Hyracotherium
This small dog-sized animal represents the oldest known horse. 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.
Hyracotherium is better known as “eohippus” - which means “the dawn horse.” The name also refers to the fact that it lived during the Eocene.
Parahippus
(This horse was the size of a German Shepard)
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.
Where & When? 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.
Why did Professor Leidy name this animal “the side horse”?
The name Parahippus implies its “side” toes (it was a three-toed horse). However, in 1858 Professor Leidy only had some teeth of this animal. Unfortunately, Leidy didn’t explain why he called it “Parahippus.” The name may refer to the “side” branches of the posterior crest of the upper molars that he noted. This feature differentiated it from species of Anchitherium.
Some species of this remarkable German Shepherd-sized horse developed teeth that were tall, or “high-crowned.” High-crowned teeth may have permitted this horse to have some grass in its diet. Because grass is an abrasive food that wears teeth down, short teeth in grass-eating animals would result in an early death from starvation.
Other advancements in the teeth include the development of cementum, a protective coating of the teeth, and the molar ridges characteristic of the modern horses.
Parahippus leonensis is the most commonly found fossil at Thomas Farm, an 18-million-year-old early Miocene locality in Florida. Virtually every participant in our Pony Express fossil camp finds specimens of this horse. It is an important species because it is believed to be a close relative to the group from which modern horses evolved.
Archaeohippus
This tiny Miocene browsing horse was about the size of a collie. It co-existed with the grazing horses that were diversifying throughout the drier open savannah habitats of this time.
Where & When? Archaeohippus fossils are found in Nebraska, Oregon, California, and Florida. Species in this genus lived from 21 - 13 million years ago.
Were all fossil horses larger than their ancestors?
Though many horses became larger than their ancestors, Archaeohippus actually became quite a bit smaller! Archaeohippus descended from the larger Miohippus. Nannippus is another example of a horse that was smaller than its ancestors.
Archaeohippus was a small, sleek, graceful animal that had an elongated snout. The molar shape (crown height) of Archaeohippus suggests that it was a forest dweller, or inhabited an open woodland. It ate a seasonally variable diet of leaves and other nonfibrous foods. Based on modern analogues, individuals were either solitary or formed territorial breeding pairs, and they had small home ranges. Archaeohippus individuals probably lived 4-5 years.
Although not as common as Parahippus, specimens of Archaeohippus blackbergi are often found during our spring Pony Express fossil camp. Skulls of this animal, however, are quite rare. A composite skeleton was mounted from bones found during these digs and will be displayed in our new fossil exhibit.
Another Tree Chart
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