It lived around 55 million years ago in the forests of North America and Europe. Hyracoth erium had four toes on the front feet and three in the back. These relatives of the modern horse came in many shapes and sizes.
Some lived in the forest, while others preferred open grassland. Here, two large Dino hippus horses can be seen grazing on grass, much like horses today. But unlike modern horses, a three -toed Hypohippus tiptoes through the forest, nibbling on leaves. A small, three -toed Nannippus , shown here eating shrubs, ate both grass and leaves. In t he background are several other large mammals alive at that time, including Procamelus, a camel relative; a herd of Dinohippus horses; Gomphotherium, a distant relative of true elephants; and Teleoceras, a hornless rhinoceros.
For more than half their history, most horses remained small, fore st browsers. But changing climate conditions allowed grasslands to expand, and about 20 million years ago, many new species rapidly evolved. Only these species survived to the present, but in the past , small and large species lived side by side. But there was not a steady increase in size over time. Little Nannippus, shown in the diorama at full adult size, was actually smaller than its predecessors. Like modern -day Equus, Dinohippus had single -toed hooves and ate mostly grass.
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The other extinct species shown here had three toes and never developed single hooves. So how did horses end up with single -toed hooves? Over millions of years, many horse species lost most of their side toes.
The middle toe evolved into a single large hoof, while the other toes became smaller and ultimately functionless. Only one spe cies in this scene, the grazing Dinohippus, has a single hoof. Hooves and long legs help horses run farther and faster on the open prairie, helping them flee from predators and find fresh grass for grazing. In the forest, where the ground is softer, many horses retained three toes. Horses that moved onto grasslands have longer legs than their forest -dwe lling ancestors. But their leg bones did not all lengthen equally. Mostly it was the bones of the foot that grew longer, with the ankle moving relatively higher up on the leg.
Small, leaf -eating horses thrived. Thereafter, dry grasslands replaced much of the North American forest, leading to rapid evolution among horses. By about nine million years ago, most forest browsers had disappeared, leaving primarily grass -eating grazers like those alive tod ay. This three -toed lineage is now exti nct, but in the past many diverse horses lived side by side. But over millions of ye ars of evolution, many horses lost their side toes and developed a single hoof. Only horses with single -toed hooves survive today, but the remains of tiny vestigial toes can still be found on the bones above their hoofs.
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Can you find the tiny side toes on these horse feet? Some individuals had three full -size toes; on others, the two small side toes only touched the ground when running. The majority of horse spec ies evolved in North America. From there, they occasionally walked to other continents.
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This map shows how horses spread around the world at three different times. Color -coded map key: [green arrows] About 20 million years ago, three -toed horses called anchitheres crossed to Asia and continued to Europe and Africa. Equus, the ancestor of all horses today, survived only in Eurasia a nd Africa.
What ended their 55 -million -year run in North America? The prime suspects are changes in the environment, disease and overhunting by humans who likely killed them for food. The first horses all had short, broad chewing teeth, like ours. Later horses had teeth three times longer. But grazing on tough grasses would quickly wear short teeth down to nothing.
Grazing is hard on teeth for two reasons. Grasses contain bits of the mineral silica that resemble glass and wear teeth away like sandpaper. Chomping grass close to the ground also picks up gritty soil that wears teeth away.
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NOOK Book. With the grim determination of an unrepentant rocker, Larry Frolick sets off on a 12,mile trek across Central Asia, brooding over the fate of its lost civilizations. From Kiev, Crimean Tartary, and Moscow, through the nomadic homelands of Uzbekistan, Kyrgizstan, Tien-Shan, and finally into distant Mongolia and Siberia, he explores a continent on the brink of a meltdown, a strange world lit harshly by the red afterglow of the Soviet collapse.
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