Equus simplicidens, also known as theHagerman horse andAmerican zebra, is an extinct species ofequine native to North America during thePliocene andEarly Pleistocene.[1] It is one of the oldest and most primitive members of the genusEquus. It is thestate fossil ofIdaho, where abundant remains of the species were discovered near the town ofHagerman in 1928.
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 theU.S. Geological Survey who then passed them on to Dr. James W. Gidley at theSmithsonian Institution. Identified as bones belonging to an extinct horse, the area where the fossils were discovered, called theHagerman Horse Quarry, was excavated and three tons of specimens were sent back to the Smithsonian in Washington, D.C.[4]
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.[4] Gidley 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.[4] A study by H. Gregory McDonald in 1996 alternatively suggested based on the age distribution of remains at the quarry, which span from newborns to adults, that a herd died in a single catastrophic event, perhaps attempting to cross a swelled/flooded river.[5]
The Hagerman horse was given the scientific name ofPlesippus shoshonensis in 1930 by aSmithsonianpaleontologist named James W. Gidley[3] 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 namedEquus simplicidens, named by paleontologistEdward Drinker Cope in 1892. Because of this similarity, the two forms were interpreted[6] to be the same species, and since the nameEquus simplicidens was the older name, it was retained following the taxonomicPrinciple of Priority. The Hagerman fossils represent some of the oldest widely accepted remains of the genusEquus.[7]
The genus placement of the species is controversial, with some authors choosing to place the species inPlesippus instead.[8] A 2019 phylogenetic analysis found it to be more closely related to livingEquus than toHippidion orDinohippus, but outside the group containing all living equines.[9]
The earliest remains ofEquus simplicidens date to the Pliocene, around 4.1-3.2 million years years ago.[10][11]Equus simplicidens has been suggested to be the ancestor of the Eurasian "stenonine" equines such asEquus stenonis, which first appeared in Eurasia at the beginning of the Pleistocene, around 2.6 million years ago, which have been proposed to be the ancestors of livingzebras andasses.[7][10] The youngest fossils ofE. simplicidens date to theIrvingtonian.[12]
The body mass of adultEquus simplicidens been estimated at around 300–400 kilograms (660–880 lb) in life, comparable to a zebra.[10] The overall form of the skull has been considered comparable to those of zebras, though the dental morphology is more primitive.[12] Thevomer bone of the skull is noticeably elongate, with the preorbital fossa being relatively large. The ramus of the mandible is angled posteriorly.[7]
Remains ofEquus simplicidens are known from across Western and Central North America, including Idaho,Arizona,California,Texas,Nebraska andKansas, southwards to Mexico.[12]
Equus simplicidens is thought to have had a highly abrasive almost exclusivelygrazing based diet, similar to that of modern equines, particularly zebras.[10][13] The species is suggested to have had a herding social structure similar to livingEquus, perhaps lead by a dominant harem stallion male similar to living horses.[5]
^abSkinner, M.F. (1972). "Order Perissodactyla". In Skinner, M.F.; Hibbard, C.W. (eds.).Early Pleistocene Preglacial and Glacial Rocks and Faunas of North-Central Nebraska. Vol. 148. New York: American Museum of Natural History. pp. 117–125.S2CID129067309.
^D.E. Savage (1951) late Cenozoic vertebrates of the San Francisco Bay region.University of California Bulletin of the Department of Geological Sciences 28: 215-314
Boss, N. H. "Explorations for Fossil Horses in Idaho".Explorations and Field Work of the Smithsonian Institution in 1931. 1932.
Gazin, C. L. '" Study of the Fossil Horse Remains from the Upper Pliocene of Idaho.Proceedings from the United States National Museum 83(2,985): 281-320. 1936.
MacFadden, Bruce J.Fossil Horses: Systematics, Paleobiology and Evolution of the Family Equidae. Cambridge Univ. Press, 1992.
McDonald, H. Gregory. "More than Just Horses",Rocks and Minerals, September/October 1993. Vol. 68:322-326.
Willoughby, David P.The Empire of Equus. A.S. Barnes and Co., 1974
