Equidae in the Eocene
(left), forelimb of Hyracotherium
Forelimb modified from MacFadden 1992.
known as Eohippus) first appeared 55 million years ago (MacFadden
1994) in North America (Harwood 1996). This genus of small, forest-dwelling
animals is generally considered to contain the first members of the family
Equidae, and the ancestor of all members of this family (MacFadden 1992).
Hyracotherium was well adapted to its habitat in the Eocene
forests of the northern hemisphere: it was digitigrade, with four hoofed
toes on the forelegs and three on the hind legs (Azzaroli 1992).
This helped it to move easily over the soft forest terrain (Draper 1992).
Its small size, 2.2-5hh (Johnson 1967),
allowed it to move easily through the dense vegetation and to hide from
predators (Benton 2000). It was also a browser, with low-crowned teeth
suitable for eating leaves, fruit, and other soft vegetation (Draper 1996).
Because of Hyracotherium's success as a forest browser, very
little evolutionary change occured within the genus during the Eocene (Hunt