The Hagerman Horse (Equus simplicidens)

Fossil leg and hoof of the Hagerman Horse. This species is the earliest-known single-toed horse.

NPS Photo/ Faith Brown

Quick Facts!

• Common name: Hagerman horse
• Scientific name: Equus simplicidens
• Geographical range: throughout Western United States and Mexico
• First appeared: Middle Pliocene (around 3.7 million years ago)
• Height: 43–57 in (1.2–1.4 m) tall at the shoulder
• Habitat: open grasslands
• Closest living relatives: living Equus species (horses, donkeys, and zebras)

Fossil Fact!

The Hagerman horse is the state fossil of Idaho!

The Tail of the Hagerman Horse

Hagerman Fossil Beds National Monument is best known for Equus simplicidens, commonly known as the Hagerman horse. Specimens from over 200 individual Hagerman horses were found, the largest single find of this species, and included individuals of all ages and both sexes. Equus simplicidens is one of the first species of the genus Equus, which makes it an ancestor of modern-day horses.

Learn more about the first fossil excavations at Hagerman

The remains of over 200 Hagerman horses uncovered by the Smithsonian remain in the Institution's collection. In the past, specimens have been on display in Washington, D.C. and in other prominent museums across the country.

Have you Herd?

Multiple individuals of Equus simplicidens of various ages were found together, which indicates that the animal was a herd species that lived together as a community. Through comparing the anatomy of Equus simplicidens to modern Equus species, scientists have determined it is more closely related to modern zebras than to modern horses.

The Mane Takeaway

The Equus simplicidens fossils found at Hagerman Fossil Beds National Monument are important because of the quantity and quality of fossils of a single species found in one place. In a field where species are often identified by a single body part, full skeletons provide a wealth of knowledge for the scientific community. When new fossils are identified and studied, previous hypotheses can be strengthened or new hypotheses can be formed based on the additional evidence. Finding hundreds of fossils from the same species in a single area allows scientists to gain information that would have not been possible with just a single specimen.

A Piece of the Puzzle

Horses have always been on the move. For millions of years, early horses, which bore little resemblance to the animals we know today, evolved and migrated between continents. Equus horses evolved in North America during the Pliocene, spread across the world, and eventually came full circle when Europeans brought their domesticated descendents back to the continent where they first originated.

There are still many unanswered questions about the evolution, migration, and eventual domestication of Equus horses. As scientists continue to investigate the complicated story of the horse, the fossils from Hagerman remain an important piece of the puzzle.

55m years ago: The earliest known members of the horse family Equidae appear in the North American fossil record. Slightly younger fossils also appear in Eurasia, indicating that horses have a long history of migration.
4–3m years ago: Herds of Equus simplicidens thrive in Pliocene Hagerman.
3–2m years ago: The first Equus horses migrate out of North America.
1.9m years ago: The ancestors of modern zebras appear in the fossil record.
~500 years ago: Europeans bring horses to the Americas.

NPS graphic. Artwork by Jay Matternes, courtesy of the Smithsonian Institution.