File:Cormohipparion occidentale (sturdy three-toed horse) in volcanic tuff (Ash Hollow Formation, Miocene, 11.83 Ma; Ashfall Fossil Beds, Nebraska, USA) 2.jpg

English: Cormohipparion occidentale (Leidy, 1856) - sturdy three-toed horse skull in volcanic tuff in the Miocene of Nebraska, USA.

Volcanic tuff is a fine-grained, clastic-textured, extrusive igneous rock - basically a solidified volcanic ash deposit. Volcanic tuff can be well to poorly indurated / lithified. This exposure is at Nebraska's Ashfall Fossil Beds. The ash is derived from the Bruneau-Jarbidge Volcanic Field in southwestern Idaho's Snake River Plain. The wind-blown ash buried a vertebrate-rich biota at what was originally a Miocene waterhole.

The skull seen here is Cormohipparion occidentale, the sturdy three-toed horse, a long-extinct species.

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From on-site info.:

THREE-TOED HORSE Cormohipparion occidentale

Notice the "side-toes" on each foot, the slender legs and light build. This is one of the larger of the several kinds of horses living in this area 11.8 million years ago.

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A PERFECTLY PRESERVED STURDY 3-TOED HORSE SKELETON FROM THE VOLCANIC ASH BED

Scientific name: Cormohipparion occidentale

This full-grown stallion (identified as male by its large canine teeth) was one of the largest horses alive 12 million years ago. Notice the good-sized "side toes" - one on each side of the main hoof - on the front and hind feet of this animal. Prior to 15 million years ago all known fossil horses had at least three hooves on each foot like this one, but as grasslands began to expand, some horse species began to lose the side toes, eventually resulting in the evolution of the modern horse foot, which has a single hoof.

This skeleton shows no reduction of the side toes. Along with other features (deep pits ahead of the eye sockets on the skull, complex folding of the enamel on the grinding teeth) this is considered to be good evidence against Cormohipparion being a likely ancestor for today's horses.

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Sturdy Three-toed Horse Cormohipparion

Nebraska-born paleontologist Morris Skinner collected and studied fossil horses for more than 50 years. One of his greatest discoveries was a series of well-preserved skulls of Nebraska horses apparently ancestral to the "Hipparion" horses of Europe and Asia. Other paleontologists had been forced to rely mostly on teeth in researching horse evolution but Skinner pointed out important details of the skull - especially the position and shape of depressions in front of the eye socket. Announcing this discovery in 1977, Skinner and a younger colleague Bruce MacFadden named Cormohipparion. The first complete skeletons of this animal have been found here at Ashfall. These show that the foot bones of Cormohipparion were shorter than those of Neohipparion, a horse with very similar teeth.

Remains of this animal have been found: X In the "RECOVERY" layer (sandstone above the ash) X In the "DISASTER" layer (volcanic ash bed) X In the "WATERHOLE" layer (sandstone below the ash)

Cormohipparion was about 3.5 feet (1.1 meters) at the shoulder.

The Ashfall species, Cormohipparion occidentale, has one of the most complicated enamel patterns on the grinding teeth of any North American horse species. This would help grind grass into small, digestible fragments.

A deep pocket in the skull far forward of the eye is characteristic of this horse.

Cormo means "stem" and Hipparion is the name of the 3-toed grazing horse of the Old World.

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Classification: Animalia, Chordata, Vertebrata, Mammalia, Perissodactyla, Equidae

Stratigraphy: Cap Rock Member, Ash Hollow Formation, Ogallala Group, Miocene, 11.83 Ma

Locality: Hubbard Rhino Barn, Ashfall Fossil Beds State Historical Park, northeastern Nebraska, USA

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Info. at: en.wikipedia.org/wiki/Cormohipparion and

en.wikipedia.org/wiki/Ashfall_Fossil_Beds