Timeline of horse evolution from 55 Million years ago to present
Species outlined in red represent those that are direct ancestors of E. ferus caballus
Hyracotherium
Hyracotherium was a small forest animal that browsed on soft foliage and fruit. It is also known as "eohippus" or dawn horse. It had 4 toes on the front feet and 3 on the back, and had pads on the bottom of its feet. It had 3 grinding molars and through much of the Eocene relatively little changed about hyracotherium and its decendants (Hunt).
Orohippus
Hyracotherium gradually and smoothly transitioned into orohippus: the feet, body structure and size stayed the same, but orohippus had 4 grinding molars on each side, and the crests became more pronounced, which shows that these animals were begining to eat tougher plant material (Hunt).
Epihippus
Epihippus had 5 grinding teeth, with the same "doggish" look to its body.
Mesohippus
Mesohippus fossils rather abruptly appear in the late Eocene. These animals were larger (about 24 " at the shoulder), had longer legs, neck, and face. There was the beginning of the depression in the skull known as facial fossa. Mesohippus had three toes on its hind feet and its front feet with a vestigial 4th toe. Mesohippus also had 6 grinding teeth, with well formed crests (Hunt).
Miohippus
Miohippus speciated from early Mesohippus and the 2 species overlapped for about 4 million years. Miohippus was larger, had a longer face and a deeper fossa. Miohippus (and all later horses) maintained their 6 grinding teeth. In the early Miocene Miohippus began to speciate rapidly into 3 different lines of evolution: the anchitheres (3 toed browsers such as Anchitherium), pygmy horses (such as Archeohippus that did not survive long), and the line that transitioned into grazing animals that took advantage of the new grasslands and plains. (Hunt).
Parahippus
Parahippus was about the same size as Miohippus and still had 3 toes. Parahippus had higher tooth crowns, and rapidly evolved into a grazing animal that mostly supported itself on its central toe.
Merychippus
Merychippus was about 40" tall at the shoulder, and stood permanently on "tiptoe" with the side toes only touching the ground when running. As time went by, these extra toes became variable with some animals having full sized toes and others having vestigial small toes. The legs became longer, the hoof wall developed, and the bones on the leg began to fuse, making Merychippus a specialized running animal. The eyes moved back in the skull to accomodate the larger tooth crowns, and the teeth began to have a layer of cement to prevent them wearing down easily. By the late miocene, Merychippus speciated rapidly and gave rise to a variety of new species including hipparions (3 toed grazers with large and elaborate fossae), protohippines (small horses rather similar to hipparions) and finally the species that led to the '"true equines" which began to lose their vestigial side toes. (Hunt)
*10 million years ago: time of most diversity for the equine family in terms of numbers and numbers of species
*10 million years ago: time of most diversity for the equine family in terms of numbers and numbers of species
Pliohippus to Dinohippus
Pliohippus shows the gradual loss of the side toes, had curved teeth, and deep facial fossae. This species gave rise to Dinohippus, which has straighter teeth, decreased facial fossae and looks incredibly similar to Equus in size, and body shape. By the end of the Pliocene, Dinohippus had gradually transitioned into Equus (Hunt).
Equus
About 4 million years ago, the species Equus finally evolved. They were about 50" at the shoulder, had a long neck and fused leg bones, stood on a single toe, and had high crowned, straight teeth meant for grazing with a thick layer of cement and sharp crests. Equus had one major speciation early on (at least 12 species developed) and as they spread, these equines became the ancestors of the zebras in Africa, the desert asses of the Mideast, and the horses of Europe, Asia, and the Americas.
As the Ice Ages began, the majority of the one toed horses in North America died off. In the late Pleistocene there were a series of extinctions that decimated the majority of large mammals. The true horses did not return in any signifigant numbers to North America until the Spanish conquistadors brought horses with them. Escaped horses thrived and formed the early herds that led to the great mustang herds in North America.
As the Ice Ages began, the majority of the one toed horses in North America died off. In the late Pleistocene there were a series of extinctions that decimated the majority of large mammals. The true horses did not return in any signifigant numbers to North America until the Spanish conquistadors brought horses with them. Escaped horses thrived and formed the early herds that led to the great mustang herds in North America.
E. ferus caballus
Today, the horse is a friend, partner, and an athlete. Humans have bred and developed the modern horse into a huge vaiety of sizes, colors and body types to fit different needs and jobs. This was all made possible by a little doggish creature that lived 55 million years ago, called Hyracotherium.
*all photographs on this page belong to the Fossil Horse Cybermuseum of the Florida Museum of Natural History with the exception of the timeline and the bottom photographs.