Apart from a couple of bothersome side branches, prehistoric horses present a neat, orderly picture of evolution in action. The basic story line goes like this: as the woodlands of North America gave way to grassy plains, the tiny proto-horses of the Eocene epoch (about 50 million years ago) gradually evolved single, large toes on their feet, more sophisticated teeth, larger sizes and the ability to run at a clip, culminating in the modern horse genus Equus. (See a gallery of prehistoric horse pictures.)
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This story is essentially true, with a couple of important "ands" and "buts." But before we embark on this journey, it's important to dial back a bit and place equines in their proper position on the evolutionary tree. Technically, horses are "perissodactyls," that is, ungulates (hoofed mammals) with odd numbers of toes. The other main branch of hoofed mammals, the even-toed "artiodactyls," are represented by pigs, deer, sheep, goats and cattle, whereas the only other significant perissodactyls beside horses are tapirs and rhinoceroses.
What this means is that perissodactyls and artiodactyls both evolved from a common ancestor, which lived only a few million years after the demise of the dinosaurs at the end of the Cretaceous period. In fact, the earliest perissodactyls (like Hyracotherium, the ancestor of all horses) looked more like small deer than majestic equines!
The Earliest Horses: Hyracotherium and Mesohippus
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.
Next Page: The First Modern Horses, and a List of Prehistoric Horse Genuses
