''Scaphohippus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

equine Equinae is a subfamily of the family Equidae, which have lived worldwide (except Indonesia and Australia) from the Hemingfordian stage of the Early Miocene (16 million years ago) onwards. They are thought to be a monophyletic grouping.B. J. Ma ...

, with two known species, known from fossils found in California, New Mexico, Montana, and Nebraska.The Paleobiology Database ''Scaphohippus'' entry
/ref>

History

Both species in the genus were originally described as members of the

wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined ...

Merychippus ''Merychippus'' is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene, 15.97–5.33 million years ago. It had three toes on each foot and is the first horse known to have grazed. Discovery and nami ...

'' in the 1915 by John Merriam. The genus was described from specimens found in the

Barstow Formation The Barstow Formation is a series of limestones, conglomerates, sandstones, siltstones and shales exposed in the Mojave Desert near Barstow in San Bernardino County, California.Dibblee, T.W., Jr. (1967). Areal Geology of the Western Mojave De ...

of southeast California. Specimens of ''Scaphohippus'' were medium-sized

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...

s with

hypsodont Hypsodont is a pattern of dentition with high-crowned teeth and enamel extending past the gum line, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritt ...

(high-crowned) teeth.

Phylogenetic relationships

The genus is closely related to ''

Protohippus ''Protohippus'' is an extinct three-toed genus of horse. It was roughly the size of a modern donkey. Fossil evidence suggests that it lived during the Late Miocene (Clarendonian to Hemphillian), from about 13.6 Ma to 5.3 Ma. Analysis of ''Proto ...

'' and ''

Callippus Callippus (; grc, Κάλλιππος; c. 370 BC – c. 300 BC) was a Greek astronomer and mathematician. Biography Callippus was born at Cyzicus, and studied under Eudoxus of Cnidus at the Academy of Plato. He also worked with Aristotle at th ...

''. In the Barstow Formation, the two species of ''Scaphohippus'' occur with only one other hypsodont horse, '' Acritohippus stylodontus''. ''Scaphohippus'' apparently developed on the

Great Plains The Great Plains (french: Grandes Plaines), sometimes simply "the Plains", is a broad expanse of flatland in North America. It is located west of the Mississippi River and east of the Rocky Mountains, much of it covered in prairie, steppe, an ...

late in the

Hemingfordian The Hemingfordian on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 20,600,000 to 16,300,000 years BP. It is usually considered to overlap the lates ...

, about 17 to 16 million years ago and invaded the

Great Basin The Great Basin is the largest area of contiguous endorheic basin, endorheic watersheds, those with no outlets, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California ...

at about 15 million years ago. The Great Plains and Great Basin were then isolated by tectonic barriers and ''Scaphohippus'' became extinct in the Great Plains, but survived in the Great Basin, until the barriers disappeared at about 13 million years ago and ''Scaphohippus'' also became extinct in the Great Basin.

Species

''Scaphohippus sumani''

''Scaphohippus sumani'', the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

for the genus, was originally described in 1915 by John Merriam as ''Merychippus sumani'' from

University of California Museum of Paleontology The University of California Museum of Paleontology (UCMP) is a paleontology museum located on the campus of the University of California, Berkeley. The museum is within the Valley Life Sciences Building (VLSB), designed by George W. Kelham and ...

specimen number 21422, consisting of four teeth. The specimen was collected from fossiliferous layers of the Barstow Formation outcropping in Rodent Hill Basin,

San Bernardino County, California San Bernardino County (), officially the County of San Bernardino, is a County (United States), county located in the Southern California, southern portion of the U.S. state of California, and is located within the Inland Empire area. As of the ...

. Specimens of the species have also been found in three other sites in California and one in Nebraska. While numerous specimens of isolated teeth and partial

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sepa ...

s have been found at the type locality, comparatively few complete skulls are known. The few examples from the location are badly crushed or have been altered due to pressure during fossilization. Detailed measurement of teeth from the type locality enabled identification of a ''S. sumani'' skull from the Punchbowl Formation, Cajon Valley, California and one from the

Olcott Formation The Olcott Formation is a geologic formation in Nebraska. It preserves fossils dating to the Barstovian stage of the Neogene period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark ...

Sioux County, Nebraska Sioux County is a County (United States), county in the U.S. state of Nebraska. As of the 2010 United States Census, the population was 1,311. Its county seat is Harrison, Nebraska, Harrison. Sioux County is included in the Scottsbluff, Nebrask ...

.The Paleobiology Database East Sand Quarry entry
/ref>

''Scaphohippus intermontanus''

''Scaphohippus intermontanus'', the second species in the genus ''Scaphohippus'', is distinguishable from ''S. sumani'' by several distinct features of the teeth. The upper cheek teeth have a height of 40–50 mm at the mesostyle and the

protocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

connects with the protoconule after approximately 30% wear. The enamel fossettes are both simpler and possess fewer plications than those of ''S. sumani''. The species was first described as ''Merychippus intermontanus'' in the same 1915 paper by John Merriam in which ''"Merychippus" sumani'' was described. The type specimen for ''S. intermontanus'' is a section of jaw containing 6 teeth found in the ''Hemicyon'' Tuff, Barstow Formation, that outcrops in Hellgate Basin, San Bernardino County. The species has been confidently identified from several California sites with possible specimens known from Montana and New Mexico.

References

{{Taxonbar, from=Q7430156 Miocene horses Miocene odd-toed ungulates Fossil taxa described in 2006 Miocene mammals of North America Serravallian extinctions Prehistoric placental genera Aquitanian genus first appearances