Evolution of the horse


Evolution of the Horse

Eohippus (Hyracotherium)

The horse has evolved over 55 million years. Eohippus (meaning "dawn horse") was the earliest-known horse, it was the size of a tiny dog. Another name for this animal was Hyracotherium (meaning "mole beast"). The Eohippus (Hyracotherium) was only 2 feet (60 cm) long and 12-14 inches high at the shoulder.   This primitive horse had 4 hoofed toes on the front feet and 3 hoofed toes on each hind foot. It had a long skull with 44 long-crowned teeth.

The Eohippus was a grazing herbivore that ate soft leaves and plant shoots. They lived during the early Eocene Epoch, about 50 million years ago. It lived in the Northern hemisphere (in Asia, Europe, and North America). The first fossils of this tiny horse were found in England by the famous palaeontologist Richard Owen in 1841 and named Hyracotherium.

Orohippus

The Orohippus lived in the woodlands of North America. It lived in the Early Eocene (52-45 million years ago). The Orohippus was about 2 feet long and 22kgs and ate mostly plants. They were small in size and had three-toed hind feet.

The Orohippus lived at about the same time as Hyracotherium, the equine ancestor also
 known as Eohippus. The only (obvious) equine characteristics of Orohippus were the slightly enlarged middle toes on its front and hind legs; other than that, this herbivorous mammal looked more like a prehistoric deer than a modern horse. 

Mesohippus

Mesohippus meaning "middle horse” in Greek lived in the woodlands of North America in the late Eocene-Middle Oligocene (40-30 million years ago). The Mesohippus was about 4 feet long and 34kgs and survived mainly on plants. They were small in size and had three-toed front feet.

The Mesohippus is similar to the Hyracotherium (Eohippus) just advanced a few million years. This prehistoric horse represented an intermediate stage between the smallish forest animals of the early Eocene epoch and the large plains browsers that dominated the Pliocene and Pleistocene epochs. About the size of a deer, Mesohippus was distinguished by its three-toed front feet (earlier horses sported four toes on their front limbs) and the wide-set eyes set high atop its long, horse-like skull.

Miohippus

The Miohippus lives on the plains of North America in the late Eocene-Early Oligocene (35-25 million years ago). The Miohippus was about 4 feet long and 45kgs and survived on plants. They were small in size and ha
d a long skull.

The Miohippus was one of the most successful prehistoric horses of the Tertiary period. This genus (w
hich was closely related to the similarly named Mesohippus) was represented by about a dozen different species, all of them indigenous to North America. Miohippus was a bit larger than Mesohippus (about 100 pounds for a full-grown adult, compared to 22 or 34 kgs); despite its name, it lived not in the Miocene but the earlier Eocene and Oligocene epochs.



Parahippus

Parahippus is Greek for "almost horse". The Parahippus lived on the plains of North America in the Miocene (25-5 million years ago). The Parahippus was about 5 feet tall and 226kgs and survived on plants. They had Long legs and skull with enlarged middle toes.

The Parahippus was an improved version of another prehistoric horse, the similarly named Miohippus. The Parahippus was slightly bigger than its immediate ancestor, and was built for speed on the open prairie, with relatively long legs and noticeably enlarged middle toes, which it put most of its weight when running. The teeth of Parahippus were also well adapted to chewing and digesting the tough grass of the North American plains.

Merychippus

Merychippus is Greek for "ruminant horse". The Merchippus lived on the plains of North America in the late Miocene (17-10 million years ago). The Merychippus was about 6 feet tall and weighed 453kgs and survived on plants. They were large with vestigial side toes on front and hind feet.

Merychippus was something of a watershed in equine evolution: this was the first prehistoric horse to bear a marked resemblance to modern horses, although it was slightly bigger and still had vestigial toes on either side of its feet, these toes didn't reach the ground, and so the Merychippus still would have run in a recognizably horse like way. Palaeontologists believe that all prehistoric horses after the late Miocene epoch, including Hipparion and Hippidion, evolved directly from Merychippus.

Pliohippus

The Pliohippus lived on the plains of North America in the late Miocene-pliocene (12-2 million years ago). The Pliohippus was about 6 feet high and 453kgs and survived on plants. They had single toed feed and had a depression in the skull above the eyes.

Like modern plains horses, Pliohippus seems to have been built for speed: this true single-toed horse roamed the grassy plains of North America between 12 million and 2 million years ago. Although Pliohippus closely resembled modern horses, there's some debate about whether the distinctive depressions in its skull, in front of its eyes, are evidence of a parallel branch in equine evolution.

Dinohippus

The Dinohippus lived on the plains of North America in the late Miocene (13-5 million years ago). The Dinohippus was about 5 feet tall and 340kgs and survived on plants. They had one and three-toed feet and the ability to stand for long periods of time

This prehistoric horse (which was once considered to be a species of Pliohippus) is now thought to have been the immediate precursor of the modern genus Equus. The giveaway is Dinohippus' primitive "stay apparatus" a tell-tale arrangement of the bones and tendons in its legs that allowed it to stand for long periods of time, like modern horses.

Equus

Equus is the only surviving genus in the once diverse family of horses. The early Equus had zebra like bodies and short donkeys like heads. They had tails although short and stiff and straight up manes. Many of the strains of horses died off for reasons unknown aside from the obvious but Equus managed to survive and is where we find the basis for the modern day horse.

Species of Equus lived from 5 million years ago until the present. Living species include horses, asses, and zebras. Fossils of Equus are found on every continent except Australia and Antarctica.
The earliest known Equus species were a set of three species known as the Equus simplicidens group. They still had some primitive traits from Dinohippus. They had zebra-like bodies, quite stocky with a straight shoulder and thick neck, and short, narrow, donkey-like skulls. They probably had stiff, upright manes, ropy tails, medium-sized ears, striped legs, and at least some striping on the back. They quickly diversified into at least 12 new species in 4 different groups, in a burst of evolution reminiscent of the great merychippine radiation. All these Equus species coexisted with other one-toed horses and with various successful hipparions and protohippines, which had been merrily evolving on their own paths.

During the first major glaciations of the late Pliocene (2.6 Ma), certain Equus species crossed to the Old World. Some entered Africa and diversified into the modern zebras. Others spread across Asia, the Mideast, & N. Africa as desert-adapted onagers and asses. Still others spread across Asia, the Mideast, and Europe as the true horse, E. caballus. Other Equus species spread into South America. The Equus genus was perhaps the most successful perissodactyl genus that ever lived, even before domestication by humans.