The dinosaurs had only been extinct about ten million years when the Eocene began. Mammals evolved larger bodies and became more diverse during the Eocene.

Plant-eating mammals included ancient hooved animals called condylarths, while meat-eaters included an extinct group of dog-like beasts called creodonts. Eight-foot-high “terror birds” also reigned during the Eocene.

Many groups of “modern” animals made their first appearance in the eocene. These include the first members of the orders of elephants, bats, whales, even-toed hooved animals (artiodactyls) and the odd-toed perissodactyls. The first horse, Hyracotherium (aka eohippus), appeared in the eocene. It is the earliest member of the family equidae, which includes all extinct horses and the modern genus Equus.

Although we can begin to recognize certain types of mammals during the Eocene, such as carnivores and perissodactyls, they bare little resemblance to their modern counterparts.

The Green River Formation of Utah-Colorado-Wyoming is a widespread, Eocene-aged lake deposit that is famous for its Eocene fish fossils.

Horses that lived during the Eocene:

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