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Origins of the Horse's Foot

Horses are hooved animals, the zoological order Ungulata meaning odd-toed. The horse family (Equidae) includes the horse, ass, zebra and quagga, all closely related anatomically. They can all interbreed, but the hybrids are almost invariably sterile, proving that the species are quite distinct.

All the members of the family are distinguished by the peculiarity that they stand on a single toe of each foot. Originally they had five toes, as is shown by the fossil remains of extinct horses - or rather ancestors of horses - but two toes have almost disappeared and the other two are now represented by the splint bones.

The horse's first direct ancestor looked like something between a fox and a hyrax.

Rock Hyrax
A Rock Hyrax

Indeed, its first and correct scientific name, Hyracotherium, was coined because it was believed to be related to the hyrax. A later name for the same ancestor, Eohippus, was used because the fossils were first discovered in deposits from the Eocene epoch. (Hippus coming from the Greek for horse.)

CRETACIC - ancestors of the horse are supposed to have had five toes on each foot.

Cretacic

EOCENE - horses have four toes in the front foot and three toes in the hind foot this being a transitional stage between five to three toes.

Eocene

Protorohippus - Middle Eocene

Hyracotherium/Eohippus first appeared some 54 million years ago and range throughout the Eocene epoch until about 38 million years ago. These horses were already relatively advanced over their ancestors in many of their skull and skeletal adaptations. The earliest known ancestor of the horse, the Hyracotherium, was no larger than a fox, but already showed a trend towards elongation of the limbs, allowing for a longer stride while running, this, of course, would be especially important in evading predators.

Fossils of Hyracotherium (there were several variations) have been found in North America and European Eocene deposits. It was widespread with characteristics that suited survival in the tropical forests, which covered most of the land at that time. It had a small head and primitive brain with eyes set forward in the skull and 44 small teeth suited to browsing on leaves and fruit, grasses not having evolved. With an arched back, four 'dog-like' pads on the front feet and three on the hind feet, which functioned like those of a dog without the spring-footed gait characteristic of horses. It was probably timid and prone to hiding, as running was not feasible in dense forest.

OLIGOCENE - horses have three toes on each foot. The side toes touch the ground.

Oligocene

Mesohippus - Upper Oligocene

Gradually, the horse's many ancestral types grew bigger and stronger, often existing at the same time. The three-toed browsers, represented by Mesohippus, appeared at the beginning of the Oligocene epoch some 38 million years ago. Mesohippus was a milestone in the late Eocene/Oligocene, its toes having reduced to three, the central one the largest, but it was still pad-footed. Its brain was more advanced that that of Hyracotherium, its teeth bigger and stronger and it was taller.

These horses show a general increase in size. The fourth toe on the front legs had gradually become reduced until it was a vestigial splint, or completely lost. These features, as well as others, showed that the horses were adapting to a diet of leafy material and a more efficient way of running. The history of the three-toed grazers is located mostly in North America, though one very successful member of this group, the Hipparions, migrated from the New World to the Old World during the late Miocene. This was some 10 to 11 million years ago.

MIOCENE - horses have three toes on each foot. The side toes do not touch the ground.

Miocene

Merychippus - Middle Miocene

One-toed horses first appeared during the middle Miocene epoch, some 17 million years ago. The transition from three-toes to one toe was the result of gradual reduction in the size of the side toes during the course of evolution until only the central toe on each leg remained. It is the development of three-toed grazers, as well as browsers that makes the Miocene an important time in the history of the horse.

This division represents an important diversification of feeding habits. Grazing generally implies the eating of grasses as opposed to leafy material, this means that a grazer's dental structure will experience a significant amount of wear, as grasses have a high content of very abrasive minerals. Thus, the three-toed grazers show an increased trend towards a more advanced skull with further deepening of the molar teeth and jaw, which apparently was as response to this then newly acquired source of food. There was also an increase in body size, with exceptions within some of the variations.

Merychippus looked rather like a 10hh pony, with grazing teeth and eyes further back and to the side of the head for a wider view of approaching predators. The familiar elongated leg structure was evolving, which included the fusing of the radius and ulna above the knee for strength. The side toes often bore no weight and it was a true spring-footed, fast runner.

PLIOCENE - some horses have three toes, others have one toe on each foot.

Pliocene

HIPPARION - Lower Pliocene. The history of the three-toed grazers is located principally in North America, though one very successful member of this group, Hipparion, migrated from North America to the European Continent during the late Miocene epoch, some 10 to 11 million years ago. Hipparions were relatively diverse in kinds and very abundant in most late Miocene locations on both sides of the Atlantic.

It was the one-toed horse Dinohippus, which first appeared in the middle Pliocene deposits in North America between 3 and 4 million years ago; that created several different species of horse during both the Pliocene and the Pleistocene epoch. This resulted in the ancestors of many of the types of horses that we know today.

AGE OF MAN

Estimated at 50,000 Years

Pleistocene and Modern

PLEISTOCENE (left) & MODERN Equus (right)

Both Pleistocene and modern horses have one toe on each foot. Pleistocene representatives of Equus were the most widespread of all fossil horses. They appeared about 3 million years ago and were very abundant on both sides of the Atlantic. But by the end of the last Ice Age about 12,000 to 15,000 years ago, many of the fossil species of Equus had become extinct and only a few survive today. The reasons for the extinction of late Pleistocene Equus are as varied as for all the other large mammals, including the mammoth, mastodon and sabre-toothed tiger that became extinct at the same time. Whatever the cause, the result was extreme, particularly in America, there horses became extinct. They were subsequently re-introduction by the Spanish in the 15th Century.