5 _Stenomylus_ had a hard, chitinous hoof, an adaptation for greater running speed, and for sure footing on rough terrain.]
Over the last 60 million years the horses have evolved from small, terrier-sized animals to the diversity of size we know today, from the huge Clydesdales to the diminutive Shetland ponies. The three-toed early horse known as _Miohippus_ was about the size of a sheep. The descendants of _Miohippus_ apparently went in two directions in their evolution: One group continued to be forest-grazing, three-toed horses that eventually reached the size of modern horses but died out later. The other group, through such intermediate forms as _Parahippus_, became grassland forms that led eventually to the modern one-toed horses. Horses became extinct in North America at the end of the Pleistocene, but no one knows why. They continued to evolve on other continents and were re-introduced in historic times.
The distance between grid lines represents five centimeters.
[Illustration: 1 The back was straighter and stiffer than in earlier horses, partly because of the increasing size of the animals and partly to allow sustained open-plains running.
2 Limbs were long in proportion to the body, an evolutionary trend in the horses for speed in open-plains running, rather than darting about in forests.
3 Most of the weight of the animal was on the middle toe, which has become a single toe in modern horses. This is an adaptation for endurance and stability in open grasslands.
4 The upright mane is a primitive horse characteristic; wild horses today have reverted to this trait.
5 The coat is shown as striped, a probable holdover from earlier horses that dwelled in forests, where a striped coat would provide camouflage.
6 A large, deep mandible supported teeth adapted to grazing and the grinding of grasses and other wild plants. The teeth were deep-rooted and continuously erupted as the surface was worn down by the grit and dirt that came with the large quantities of plant food consumed daily.]
_Palaeocastor_ was an ancient beaver whose mode of life was like that of a modern prairie dog—land-oriented instead of water-oriented. _Palaeocastor_ was small, about 12 centimeters (5 inches) high, and about 30 centimeters (12 inches) long. Its fossilized spiral burrows, called _Daemonelix_, survive to tell us what its habitation was like, a feature unique to _Palaeocastor_ among all the fossil beavers. The _Daemonelix_ shown here dwarfs a member of Olaf A. Peterson’s field crew from the Carnegie Museum. The bones of a _Palaeocastor_ and one of its predators were found at the bottom of one such burrow, helping to prove that _Palaeocastor_ was responsible for making these corkscrew holes in the ground.