals as seen above. The odd-toed perissodactyls, such as the Palaeosyops in the rear center had already appeared. In the trees next to them are the primitive rodent Ischyrotomys and the primate Smilodectes.
A group of primitive tapirs (Helatetes) are in the center of the frame. Just in front of them, looking at a group of Orohippus (a perrisodactyl), is a predatory Patreofelis. Just below a saber-toothed Machaeroides defends its kill (a lizard, Echmatemys) from the fox-size carnivore, Sinopa.
A couple of artiodactyls, the tiny Homacodon and Helohyus, are seen just above an edentate Metacheiromys leaving the frame above the saber-tooth and its kill.
The global cooling that eventually leads to later ice ages begins during this epoch. Forests begin to shrink and grasslands expand at the expense of forests.
