Evolution of Horses from 54 Million years ago (initiate & plan #1)
Hyracotherium (Eohippus): This small dog-sized animal, (0.25 - 0.45m in height), is the oldest found horse ancestor that lived about 54-55 million years ago in the early Eocene time period. It had an arched back, short neck, short snout, short legs, and long tail. It browsed on fruit and fairly soft foliage; it was slow, and not very agile. It had a primitive short face, with eye sockets in the middle and a short diastema (the space between the front teeth and the cheek teeth). Although it had low-crowned teeth, the beginnings of the characteristic horse-like ridges on the molars were beginning to develop. This animal looked nothing like a horse. It is most commonly known as “Eohippus” which means “dawn horse”. At this point in the early Eocene, Equidae were not yet very different from the other perissodactyl groups; the Hyracotherium genus includes some species closely related to (or ancestral to) rhinos and tapirs, as well as species that are distinctly equine. Hyracotherium was a very successful animal in its time; it had found a nice stable niche for itself. Throughout most of the Eocene (over 20 years), only minor evolutionary changes took place in Hyracotherium and its close descendants. The body and feet stayed mostly the same, with slight changes in the toes. The major change was in the teeth; as Eocene Equidae started to eat more plant browse and less fruit, they developed more grinding teeth to deal with the slightly tougher food.
Orohippus: In the early to mid Eocene, (about 50 million years ago), the Eohippus slowly transitioned into the Orohippus, meaning “mountain horse” although it did not live in the mountains. It was about the same size as Eohippus, but it had a slimmer body, slimmer forelimbs, an elongated neck, and longer hind legs, which are characteristics of a good jumper. Orohippus still had padded feet, but the vestigial outer toes of Eohippus were not present in Orohippus. It had four toes on each fore leg and three each on the hind leg. The most dramatic change between Eohippus and Orohippus was in the teeth: the first premolar teeth were smaller, the last premolar shifted in shape and function into a molar, and the crests on the teeth became more pronounced. All these factors gave the teeth of Orohippus greater grinding ability, which suggests that Orohippus ate tougher plant material.
Mesohippus: As the Oligocene period gets closer, horses start to change. The North American climate became drier, grasses were starting to evolve, and the vast forests started shrinking. The late Eocene horses adapted to the changes by developing stronger teeth and becoming a bit larger and a bit faster. The Mesohippus species (also known as “middle horse”) appeared suddenly in the late Eocene period approximately 40 million years ago as result of its response to strong selective pressures. Mesohippus is in between the Eohippus horses and the more “modern hoses”. It was slightly larger than the Epihippus, 24’’ at the shoulder. It began to look more like a horse: the legs and neck were a bit longer, the back was less arched, and the face and snout was particularly longer. Mesohippus had a shallow facial depression on the skull. It had three toes on its front and hind feet, the 4th toe became a vestigial lump.
As millions of years progressed, the various species of horses evolved; they became larger, taller, the number and size of their teeth increased, and each species looked more distinctively like a modern day horse.
Equus: This is the genus of all modern horses that appeared about 4 million years ago and are still alive till this day. It is the only surviving genus in the previous diverse family of horses. The first Equus were pony-sized with features of a horse: it had a rigid spine, long neck and legs, a long nose, deep jaw, and a flexible muzzle. Its brain was a bit larger than in the early Dinohippus. Equus had one toe with side ligaments that prevented the twisting of the hoof. It also had straight grazing teeth with high crowns and strong crests. Members of Equus still have the genes of “side toes” that are usually seen as vestigial splint bones of the 2nd and 4th toe around the large central toe. It is rare for a modern Equus to be born with small and fully-formed side toes. Equus were domesticated about 3,000 years ago and have had a big impact on human history in areas like farming, sport, communication, travel, warfare, and migration.
Horse Evolution Time Line
Orohippus: In the early to mid Eocene, (about 50 million years ago), the Eohippus slowly transitioned into the Orohippus, meaning “mountain horse” although it did not live in the mountains. It was about the same size as Eohippus, but it had a slimmer body, slimmer forelimbs, an elongated neck, and longer hind legs, which are characteristics of a good jumper. Orohippus still had padded feet, but the vestigial outer toes of Eohippus were not present in Orohippus. It had four toes on each fore leg and three each on the hind leg. The most dramatic change between Eohippus and Orohippus was in the teeth: the first premolar teeth were smaller, the last premolar shifted in shape and function into a molar, and the crests on the teeth became more pronounced. All these factors gave the teeth of Orohippus greater grinding ability, which suggests that Orohippus ate tougher plant material.
Mesohippus: As the Oligocene period gets closer, horses start to change. The North American climate became drier, grasses were starting to evolve, and the vast forests started shrinking. The late Eocene horses adapted to the changes by developing stronger teeth and becoming a bit larger and a bit faster. The Mesohippus species (also known as “middle horse”) appeared suddenly in the late Eocene period approximately 40 million years ago as result of its response to strong selective pressures. Mesohippus is in between the Eohippus horses and the more “modern hoses”. It was slightly larger than the Epihippus, 24’’ at the shoulder. It began to look more like a horse: the legs and neck were a bit longer, the back was less arched, and the face and snout was particularly longer. Mesohippus had a shallow facial depression on the skull. It had three toes on its front and hind feet, the 4th toe became a vestigial lump.
As millions of years progressed, the various species of horses evolved; they became larger, taller, the number and size of their teeth increased, and each species looked more distinctively like a modern day horse.
Equus: This is the genus of all modern horses that appeared about 4 million years ago and are still alive till this day. It is the only surviving genus in the previous diverse family of horses. The first Equus were pony-sized with features of a horse: it had a rigid spine, long neck and legs, a long nose, deep jaw, and a flexible muzzle. Its brain was a bit larger than in the early Dinohippus. Equus had one toe with side ligaments that prevented the twisting of the hoof. It also had straight grazing teeth with high crowns and strong crests. Members of Equus still have the genes of “side toes” that are usually seen as vestigial splint bones of the 2nd and 4th toe around the large central toe. It is rare for a modern Equus to be born with small and fully-formed side toes. Equus were domesticated about 3,000 years ago and have had a big impact on human history in areas like farming, sport, communication, travel, warfare, and migration.
Horse Evolution Time Line