Horses

Horses have gone through their own distinct set of changes since they first appeared in the forests, 55 million years ago in the  Eocene epoch. Changes in body size, and number of toes are just two patterns among fossil horses, as vegetation and climate changes have helped to create today's living world.

Hyracotherium

Found in North America and Europe, smaller than a domestic cat, Hyracotherium was a browser, feeding on seeds, fruits and young leaves. It was not designed for high speeds, but padded about on four-toed feet on the forest floor. For the next 55 million years, horses developed longer teeth to deal with tough, abrasive grasses.

Hipparion

In the progression to fewer toes, toe three became larger and longer, until it became as broad as the horse's leg. Side toes were reduced and eventually lost in modern horses. Three-toed Hipparion placed most of its weight on its large middle toe, and probably managed a running-walk of speeds up to 15km per hour (9 mph).

During the early history of the horse, changes in climate reduced the area of forests. About 23 million years ago grass plains spread over wide areas. Horses such as Hipparion adapted to a grazing habit by the development of long, heavily-ridged teeth. The ridges on the grinding surfaces were made of hard, resistant enamel. Like coarse files, the ridged tooth rows could grind the tough cellulose of grass plants.

The Hipparion lived about 15 million years ago in Europe, Asia, Africa and North America.