Horses in Wyoming are nothing new. In fact, the ancestors of the horses we know today were roaming what are now the plains 50 million years ago. Called Hyracotherium, this mammal was small, the size of a small dog or fox. But fossils of these tiny pre-equines are rare. One of the most recent finds occurred in September, 2007 when fossil hunter Jim Tynsky though he’d found the fossil of a turtle on the dried up bed of Fossil Lake near Kemmerer, Wyoming.
Tynskly was looking for fossil fish when he spied a small, pointy foot. Since Tynsky could only clearly see the foot while the rest of the horse was covered in a thin layer of rock with only a hint of the horse’s bones showing, he drove the whole slab of rock to South Dakota. He thought he’d found a prehistoric horse.
The animal had four toes on the front feet and three on the back feet, not three in front like a tapir, plus 44 teeth and a six-inch skull. The animal would have stood about 12 inches high. While there was originally some dispute over whether this fossil showed a horse or a tapir, finally the experts agreed it truly was an early form of the horse from the Eocene period, 45 million to 55 million years ago.
It is the most complete horse of its kind ever found since the first partial specimen was discovered about 150 years ago. It’s excellent state of preservation can likely be attributed to one of the greatest mysteries of the find: it’s location in a marine environment. The tiny prehistoric equine had probably fallen into the lake while still alive. Whether it had been swimming and drowned or dropped into the lake by a large bird, can only be guessed.
Since the fossil was found on property Tynsky leases from the Lewis Horse Ranch near Kemmerer, he said “that makes it the oldest horse ranch” of any in the world since the little horse is dated at 50 million years old.
A cast of Tynksy’s Dawn horse is on exhibit in his fossil shop. The fossil itself is too valuable to display.
So, how did Hyracotherium evolve from a knee high toed mammal to the horse we ride today?
The Hyracotherium were part of the order of Perissodactyla, or odd-toed ungulates. They appeared less than 10 million years after the extinction of the dinosaurs and originally lived in tropical forests. Other members of this group included tapirs and rhinoceroses. These mammals had an odd number of toes on each hoof (including a larger, middle toe) and were hind gut fermentors.
Whereas tapirs and rhinoceroses continued to live in jungle-like environments, Hyracotherium adapted to live in dryer climates. According to Wikipedia:
The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist grounds of primeval forests. As grass species began to appear and flourish, the equids’ diets shifted from foliage to grasses, leading to larger and more durable teeth. At the same time, as the steppes began to appear, the horse’s predecessors needed to be capable of greater speeds in order to outrun predators. This was attained through the lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third.
It wasn’t until the Pliohippus, which lived 12-6 million years ago that fossil remains start to look more like the modern horse. This was the first single-hoofed horse. It was about the size of a donkey, had a longer neck than its ancestors and eyes set more to the sides of its head, giving it the ability to see both in front and behind itself.
The Dinohippus is even closer to the modern form of Equus as it was the first to exhibit rudimentary forms of the “stay apparatus”, the system of muscles, tendons, and ligaments in the horse’s leg that work together with the suspensory apparatus to allow the horse to “lock” its lower leg joints with no muscular effort. This mechanism makes it possible for the horse to sleep while standing.
As for the genus Equus, it first appeared about 5 million years ago, the only surviving genus in the originally diverse family of horses.