Orohippus, which translated from Greek means “mountain horse”, is the second oldest known ancestor of the horse. Only Hyracotherium existed before and it is from this that Orohippus originated.
Taxonomic Classification
Rich | Animalia | Animals |
tribe | Chordata | Vertebrates |
Class | Mammalia | Mammals |
Order | Perissodactyla | Odd-toed ungulates |
Family | Equidae | Equines |
Sex | Orohippus | Marsh 1872 |
Species type | Orohippus pumillus |
Species:
- agilis
- major
- progressus
- proteros
- pumillus
- sylvaticus
(Mac Fadden, 1998)
Discovery
Othniel C. Marsh discovered fossils of Orohippus in Eocene deposits from about 52 – 54 million years ago in Wyoming and Oregon. The earliest evidence of the occurrence of Orohippus has been found in layers about 2 million years younger than the one in which the earliest evidence of the occurrence of Hyracotherium was found. The two different ancestors both lived during the Eocene and yet the fossils of Orohippus have not been recovered as numerous and widespread as those of Hyracotherium.
Orohippus, sometimes also called Protorohippus, is the name that Professor OC Marsh gave the animal in 1872. He never really explained this name, because contrary to what the name (Orohippus is translated as mountain horse) suggests, the animal did not live in the mountains. Possibly it could refer to the location of the fossils in Wyoming, USA, namely: Grizzly Butte. A “butte” is a single hill or mountain in an otherwise flat landscape. If Marsh got his inspiration from the Grizzly piece of the name, we would be talking about Ursahippus.
Description
Hyracotherium underwent minor evolutionary changes to give rise to Orohippus.
Hyracotherium and Orohippus were about the same size, Orohippus slightly larger. Furthermore, Orohippus had a slimmer body, a longer head, a slimmer forequarters and longer hind legs. These are all qualities of a good jumper.
Orohippus still had sole pads like Hyracotherium but the rudimentary outer toes had already disappeared from Orohippus. So he had 4 toes on each of the front limbs and 3 toes on each of the hind limbs. Orohippus also had longer middle toes than Hyracotherium, on both the fore and hind limbs.
The most drastic change was found in the teeth: the first premolar of Orohippus was reduced in size and the last (second) premolar changed both in shape and function into a true molar. Both factors made Orohippus’ teeth better adapted to grinding food. Orohippus also had clearer grooves in his molars. From all these things you can deduce that Orohipus ate tougher plants than Hyracotherium.
Evolution
Thus, Orohippus evolved from Hyracotherium, after it underwent some evolutionary changes. They lived for a time at a time. Orohippus evolved into Epihippus, which also shows the greatest evolutionary difference in the teeth.