Hagerman "Horse" - Equus simplicidens
What makes the Hagerman Horse so important? First, the discovery from Hagerman is the largest sample of this extinct species from one locality. Over two hundred individuals of both sexes and all ages were recovered by the Smithsonian. Included are complete skeletons as well as skulls, jaws and detached bones. They were about the size of the present day Arabian horse, and had a single toe (hoof). Vertebrate paleontologists must often work with single, isolated bones or teeth. So it is often difficult to assign them to an already described species when differences in sex or age of an individual are taken into consideration. The large number of individuals recovered at the Hagerman Horse Quarry simplifies this problem here.
Despite the popular use of the name, Hagerman Horse, it is actually more closely related to the zebras. Although we don't have fossil evidence of stripes, the pattern of the chewing surfaces of the teeth and details of the skull and rest of the skeleton indicate that this animal was more closely related to the living Grevy's Zebra of Africa than to horses. So the next time you're at the zoo, take a good look at the zebras on display and you'll have an opportunity to see a close relative of one of the earliest residents of the Hagerman area.
Many different scientific names have been applied to this horse. James W. Gidley, the Smithsonian paleontologist, who led the initial excavations at Hagerman in 1929, felt that the horse being uncovered was different enough in its skeleton that it represented a new species distinct from any other known fossil horse. He proposed the name Plesippus shoshonensis. By placing the Hagerman horse in the genus Plesippus, he considered it to be closely related to another fossil species, Plesippus simplicidens, from Texas. Although another horse, Equus idahoensis, had been described from elsewhere in this region, Dr. Gidley considered his new species to be more primitive. Since the early work of Gidley, many other studies on fossil horses have been made and the consensus is that the horse at Hagerman does belong in the modern genus Equus and that it is the same as the extinct species from Texas, simplicidens. So today most paleontologists refer to the Hagerman Horse by the scientific name of Equus simplicidens.
The Hagerman Horse also has the distinction of being the earliest record of Equus, the genus that includes all modern horses, donkeys, and zebras.
Even though the species found at Hagerman, Equus simplicidens, is known from elsewhere such as Nebraska, Florida, and Texas, all of the other records are much younger, making the sample from Hagerman the oldest.
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This article originally appeared in The Fossil Record, March 1993
Last updated: November 22, 2016