Eohippus, the "First Horse"

Skeleton of Eohippus
National Museum of Natural History

In paleontology, correctly naming a new genus of an extinct animal can often be a long, tortured affair. Eohippus, aka Hyracotherium, is a good case study: This prehistoric horse was first described by the famous 19th century paleontologist Richard Owen, who mistook it for an ancestor of the hyrax, a small hoofed mammal—hence the name he bestowed on it in 1876, Greek for "hyrax-like mammal."

A few decades later, another eminent paleontologist, Othniel C. Marsh, gave a similar skeleton discovered in North America the more memorable name Eohippus, or "dawn horse."

Since Hyracotherium and Eohippus were considered for a long time to be identical, the rules of paleontology dictated that this mammal be called by its original name, the one bestowed by Owen. Never mind that Eohippus was the name used in countless encyclopedias, children's books, and TV shows.

Now, the weight of opinion is that Hyracotherium and Eohippus were closely related, but they were not identical. The result is that it's once again kosher to refer to the American specimen, at least, as Eohippus.

Amusingly, the late evolutionary scientist Stephen Jay Gould railed against the depiction of Eohippus in the popular media as a fox-size mammal, when in fact it was the size of a deer.

Ancestor of Modern Horses

There's a similar amount of confusion about whether Eohippus or Hyracotherium deserves to be called the "first horse." When you go back in the fossil record 50 million or so years, it can be difficult, verging on impossible, to identify the ancestral forms of any given extant species.

Today, most paleontologists classify Hyracotherium as a "palaeothere," that is, a perissodactyl, or odd-toed ungulate, ancestral to horses and the giant plant-eating mammals known as brontotheres typified by Brontotherium, the "thunder beast." Its close cousin Eohippus, on the other hand, seems to deserve a place more firmly in the equid than in the palaeothere family tree, though, of course, this is still up for debate.

Whatever you choose to call it, Eohippus was clearly at least partly ancestral to all modern-day horses, as well as to the numerous species of prehistoric horse, such as Epihippus and Merychippus, that roamed the North American and Eurasian plains of the Tertiary and Quaternary periods. As with many such evolutionary precursors, Eohippus didn't look much like a horse, with its slender, deerlike, 50-pound body and three- and four-toed feet.

Also, judging by the shape of its teeth, Eohippus munched on low-lying leaves rather than grass. In the early Eocene epoch, during which Eohippus lived, grasses had yet to spread across the North American plains, which spurred the evolution of grass-eating equids.

Facts About Eohippus

Eohippus, Greek for "dawn horse," pronounced EE-oh-HIP-us; also known (possibly not correctly) as Hyracotherium, Greek for "hyrax-like beast," pronounced HIGH-rack-oh-THEE-ree-um

Habitat: Woodlands of North America and Western Europe

Historical Epoch: Early-Middle Eocene (55 million to 45 million years ago)

Size and Weight: About two feet high and 50 pounds

Diet: Plants

Distinguishing Characteristics: Small size; four-toed front and three-toed back feet