Orohippus

Orohippus Temporal range: early to middle Eocene
Skeleton of Orohippus pumillus at the Smithsonian Museum of Natural History.
Conservation status
Extinct (IUCN 2.3)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Equidae
Subfamily:	Hyracotheriinae
Genus:	†Orohippus Marsh, 1872
Type species
†Orohippus pumillus
Species
†O. agilis †O. major †O. progressus †O. proteros †O. pumillus †O. sylvaticus

Orohippus (Gr., "mountain horse") is an extinct ancestor of the modern horse that lived in the Eocene (about 50 million years ago).

Restoration by Charles R. Knight

Orohippus skeleton

Restoration

It is believed to have evolved from palaeotheres such as Hyracotherium, as the earliest evidence for Orohippus appears about 2 million years after the first appearance of Hyracotherium. The anatomical differences between the two are slight: they were the same size, but Orohippus had a slimmer body, a more elongated head, slimmer forelimbs and longer hind legs, all of which are characteristics of a good jumper. The upper premolars of Orohippus are more molariform (flat-surfaced) than in Hyracotherium, giving Orohippus more teeth for grinding, and the crests on the teeth are more pronounced, indicating that Orohippus probably fed on tougher plants. The outer toes of Hyracotherium are no longer present in Orohippus, hence on each forelimb there were four fingers (toes) and on each hind leg three toes.

Orohippus has also been referred to as Protorohippus.

See also

• Evolution of the horse

References

• MacFadden, B.J., 1998. Equidae. pp. 537–559 in C.M. Janis, K.M. Scott, and L.L. Jacobs (eds.) Evolution of Tertiary Mammals of North America Cambridge University Press, Cambridge.
• Orohippus, Index Fossils and the Tertiary
• The Evolution of the Horse
• National Center for Science Education