Horse Evolution

The horse family is a classic example of evolution and is supported by extensive fossil evidence. It has been comprehensively studied to form possibly the most complete evolution of any animal after humans. Horses are believed to have first appeared around 55 million years ago in the form of the Hyracotherium, or ‘hyrax-like beast’ (The World Book Encyclopedia). They originally inhabited North America and Europe, where the oldest fossils of possible ancestors were found (Encyclopædia Britannica, 2007). The Przewalski’s Horse, Equus ferus przewalskii or Equus caballus przewalskii, belongs to the family Equidae in the order Perissodactyla, which means odd-toed ungulates (Wikipedia, 2007). Animals in this order all have hoofed feet with an odd number of toes, mobile upper lips and similar tooth structure. Other animal families in this order are tapirs and rhinoceroses.

In the evolution of the horse, the change of the teeth, toes, legs and overall shape of the body are of particular significance. These characteristics evolved as adaptations to changes in their environment and the progress from a smaller minor species to one of the largest species today. The equids diet began to change from foliage to grasses when grasses first began to flourish, causing the equids to have larger and more durable teeth. Equids also became further under threat from predators as new species evolved, leading them to be capable of greater speeds. Equids adapted by lengthening their limbs and shifting the weight of the body to the third toe, the longest of the toes.

The Hyracotherium, also known as the Eohippus, originally developed around 55 million years ago; 10 million years after the dinosaurs became extinct (Wikipedia, 2007). Fossil records show that the Hyracotherium was around the size of a fox, around 25 to 50 cm in height, and lived in North America. They had a short head and neck, with an arched back. They had 44 low-crowned teeth in an arrangement typical of an omnivorous mammal (Critters-2-Go, 2007). Their molars were small and bumpy and mainly used for grinding foliage. The Hyracotherium had a small brain with considerably small frontal lobes. They had long, fleshy tails, which is different to horses today, which have short tails and long hair (Encyclopædia Britannica, 2007). Their limbs were long in relation to their body size, presumably to assist running, and were more flexible. The forelimbs developed with five toes; four equipped with small proto-hooves, and the fifth toe off the ground. On the hind limbs three toes were equipped with proto-hooves, while the vestigial first and fifth toes were also off the ground. The feet were padded, like a dog’s, and adapted to soft, moist grounds of forests.

Close to 53 million years ago, the Hyracotherium evolved into the Orohippus. They were similar in size; yet the Orohippus had a slimmer body and forelimbs, and a longer head and hind legs. These are characteristics of a good jumping horse, and may be attributed to increasing threat of predators (Draper, J. 1999). The outer toes had become redundant and were no longer present, with four toes on each forelimb and three on each hind leg. The premolar teeth reduced in size; the last premolar became a molar and the crests on the teeth became more evident. This resulted in a better grinding ability, suggesting the Orohippus ate tougher plant materials.

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Three million years later, after the first Orohippus, evolved a new species called the Epihippus. It continued the trend of greater teeth grinding ability. It had five grinding, low-crowned teeth with defined crests. 

As the climate became drier in North America 40 million years ago, new environments began to form and grasses evolved (Critters-2-Go, 2007). About 40 – 37 million years ago the Mesohippus developed in response to these new environmental pressures. It was one of the most widespread mammals in North America and was slightly larger than the Epihippus, at 61 cm in height. It had a straighter back and ...

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