''Equus simplicidens'', also known as the Hagerman horse and American zebra, is an extinct species of

equine Equinae is a subfamily of the family Equidae, known from the Hemingfordian stage of the Early Miocene (16 million years ago) onwards. They originated in North America, before dispersing to every continent except Australia and Antarctica. They are ...

native to North America during the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Early Pleistocene The Early Pleistocene is an unofficial epoch (geology), sub-epoch in the international geologic timescale in chronostratigraphy, representing the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently esti ...

. It is one of the oldest and most primitive members of the genus '' Equus.'' It is the

state fossil Most states in the US have designated a state fossil, many during the 1980s. It is common to designate a fossilized species, rather than a single specimen or a category of fossils. State fossils are distinct from other state emblems like state d ...

Idaho Idaho ( ) is a landlocked U.S. state, state in the Pacific Northwest and Mountain states, Mountain West subregions of the Western United States. It borders Montana and Wyoming to the east, Nevada and Utah to the south, and Washington (state), ...

, where abundant remains of the species were discovered near the town of Hagerman in 1928.

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 The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on March ...

who then passed them on to Dr. James W. Gidley at the

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

. 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 with a backwall high. Ultimately five nearly complete skeletons, more than 100 skulls, and forty-eight lower jaws as well as numerous isolated bones were found. 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. 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.

Classification

The Hagerman horse was given the scientific name of ''Plesippus shoshonensis'' in 1930 by a Smithsonian

paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

named James W. Gidley (1930) A new Pliocene horse from Idaho. ''Journal of Mammalogy'' 11: 300–303 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 Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontology, paleontologist, comparative anatomy, comparative anatomist, herpetology, herpetologist, and ichthyology, ichthyologist. Born to a wealthy Quaker fam ...

in 1892. Because of this similarity, the two forms were interpreted (1951) late Cenozoic vertebrates of the San Francisco Bay region. ''University of California Bulletin of the Department of Geological Sciences'' 28: 215–314 to be the same species, and since the name ''Equus simplicidens'' was the older name, it was retained following the taxonomic

Principle of Priority Priority is a principle in Taxonomy (biology), biological taxonomy by which a valid scientific name is established based on the oldest available name. It is a decisive rule in Botanical nomenclature, botanical and zoological nomenclature to recogn ...

. The Hagerman fossils represent some of the oldest widely accepted remains of the genus ''Equus''. The genus placement of the species is controversial, with some authors choosing to place the species in '' Plesippus'' instead. A 2019 phylogenetic analysis found it to be more closely related to living ''Equus'' than to ''

Hippidion ''Hippidion'' (meaning ''little horse'') is an extinct genus of equine that lived in South America from the Late Pliocene to the end of the Late Pleistocene (Lujanian), between 2.5 million and 11,000 years ago. They were one of two lineages of eq ...

'' or ''

Dinohippus ''Dinohippus'' ( Greek: "Terrible horse") is an extinct equid which was endemic to North America from the late Hemphillian stage of the Miocene through the Zanclean stage of the Pliocene (10.3—3.6 mya) and in existence for approximately . Foss ...

'', but outside the group containing all living equines.

Evolution

The earliest remains of ''Equus simplicidens'' date to the Pliocene, around 4.1-3.2 million years ago. ''Equus simplicidens'' has been suggested to be the ancestor of the Eurasian "stenonine" equines such as '' Equus 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 living

zebras Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped Animal coat, coats. There are three Extant taxon, living species: Grévy's zebra (''Equus grevyi''), the plains zebra (''E. quagga''), and t ...

and asses. The youngest fossils of ''E. simplicidens'' date to the

Irvingtonian The Irvingtonian North American Land Mammal Age on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), spanning from 1.8 million – 250,000 years Before Present, BP.

Description

The body mass of adult ''Equus simplicidens'' been estimated at around in life, comparable to a zebra. The overall form of the skull has been considered comparable to those of zebras, though the dental morphology is more primitive. The

vomer The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...

bone of the skull is noticeably elongate, with the preorbital fossa being relatively large. The ramus of the mandible is angled posteriorly.

Distribution

Remains of ''Equus simplicidens'' are known from across Western and Central North America, including Idaho,

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

Nebraska Nebraska ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Ka ...

and

Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...

, southwards to Mexico.

Ecology

''Equus simplicidens'' is thought to have had a highly abrasive almost exclusively

grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to free range (roam around) and consume wild vegetations in order to feed conversion ratio, convert the otherwise indigestible (by human diges ...

based diet, similar to that of modern equines, particularly zebras. The species is suggested to have had a herding social structure similar to living ''Equus'', perhaps lead by a dominant harem stallion male similar to living horses.

References

The Hagerman Horse Quarry

Dr. Greg McDonald

External links

{{Taxonbar, from=Q284310 Pliocene horses Prehistoric mammals of North America Pliocene Perissodactyla Pliocene first appearances Pleistocene extinctions Pleistocene horses Pliocene mammals of North America Pleistocene mammals of North America Equus (genus) Extinct animals of the United States Taxa named by Edward Drinker Cope Fossil taxa described in 1892 Symbols of Idaho