File:Pliohippus pernix (stout one-toed horse) foot bones (Ash Hollow Formation, Miocene, 11.83 Ma; Ashfall Fossil Beds, Nebraska, USA).jpg

English: Pliohippus pernix Marsh, 1874 - stout one-toed horse foot bones from the Miocene of Nebraska, USA.

This fossil horse foot was excavated from a volcanic ash deposit at Nebraska's Ashfall Fossil Beds. The ash was 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.

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From exhibit info.:

EVOLUTION IN ACTION - HORSE FEET -

Was Darwin right? Today's horses have a single toe (hoof) on each foot. Scientists of Darwin's day predicted that ancestral horses with more toes would someday be found as fossils.

Ashfall proved them right! As you can see in these front feet of horse skeletons from the ash bed, most kinds had 3 toes but one has a single hoof.

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Stouth One-toed Horse Pliohippus

The largest single-toed horse found at Ashfall, Pliohippus was about the size of a big white-tail or mule deer. Like the other four kinds of horses excavated from the ash bed, Pliohippus was a grazer that probably lived in social herds as modern horses do. When the first partial skeleton of Pliohippus was discovered near Valentine, Nebraska, in 1873, it created excitement in the world of paleontology because it was the last major "missing link" in the evolution of the horse to be discovered. By then, the tiny horses from 30 to 50 million years ago were already known and sheep-sized 3-toed horses had been collected from 15 to 20 million-year-old beds but no fossils were known that demonstrated how the vestigial "extra" toes were eliminated.

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)

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

A pair of deep, pocket-like depressions in the skull ahead of the eye socket distinguishes Pliohippus from other horses.

Hippos means horse and Plio refers to the Pliocene Epoch of geologic time, 2 to 5 million years ago.

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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: Ashfall Fossil Beds State Historical Park, northeastern Nebraska, USA

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

en.wikipedia.org/wiki/Ashfall_Fossil_Beds