Hyracotherium

Hyracotherium ("Hyrax-like beast") is considered to be the earliest known member of the horse familyFlorida Museum of Natural History and the National Science Foundation: Fossil Horses In Cyberspace Hyracotherium, page 1. It was dog-sized and four-toed and lived in the Northern Hemisphere (in Asia, Europe, and North America) during the Eocene, between 60 and 45 million years ago. Florida Museum of Natural History and the National Science Foundation: Fossil Horses In Cyberspace Hyracotherium, page 2

The first fossils of this animal were found in England by the paleontologist Richard Owen in 1841, who suspected that it was a primate due to its teeth. He did not have a full specimen and called it "Hyrax-like beast". In 1876, Othniel C. Marsh found a full specimen in America, which he named Eohippus ("dawn horse"). When it became clear that the two finds were the same species, and the first published name (Hyracotherium) has priority as the official name and Eohippus is considered a synonym.

Hyracotherium averaged only 2 feet (60 cm) in length and averaged 8 to 9 inches (20 cm) high at the shoulder. It had 4 hoofed toes on the front feet and 3 hoofed toes on each hind foot. The skull was long, having 44 long-crowned teeth. Hyracotherium is believed to have been a browsing herbivore that ate soft leaves and plant shoots.

It is believed by some scientists that the Hyracotherium was not only ancestral to the horse, but to other Perissodactyls such as rhinos and tapirs. Florida Museum of Natural History and the National Science Foundation: Fossil Horses in Cyberspace Hyracotherium, page 3

In elementary level textbooks, Hyracotherium is commonly described as being "the size of a small fox terrier", which is about twice the size of the Hyracotherium. This arcane analogy was so curious that Stephen Jay Gould wrote an essay about it ("The Case of the Creeping Fox Terrier Clone").

References

Prehistoric horses | Eocene

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