394 



AN INTRODUCTION TO THE STUDY OF FOSSILS 



EQUUS 



PROTOHIPPUS 



MESOHIPPUS 



to bony splints at the sides of 

 the leg bone. The embryo of 

 the modern horse passes through 

 at least some of these stages 

 in its development. For ex- 

 ample, a horse embryo fourteen 

 inches long has three toes with 

 well-developed metacarpal and 

 phalangeal structures, resem- 

 bling Miohippus. Merychippus 

 shows w^ell its intermediate posi- 

 tion between the browsing and 

 grazing type of horse in its 

 teeth ; its milk teeth are short- 

 crowned and without a covering 

 of cement like its ancestors ; its 

 permanent set are long-crowned 

 with cement filling the spaces 

 fl between the enamel ridges, as 

 in its descendant Equus. This 

 change from the little Eohippus, 

 the '' dawn horse," to the large 

 modern horse took place through 

 changes in all parts of the 

 skeleton. Besides increase in 

 size of most bones in the body, the 

 succeeding descendants walked 

 more and more upon the ends of 

 the toes. Since the series began 

 by having the middle digit the 

 longest, this digit gradually as- 

 sumed the weight of the body. 

 The arched back likewise became straight, and the molar teeth 

 changed from a rooted, low-crowned, cement-free type adapted 

 only to browsing, to one growing until an advanced age and hence 



OROHIPPUS 



Fig. 174. — Evolution of the horse. 

 II. (Not drawn to scale.) In the 

 course of evolution from Orohippus 

 (Upper Eocene) to Equus, the bones 

 of the fore arm (radius and ulna) 

 gradually became consolidated into 

 one bone, resulting in the disappear- 

 ance of the lower end of the ulna. 

 In the fore leg (fibula and tibia) 

 the fibula was gradually obliterated. 

 These changes resulted in a more 

 slender limb and hence greater speed. 

 j2., fibula; m., radius; ^f., tibia; til., 

 ubia. (After Marsh.) 



