AskDefine | Define eohippus

Dictionary Definition

eohippus n : earliest horse; extinct primitive dog-sized 4-toed Eocene animal [syn: dawn horse]

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English

Noun

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Extensive Definition

Hyracotherium ("Hyrax-like beast") (also known as Eohippus) 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. It was once considered to be the earliest known member of the horse family before being reclassified as a palaeothere, of a perissodactyl family related to both horses and brontotheres.

Discovery

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.

Description

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.

Evolutionary role

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. It is now regarded as a paleothere, rather than a horse proper, but this is only true of the type species, H. leporinum. 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. At one time, Xenicohippus was regarded as an early brontothere.

Miscellaneous

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.

References

eohippus in Finnish: Hyracotherium
eohippus in Czech: Hyracotherium
eohippus in German: Hyracotherium
eohippus in Spanish: Hyracotherium
eohippus in French: Hyracotherium
eohippus in Italian: Hyracotherium
eohippus in Hebrew: איוהיפוס
eohippus in Dutch: Hyracotherium
eohippus in Polish: Eohippus
eohippus in Portuguese: Hyracotherium
eohippus in Swedish: Hyracotherium
eohippus in Chinese: 始祖馬
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