| Hyracotherium Temporal range: Early Eocene–Mid Eocene |
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| Mounted replica of a Hyracotherium vasacciensis skeleton | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Perissodactyla |
| Family: | †Palaeotheriidae |
| Genus: | †Hyracotherium Owen, 1841 |
| Species: | †H. leporinum |
| Binomial name | |
| Hyracotherium leporinum Owen, 1841 |
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| Synonyms | |
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?Eohippus Marsh, 1876 |
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Hyracotherium ( /ˌhaɪərəkɵˈθɪəriəm/ hy-rək-o-theer-ee-əm; "Hyrax-like beast"), also known as Eohippus or the dawn horse, is an extinct genus of very small (about 60 cm in length) perissodactyl ungulates that lived in the woodlands of the northern hemisphere, with species ranging throughout Asia, Europe, and North America during the early Tertiary Period and the early to mid Eocene Epoch, about 55—45 million years ago[1] with the earliest fossil specimen found at the Tsagan Khushu Quarry 1 site, Mongolia averaging about 60 cm in length and weighing around 15–16 kg (36 lbs).[2] This small, dog-sized animal is the oldest known horse and was once considered to be the earliest known member of Equidae[3] before the type species was reclassified as a palaeothere, of a perissodactyl family related to both horses and brontotheres.
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The first fossils of this genus were found in England and described by the paleontologist Richard Owen in 1841. Suspecting that his species was a hyrax due to its teeth, but lacking parts of the skeleton, Owen called it a "Hyrax-like beast" and placed it in the new genus Hyracotherium. In 1876 in America Othniel C. Marsh found a full skeleton, which he placed in another new genus Eohippus, from the Greek ηώς (eōs, "dawn") and ιππος (hippos, "horse"), meaning "dawn horse". When it became apparent that the two genera were likely one and the same, Eohippus for a time became a synonym of Hyracotherium, the genus with the earlier date of publication.
Hyracotherium averaged two feet (60-cm) in length and eight to 14-inches (20-cm) high at the shoulder and weighed about 50 pounds. It had four-hoofed toes on each front foot and three-hoofed toes on each hind foot. Each toe had a pad on its underside, similar to those of a dog. It had a primitive, short face with eye sockets in the middle and a short diastema (the space between the front teeth and the cheek teeth). The skull was long, having 44 low-crowned teeth. Although it had low-crowned teeth, the beginnings of the characteristic horse-like ridges on the molars can be seen. Hyracotherium is believed to have been a browsing herbivore that ate primarily soft leaves as well as some fruits and nuts and plant shoots.[4]
It is proposed by some scientists that the Hyracotherium was not only ancestral to the horse, but to other perissodactyls such as rhinos and tapirs.[5] It is now regarded as a paleothere, rather than a horse proper, but this is only true of the type species, H. leporinum.[6][7] Most other species of Hyracotherium are still regarded as equids, but they have been placed in several other genera, such as Arenahippus, Minippus, Pliolophus, Protorohippus, Sifrhippus, Xenicohippus, and even Eohippus.[7] At one time, Xenicohippus was regarded as an early brontothere. The main stream of horse evolution occurred on the North American continent.
In elementary level textbooks, Hyracotherium is commonly described as being "the size of a small Fox Terrier", despite the Fox Terrier being about half the size of Hyracotherium. This arcane analogy was so curious that Stephen Jay Gould wrote an essay about it ("The Case of the Creeping Fox Terrier Clone", essay #10 in his book, Bully for Brontosaurus), 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. While the Hyracotherium was twice the size of a terrier, later horse genera like Propalaeotherium could indeed be terrier sized, down to 30 cm at the shoulder.[8]