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Anything and Everything about Horses.
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Prehistoric Horses - The Story of Equine Evolution
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Until an even earlier candidate is found, paleontologists agree that the ultimate ancestor of all modern horses was Hyracotherium, a tiny (no more than 50 pounds), deer-shaped herbivore with four toes on its front feet and three toes on its back feet. (If the name Hyracotherium sounds unfamiliar, that's because this mammal was once known by the more fitting Eohippus, or "dawn horse.") The key to Hyracotherium's classification as an early horse was its posture: it appears to have put most of its weight on a single toe of each foot, anticipating later equine developments. Hyracotherium (its name, "hyrax-like beast," is a complete misnomer) was closely related to another early ungulate, Palaeotherium. This tapir-like mammal (which may or may not have sported a flexible trunk) is important because it shows that not all early 'dactyls were directly related to modern horses; Palaeotherium inhabited only a distant side branch on the evolutionary tree. Five to ten million years after Hyracotherium, the most important ancient horses were the similarly named Orohippus ("mountain horse"), Mesohippus ("middle horse"), and Miohippus ("Miocene horse," even though it went extinct long before the Miocene epoch). These perissodactyls were about the size of large dogs, and sported slightly longer limbs with enhanced middle toes on the feet. They probably spent most of their time in dense woodlands, but may occasionally have ventured out onto the grassy plains. Toward True Horses: Epihippus, Parahippus and Merychippus During the Miocene period, North America saw the evolution of "intermediate" horses, bigger than Hyracotherium and its ilk but smaller than the large equines that followed. One of the most important of these was Epihippus ("marginal horse"), which was heavier (possibly weighing a few hundred pounds) and equipped with more robust grinding teeth than its ancestors. As you might expect, Epihippus also continued the trend toward enlarged middle toes. Crucially, Epihippus appears to have been the first prehistoric horse to spend more time feeding in meadows than in forests. Similar to Epihippus were two more "hippi," Parahippus and Merychippus. Parahippus ("almost horse") can be considered a next-model Miohippus, slightly bigger than its ancestor and (like Epihippus) sporting long legs, robust teeth, and enlarged middle toes. Merychippus ("ruminant horse") was the biggest of all these intermediate equines, about the size of a modern horse (1,000 pounds) and blessed with an especially fast gait. At this point, it's worth asking the question: what drove the evolution of horses in the fleet, single-toed, long-legged direction? During the Miocene epoch, waves of tasty grass covered the North American plains, a rich source of food for any animal well-adapted enough to graze at leisure and run quickly from predators if necessary. Basically, prehistoric horses filled this open evolutionary niche, as modern equines continue to do today Following the success of intermediate equines like Parahippus and Merychippus, the evolutionary stage was set for the emergence of bigger, more robust, more "horsey" horses. Chief among these were two similarly named genuses, Hipparion ("like a horse") and Hippidion ("like a pony"). Of the two, Hipparion is the better known. This was the most successful horse of its day, spreading out from its original North American habitat (by way of the Siberian land bridge) to Africa and Eurasia. Hipparion was about the size of a modern horse, and only a trained eye might have noticed the two vestigial toes surrounding its single hooves. Otherwise, Hipparion appears to have run much like a modern thoroughbred. Lesser known than Hipparion, but perhaps more interesting, was Hippidion, one of the few horses to have colonized South America (where it persisted until historical times). The donkey-sized Hippidion was distinguished by its prominent nasal bones, which indicate that it had a highly developed sense of smell. Oddly, there are some hints that Hippidion might actually have been a species of Equus, making it more closely related to modern horses than Hipparion was. Speaking of Equus, this genus--which comprises modern horses, as well as zebras and donkeys--evolved in North America during the Pliocene epoch (about 4 million years ago), and then, like Hipparion, took the land bridge to Eurasia. In one of the ironies of evolution, the Ice Age saw the extinction of both North and South American horses, which disappeared from both continents by about 10,000 B.C. However, Equus continued to flourish on the plains of Eurasia, and was reintroduced to the Americas by the European colonizing expeditions of the 15th and 16th centuries A.D. Here's a list of the most notable prehistoric horse genuses. Anchitherium- A long-lived "side branch" on the equine tree of life. Dinohippus This prehistoric horse wasn't quite as fearsome as its name. Epihippus- This tiny, prehistoric horse lived about 30 million years ago. Hipparion- One of the most successful horses of the Miocene epoch. Hippidion -This donkey-sized horse had a prominent snout. Hyracotherium -The horse formerly known as Eohippus. Merychippus- An important intermediate step in equine evolution. Mesohippus- This "middle horse" was about the size of a deer. Miohippus- This "Miocene horse" actually lived much earlier. Orohippus -This prehistoric horse was a close relative of Hyracotherium. Palaeotherium- This tapir-like beast was remotely related to modern horses. Parahippus- This "almost horse" had noticeably enlarged middle toes. Pliohippus -This prehistoric horse was built for speed.
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