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When horses had toes

Posted in Animals, Historical articles, Prehistory, Wildlife on Tuesday, 31 May 2011

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This edited article about the prehistoric horse originally appeared in Look and Learn issue number 953 published on 26 April 1980.

merychippus, picture, image, illustration

The Merrychippus (below) is the ancient ancestor of the modern-day stallion, by Angus McBride

Of the scores of breeds of horses in the world today, none runs truly wild. Przewalski’s horse, a small animal only 120 cm or 12 hands tall, continued to roam the steppes of Mongolia into the 20th century, but has now become extinct in the wild. Zoos are well supplied. It represents the species Equus caballus, to which all domestic breeds belong.

In the past few thousand years the horse has changed a great deal, mainly through man’s selective breeding. If you look, for example, at statues and paintings of horses from our immediate past, you will see animals that do not look like the horses we knew today.

The great bronze horses of Venice or the magnificent animal that bears Charles I in Van Dyck’s portrait make you mistrust the artists’ dimensions: large bodies carry enormously powerful chests which in turn curve up to heads that seem too small. This kind of horse was bred to have its head pulled back proudly in battle and, of course, there is no longer any need for it. Instead, we have sleek racehorses, whose necks follow the line of their bodies and whose heads are kept low to reduce wind resistance.

If such a change can occur in only a few hundred years, then you may well imagine the number of changes since the first horse developed from the small creatures that lived in the age of the dinosaur. Hundreds of different kinds of horse fossils, studding the rocks of Europe, Asia and especially North America, allow us to chart those changes.

The Equus caballus is an ungulate, a name which comes from the Latin ungula, meaning hoof. It applies to all animals which have a sheath of horn covering their feet. The development of the hooves tells us to which era the fossils belong. The earliest ones to be found belong to the Dawn Horse (Eohippus) which lived in the Eocene period around 50 million years ago. But already one toe on its forefeet and two on its hind-feet had become obsolete since its five-toed ancestors, the Condylarthra, had crawled into the Tertiary era after the dinosaurs died out, 65 million years ago.

At each stage of evolution, the other toes have become increasingly redundant, losing their importance and their size. Finally, with Przewalski’s horse, there was only the middle toe left, encased in a hoof.

The first horse for whose existence we have physical, as opposed to hypothetical, evidence, the Dawn Horse, ranged in size from that of a fox terrier to that of a Shetland pony, with a head barely recognisable as a horse’s. It was widely distributed throughout Europe and North America.

In the millions of years that followed it developed on both sides of the Atlantic in many different shapes and forms. To make life a little easier, the main species – Eohippus, (the Dawn Horse), Miohippus and Pliohippus – have been named after the age in which they lived – Eocene, Miocene and Pliocene.

Of course, there have to be exceptions to the rule, such as Mesohippus, the middle horse, which followed the Dawn Horse in North America. This was a speedy animal built on the lines of a greyhound, with long, slender legs.

The American line then produced the Miohippus, which was slightly bigger – about the size of a sheep. Like its predecessor it had three toes on each foot and, although the other toes were still visible, they were starting to grow smaller. The species that followed reached almost the size of a modern horse, but lived in forests and looked more like a deer.

In the meantime another line had branched off, moving more directly towards the modern horse, Still exclusive to North America, this was the Merychippus, which lived in herds on the grasslands and became adept at running on hard, flat ground.

It marked a parting of the ways for these ungulates, splitting them into at least six further lines, of which two were successful. One is typified by the Hipparion, which made its way across to Asia and Europe and down to Africa, where it took over from the original Dawn Horse and survived until about three million years ago. The second branch produced Pliohippus, from which Equus, the genus of modern horses, developed about a million years ago.

By this time, the end of the Pliocene period when the Hipparion was becoming extinct, Equus caballus was emerging in North America as Przewalski’s horse. The only two side toes remaining were covered in skin, as in modern breeds of horse.

Equus caballus made its way across the Bering Strait and moved down into the steppes of Europe and Asia, pushed south by the encroaching Ice Age. Now it was North America’s turn to be deprived of the horse, and it died out there not more than ten thousand years ago. Not until the Spanish conquistadors crossed the Atlantic in the 16th century did the horse return to the lands where it had developed.

Although a great friend of man, the horse was one of the last of the domesticated animals to be tamed, much later than dogs and cattle. There is no evidence as to when this took place, but tablets written in 1500 BC tell of “trainers” who looked after the stables for Assyrian kings.

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