Evolution 33S 



Mesohippus. — Mesohippus, the first three-toed horse, lived during 

 the early and middle Oligocene. Numerous fossils of this form are 

 found in the Badlands of South Dakota. Mesohippus would look much 

 more familiar than Eohippus inasmuch as it looked almost like a minia- 

 ture horse. This appearance, however, was only superficial, for it had 

 innumerable differences. 



Mesohippus integrated almost imperceptibly with another horse of 

 the Oligocene, Miohippus. This latter lived into the early Miocene. 

 Following this period, many diverging lines developed, making for 

 numerous complications in the history of the modern horse. 



Anchitherium which first appeared in North America soon mi- 

 grated to the Old World where it became very abundant. Another 

 horse, closely related to Anchitherium remained in America to give rise 

 to a form known as Hypohippus. Hypohippus was larger than Anchithe- 

 rium and it gave rise to others which have died out. All of these were 

 off the main line of evolution to the modern horse, but should be 

 mentioned. 



Merychippus. — Coincident with the evolution of grasses in our 

 plains areas was the evolution of horses suited to graze upon them. 

 The first of the horses with dentition suitable for grinding the abrasive 

 grasses was Merychippns which lived during the Miocene. 



The teeth of this form were still low, but much higher than those 

 of its predecessors ; further, they were supplied with cement. Changes 

 also occurred in other parts of the body. For instance, there was an 

 increase in size, with some forms reaching the height of a small pony, 

 and the skull became very similar to that of the modern horse. Also 

 the proportions of the legs were quite like those of modern Equiis. 



The feet of Merychippns still had three toes, but in more advanced 

 forms, the side toes were decreasing in size. The central toes which 

 carried the main weight of the body ended in a large convex hoof. 



Following Merychippns, six lines developed. The most important 

 of these was that resulting in Pliohippus of the late Miocene. 



Pliohippus.— F//o/i//>/'».y is the first one-toed horse having the 

 second and fourth toes represented only by splints. The teeth were 

 much higher than those of Merychippns.. In most details, Pliohippus 

 was very like modern Equus. 



Equus. — The earliest specimens of modern horses probably arose 

 at the end of the Pliocene from an advanced type of Pliohippus. Its 



