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The “middle horse” earned its name. Mesohippus is intermediate between the Eohippus-like horses of the Eocene, which don’t look much like our familiar “horse,” and more “modern” horses.

Mesohippus means "middle horse"Where & When?

Fossils of Mesohippus are found at many Oligocene localities in Colorado and the Great Plains of the US, including Nebraska and the Dakotas, and Canada. This genus lived about 37-32 million years ago.

black and white drawing of Mesohippus horses
Mesohippus are browsing in their forest habitat in this 1913 painting by Bruce Horsfall.


How was Mesohippus intermediate between the ancient Eocene horses and the later, more modern forms?

The Eocene predecessors of Mesohippus had four toes on their front feet, but Mesohippus lost the fourth toe.

Hyrocotherum vs. Mesohippus
At left, the front foot of Hyracotherium. At right, the front foot of Mesohippus.

Also, Mesohippus‘ premolar teeth became more like molars. These premolars are said to be “molariform.” The primitive triangular premolar pulps food, while the squared molariform teeth crush and grind food. This might reflect a shift from a more diverse diet including fruit to a more limited diet of leaves and possibly grass

The first upper premolar is never molarized. It is only occasionally present in modern horses. It is popularly called the “wolf-tooth” by horse-breeders.

diagram comparing hyracotherium and mesohippus teeth rows
The premolars of Eocene horses, like Hyracotherium, Orohippus and Epihippus, were small and triangular compared to their molars (left). Note how the Mesohippus premolars look very similar to the molars (right).

 

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