Family Equidae

Equids have their origins in small, scampering animals, no larger than small terriers, which browsed in the forests of Early Eocene times. With the arrival of the generally drier climate of the Miocene, around 20 million years ago, humid forests began to give way to more open country, and in some parts of the world, notably in North America, to vast grassy plains. It is to such conditions that the modern equids — horses, zebras and asses — are so well adapted.

palaeotherium palaeotherium

Keluarga Diadiaphorus

name: Hyracotherium time: Early Eocene locality: Widespread in Asia,

Europe, and North America size: 8 in/20 cm high at the shoulder In spite of its name, Hyracotherium is not a close relative of the hyraxes (see pp. 234—237) — this is the result of mistaken identification in the last century. A more evocative name, Eohippus, the "dawn horse," has been suggested, but science retains the earlier term.

Hyracotherium, the earliest known equid, is believed to be ancestral to the rest of the horse line, and possibly to the palaeotheres as well. It was tiny compared to modern horses, only about 2 ft/60 cm in length. Its skull was elongated and the mouth had a full complement of 44 teeth, indicating how primitive the animal was. The teeth were low-crowned and suitable only for chewing the soft leaves of tropical forest trees and shrubs.

As with Palaeotherium, there were 4 toes at the front and 3 at the back, making the feet large and unhorselike. Most of the weight was carried on the third toe, anticipating the evolutionary pattern to come. The body was long and arched, giving the animal a hunched appearance. The relative size and complexity of the brain suggests that Hyracotherium was alert and intelligent, and this may have been a factor in the survival of the horse line as a whole.

Hyracotherium was widespread in Eocene times, but when the horse line died out in Europe and Asia around during the Early Oligocene around 35 million years ago, further evolution took place on the North American continent.


merychíppus frn ^w4

name: Mesohippus time: Middle Oligocene locality: North America size: 2 ft/60 cm high at the shoulder

In areas where the forests were giving way to more open country, the horses were no longer confined to scampering among the undergrowth and began to develop the capacity to trot and run.

About the size of a greyhound, with a body some 4 ft/ 1.2 m long, Mesohippus was larger than its predecessors, but had rather lighter 3-toed feet. The central toe was larger than the others. The premolar teeth began to resemble the molars, which increased the chewing surface and hence their efficiency, but they were still low-crowned. Such teeth needed only a shallow jaw, so the head was quite long and pointed.

name: Anchitherium time: Early to Late Miocene locality: North America and later

Asia and Europe size: 2 ft/60 cm high at the shoulder The evolution of the horse was not a simple "straight line" affair, and a number of side-branches developed which have left no descendants today. Anchitherium represents a very successful but conservative offshoot.

Anchitherium evolved in North America in the Early Miocene period, around 25 million years ago. A 3-toed browsing horse, much like Mesohippus in size and shape, Anchitherium browsed the tender vegetation of humid forests. Crossing the Bering land bridge that once joined modern Alaska to Siberia, it spread across Asia and Europe. There, it survived long after it had been replaced in North America by the first grazing horses during the Middle Miocene, some 15 million years ago. Anchitherium did not become extinct in China until the end of Miocene times, some 5 million years ago.

name: Parahippus time: Early Miocene locality: North America size: 3 ft 3 in/1 m high at the shoulder

Parahippus represents an intermediate stage in the evolution of the horse. There were still 3 toes on the feet, and in appearance it was very similar to its ancestor Mesohippus. Its body was larger, though, as were its molars, which came to resemble millstones.

This latter change is highly significant. The newly evolved grasses contained abrasive silica in their cell walls, which made them difficult to crop and masticate. Teeth would have worn down even faster had it not been for the acquisition of hard-wearing "cement" that coated their enamel crests and outer sides. It seems likely, therefore, that Parahippus ventured out from the woodlands to take some advantage of the major new source of nutrition represented by the spreading grasslands.

name: Merychippus time: Middle to Late Miocene locality: North America (Nebraska) size: 3 ft 3 in/1 m high at the shoulder

Herds of Merychippus, the earliest horse to feed exclusively on grass, once roamed the prairies of what is now Nebraska. Their teeth were tall-crowned — dental growth continuing for longer than hitherto — and interlined with cement. Continuing the development seen in Mesohippus, the premolars now had the same grass-grinding design as the molars. The tall teeth needed a deep jaw to contain them, so the head developed the heavy jawline of the modern horse.

Merychippus had a longer neck than its browsing ancestors, since it spent much of its time with its mouth down in the grass. It also developed a strong ligament along the neck from the skull to the shoulder. The springiness of this ligament enabled the heavy head to be raised with little effort, which meant that Merychippus could stay alert to threats from swift predators such as the early dogs and cats.

Merychippus still had 3 toes but now only the middle toe was used to bear the weight (the 2 side-toes did not reach the ground). In addition, an elastic tendon in the leg linking muscle to bone acted like a spring, further increasing the efficiency of movement and making possible a progressively lighter foot and lower leg. The compression of this tendon absorbed the shock of each step, thus saving the ankle from damage, then released the energy stored up in the process to spring the foot up into its next stride.

name: Hipparion time: Middle Miocene to Pleistocene locality: Widespread in North

America, Europe, Asia and Africa size: 4 ft 6 in/1.4 m high at the shoulder

Once the plains-living grazing horses had evolved they, too, radiated into many different types. Of these, all but the Equus species are now extinct.

Hipparion represents one of the many grazing horses that evolved during the Miocene, around 15 million years ago. It was particularly successful, spreading during the Miocene from North America into Asia, Europe and Africa. In Africa, it survived until the Pleistocene about 2 million years ago. This elegant creature resembled the modern horse, but like Merychippus had 3 toes, 2 of which were much reduced and did not touch the ground.

name: Hippidion time: Pleistocene locality: South America size: 4 ft 6 in/1-4 m high at the shoulder

There seem to have been no horses in South America throughout the Tertiary. However, it cannot have been the environment that prevented their colonization, since conditions in South America were able to support the evolution of horselike litopterns such as Diadiaphorus {see pp. 246-249). When a land connection was re-established between North and South America during the Pliocene, 5 million years ago, horses were able to migrate and thrive there.

Hippidion, probably a descendant of Merychippus, was one of these South American horses. It probably resembled a small donkey, with a fairly large head. However, its long, delicate nasal bones were quite distinct from those of other horses, suggesting that Hippidion continued to evolve in isolation from the mainstream of horse evolution in North America until it became extinct around 8000 years ago.

The modern genus Equus, which includes the zebras and asses as well as the wild and domestic horses, seems to have evolved about 4 million years ago in North America, from where it migrated to Asia, Africa and Europe. Curiously, all the horses in the Americas died out about 8000 years ago, and did not reappear there until about 400 years ago — and then only as a result of deliberate introduction by humans. Some paleontologists have speculated that their demise was caused by some devastating epidemic disease, possibly like myxomatosis.

hipparion hippidion "V""

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