Wild Horses: Evolution Timeline
Origins of the Horse
Click on each part of the HORSE EVOLUTION TIMELINE by paleontologist Kathleen Hunt
to see a larger version.
The first equid was Hyracotherium, a small forest animal of the early Eocene. It looked nothing at all like a horse (10 – 20” hight). It resembled a dog with an arched back, short neck, short snout, short legs, and long tail. It browsed on fruit and soft foliage and probably would have had mannerisms more like that of a deer (timid, flighty, etc.). This famous little equid was once known as Eohippus or “Dawn Horse”.
The species Mesohippus celer appears suddenly in the late Eocene, approximately 40 million years ago. This animal was slightly larger than Eohippus, standing 24” at the shoulder. It no longer looked like as much like a dog. The back was less arched, the legs and neck a bit longer, and the stout and face distinctively longer. It had a shallow, facial fossa, and a depression on the skull. Mesohippus had three toes on all its feet and was still pad-footed. The fourth front toe was reduced to a vestigial stub.
Soon after Mesohippus celer and its very close relative Mesohippus westoni appeared, a similar animal called Miohippus assiniboiensis arose, approximately 36 million years ago. A typical Miohippus was distinctly larger and had a slightly larger skull than a typical Mesohippus. The facial fossa was deeper and more expanded. In addition, the ankle joint had changed subtly. Miohippus also began to show a variable crest on its upper cheek teeth. In later horse species, this crest became a characteristic feature.
Seventeen million years ago, Merychippus joined the equine line and was about 10 hands or 40” tall, the tallest yet. The muzzle became elongated, the jaw deeper, and the eyes moved farther back to accommodate the large teeth roots. The brain was notably larger, with a fissured neocortex and a larger cerebellum, making Merychippus smarter and more agile than earlier horses. Overall, this species was distinctly recognizable as a horse with a very “horsey” head.
About 15 million years ago in the middle Miocene epoch arose Pilohippus, a three-toed horse. Gradual loss of the side toes is seen in this species through three successive strata. Pliohippus was very similar to and, until recently, thought to be the direct ancestor of Equus, except for two significant differences. First, Pliohippus’ skull has deep facial fossa whereas Equus has no facial fossae at all. Second, Pliohippus’ teeth are strongly curved while Equus’ teeth are very straight. Though Pliohippus is obviously related to Equus, the former probably did not give rise to the latter.
The genus of all modern equines, the first Equus was about 13.2 hands tall, pony size, with a classic “horsey” body – rigid spine, long neck, legs, and nose, and fused leg bones with no rotation. The brain was a bit larger than in early Dinohippus. Like Dinohippus, Equus was (and still is) one-toed, with side ligaments that prevent twisting of the hoof. This species has high-crowned, straight, grazing teeth with strong crests lined with cement.
ORIGINS OF THE HORSE