Hagerman horse Temporal range: Pliocene-Early Pleistocene
Mounted skeleton of a Hagerman horse
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Equidae
Genus:	Equus
Subgenus:	Dolichohippus
Species:	†E. simplicidens
Binomial name
†Equus simplicidens Cope, 1892[1]
Synonyms
Plesippus shoshonensis Gidley, 1930[2]

Equus simplicidens, sometimes known as the Hagerman horse or the American Zebra is an extinct species of equine native to North America during the Pliocene and Early Pleistocene. It is one of the oldest and most primitive members of the genus Equus. Abundant remains of it were discovered in 1928 in Hagerman, Idaho. It is the state fossil of Idaho.^[3]

Classification

The Hagerman horse was given the scientific name of Plesippus shoshonensis in 1930 by a Smithsonian paleontologist named James W. Gidley^[2] who led the initial excavations at Hagerman that same year.

However further study by other paleontologists determined that fossils closely resembled fossils of a primitive horse from Texas named Equus simplicidens, named by paleontologist Edward Drinker Cope in 1892. Because of this similarity, the two forms were interpreted^[4] to be the same species, and since the name Equus simplicidens was the older name, it was retained following the taxonomic Principle of Priority. The Hagerman fossils represent some of the oldest widely accepted remains of the genus Equus.

Discovery

A cattle rancher named Elmer Cook discovered some fossil bones on this land in Hagerman, Idaho. In 1928, he showed them to Dr. H. T. Stearns of the U.S. Geological Survey who then passed them on to Dr. James W. Gidley at the Smithsonian Institution. Identified as bones belonging to an extinct horse, the area where the fossils were discovered, called the Hagerman Horse Quarry, was excavated and three tons of specimens were sent back to the Smithsonian in Washington, D.C.

Excavation of the fossils continued into the early 1930s. The Hagerman Horse Quarry floor grew to 5,000 square feet (460 m²) with a backwall 45 feet (14 m) high. Ultimately five nearly complete skeletons, more than 100 skulls, and forty-eight lower jaws as well as numerous isolated bones were found. Some paleontologists believed that such a large amount of fossils found in one location was because of the quarry area being a watering hole at one point. The waterhole could have been where the bones of the Hagerman horses accumulated as injured, old, and ill animals, drawn to water, died there. Other paleontologists think that an entire herd of these animals drowned attempting to ford a flooded river and were swept away in the current and ended up buried in the soft sand at the bottom.

Taxonomy

The genus placement of the species is controversial, with some authors choosing to place the species in Plesippus instead.^[5] A 2019 phylogenetic analysis found it to be more closely related to living Equus than to Hippidion or Dinohippus, but outside the group containing all living equines.^[6] Some authors have argued for a close relationship of Equus simplicidens with living zebras to the exclusion of other living equines, but these claims have been considered equivocal by others, who note that many "stenonine horses" from Eurasia exhibit similarities to zebras.^[7]