Evolution

LET’S  BEGIN  WHIT THE   EVOLUTION

Imagine a world in which horses of all colors, shapes, and sizes roamed the world, some barely larger than a small dog. That world no longer exists--but once it was real. Today's horses represent just one tiny twig on an immense family tree that spans millions of years. All the other branches of the horse family, known as Equidae, are now extinct. The earliest known horses evolved 55 million years ago and for much of this time, multiple horse species lived at the same time, often side by side, as seen in this diorama.

This small dog-sized animal is the oldest found horse ancestor that lived about 55 million years ago.  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 has low-crowned teeth, we see the beginnings of the characteristic horse-like ridges on the molars.

The earliest evidence of this “little horse” is found in the middle Eocene of Wyoming, about 2 million years after the first appearance of Hyracotherium.  The two genera coexisted during the Eocene, although Orohippus fossils are not as numerous or as geographically widespread as those of Hyracotherium. Fossils of Orohippus have been found in Eocene sediments in Wyoming and Oregon, dating from about 52-45 million years ago.

The "middle horse" earned its name.  Mesohippus is intermediate between the eohippus-like horses of the Eocene, (which don't look much like our familiar "horse") and more "modern" horses. Fossils of Mesohippus are found at many Oligocene localities in Colorado and the Great Plains of the US (like Nebraska and the Dakotas) and Canada.  This genus lived about37-32 million years ago.

Species of Miohippus gave rise to the first burst of diversity in the horse family.  UntilMiohippus, there were few side branches, but the descendants of Miohippus were numerous and distinct.  During the Miocene, over a dozen genera existed. Fossils of Miohippus are found at many Oligocene localities in the Great Plains, the western US and a few places in Florida. Species in this genus lived from about 32-25 million years ago.

Parahippus appears to be the evolutionary “link” between the old forest-dwelling horses and the modern plains-dwelling grazers.   It has 3 toes, like primitive horses, but the side toes are smaller.   They are "horse-faced," or long-headed with the eye socket well back from the middle of the skull. Fossils of Parahippus are found at many early Miocene localities in the Great Plains and Florida.   Species in this genus lived from 24-17 million years ago.

Merychippus represents a milestone in the evolution of horses.   Though it retained the primitive character of 3 toes, it looked like a modern horse.   Merychippus had a long face.   Its long legs allowed it to escape from predators and migrate long distances to feed.   It had high-crowned cheek teeth, making it the first known grazing horse and the ancestor of all later horse lineages.  Fossils of Merychippus are found at many late Miocene localities throughout the United States.   Species in this genus lived from 17-11 million years ago. 

"Grandfather" to the modern horse, Pliohippus appears to be the source of the latest radiation in the horse family.  It is believed to have given rise to Hippidion and Onohippidion, genera that thrived for a time in South American, and to Dinohippus which in turn led to Equus. Fossils ofPliohippus are found at many late Miocene localities in Colorado, the Great Plains of the US (Nebraska and the Dakotas) and Canada.  Species in this genus lived from 12-6 million yearsago.

 Dinohippus is believed to be the closest relative to Equus, the genus that includes the living horses, asses and zebras.  Dinohippus fossils are found in the Upper Miocene of North America and date from 13 - 5 million years ago.

 Equus is the only surviving genus in the once diverse family of horses.   Domesticated about 3,000 years ago, the horse had a profound impact on human history in areas such as migration, farming, warfare, sport, communication, and travel.  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.