Hyracotherium sp.
Early Eocene
Willwood Formation
Powell, Wyoming, USA

55mm jaw section with 3 teeth

Hyracotherium is the earliest known genus in the horse family, Equidea. It’s the same animal that has been called “Eohippus.” Continuing discussion among researchers has generated even more names for early Eocene horse remains. It is possible that at least one species could be reassigned to Eohippus and it appears Hyracotherium may be retained only for European forms. For the time being we’ll stick with Hyracotherium.

These early horses show the basics of horse evolution and exhibit a size range from that of a housecat to that of a collie. Their front feet retained four toes while their back feet had three. Horses of the late Eocene would bear three toes on their front and hind feet. The middle toe would bear most of the animal’s weight in later horses on the way to the one-toed horse, Equus, we know today.

Teeth of Eocene horses are not common on the market and Hyracotherium/Eohippus is the best-known of the early mammals so it’s a must-have in any mammal collection.