Hyracotherium

Hyracotherium ("Hyrax-like beast") was once considered to be the earliest known member of the horse family^[1]. Now, though, it is considered to be a palaeothere, of a perissodactyl family related to both horses and brontotheres. Hyracotherium was a dog-sized perissodactyl ungulate that lived in the Northern Hemisphere, with species ranging throughout Asia, Europe, and North America during the Early to Mid Eocene, about 60 to 45 million years ago. ^[2]   The first fossils of this animal were found in England by the paleontologist Richard Owen in 1841, who suspected that it was a hyrax due to its teeth. He did not have a full skeleton and called it "Hyrax-like beast". In 1876, Othniel C. Marsh found the full skeleton in America, which he named Eohippus ("dawn horse"). When it became clear that the two finds were closely related, the first published name (Hyracotherium) became official and Eohippus came to be a synonym.

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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 low-crowned teeth. Hyracotherium is believed to have been a browsing herbivore that ate primarily leaves as well as some fruits and nuts.^[3]

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. ^[4] It is now regarded as a paleothere, rather than a horse proper, but this is only true of the type species, H. leporinum.^[5][6] Most other species of Hyracotherium are still regarded as equids, but they have been placed in several other genera: Arenahippus, Minippus, Pliolophus, Protorohippus, Sifrhippus, Xenicohippus, and even Eohippus.^[6] At one time, Xenicohippus was regarded as an early brontothere.

In elementary level textbooks, Hyracotherium is commonly described as being "the size of a small Fox Terrier", which is actually 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"), in which he concluded that Henry Fairfield Osborn had so described it in a widely distributed pamphlet, Osborn being a keen fox hunter who made a natural association between horses and the dogs that accompany them.

See also

• Evolution of the horse
• Mesohippus
• Merychippus
• Equus

References