Orohippus

Orohippus; Temporal range: early to middle Eocene
	Skeleton of Orohippus pumillus at the Smithsonian Museum of Natural History.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Equidae
Genus:	†Orohippus; Marsh, 1872
Type species
	†Orohippus pumillus;
Species
	†O. agilis; †O. major; †O. progressus; †O. proteros; †O. pumillus; †O. sylvaticus;

Orohippus (from the Greek ὄρος óros, 'mountain' and ἵππος híppos, 'horse'^[3]) is an extinct equid that lived in the Eocene (about 50 million years ago). Its fossils have been unearthed in Oregon, Colorado, Utah, and Wyoming.^[4]^[5]^[6]^[7]^[8]^[9]^[10]

Description

It is believed to have evolved from equids such as Eohippus, as the earliest evidence for Orohippus appears about 2 million years after the first appearance of Eohippus.^[11] 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. Its teeth were brachydont in height, but the development of flattened surfaces and shearing lophs on their molars suggests they were more a browser than a frugivore.^[12] The outer toes of Eohippus are no longer present in Orohippus, hence on each forelimb there were four fingers (toes) and on each hind leg three toes.

Species of Orohippus has also been referred to Protorohippus.

Notes

^ MacFadden, 1998, p.554
^ MacFadden, 1998, p.543
^ "Glossary. American Museum of Natural History". Archived from the original on 20 November 2021.
^ Korth, William W.; Evander, Robert L. (1982). "A New Species of Orohippus (Perissodactyla, Equidae) from the Early Eocene of Wyoming". Journal of Vertebrate Paleontology. 2 (2): 167–171. ISSN 0272-4634.
^ Nelson, Michael E. (1973-04-01). "Age and stratigraphic relations of the Fowkes Formation, Eocene, of southwestern Wyoming and northeastern Utah". Rocky Mountain Geology. 12 (1): 27–31. ISSN 1555-7332.
^ Stucky, Richard K.; Prothero, Donald R.; Lohr, Walter G.; Snyder, Jennifer R. (1996-06-13), "Magnetic stratigraphy, sedimentology, and mammalian faunas of the early Uintan Washakie Formation, Sand Wash Basin, northwestern Colorado", The Terrestrial Eocene-Oligocene Transition in North America, Cambridge University Press, pp. 40–51, retrieved 2025-03-09
^ Farr, Marcus S. (1896). "Notes on the Osteology of the White River Horses". Proceedings of the American Philosophical Society. 35 (151): 147–175. ISSN 0003-049X.
^ Turnbull, William D. (2002). The mammalian faunas of the Washakie Formation, Eocene age, of southern Wyoming. Chicago, Ill: Field Museum of Natural History.
^ Marsh, O. C. (2012-03-01), ""Fossil Horses in America," American Naturalist (1874)", An Anthology of Nineteenth-Century American Science Writing, Anthem Press, pp. 177–183, retrieved 2025-03-09
^ Marsh, O. C. (1874-03-01). "Notice of new equine mammals from the Tertiary formation". American Journal of Science. s3-7 (39): 247–258. doi:10.2475/ajs.s3-7.39.247. ISSN 0002-9599.
^ Matthew, W. D. (April 1926). "The Evolution of the Horse: A Record and Its Interpretation". The Quarterly Review of Biology. 1 (2): 139–185. doi:10.1086/394242. ISSN 0033-5770.
^ Kitts, D. B. 1957. A Revision of the Genus Orohippus (Perissodactyla, Equidae). American Museum Novelties, 1864:1–40.

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