CENOZOIC ERA 203 



From Hyracotherium to Equus 



We have sketched above the beginning and the ending of horse evolution. 

 What occurred in the milHons of years separating Hyracotherium from 

 its modern descendant, Equus? Fortunately the intervening history is well 

 documented by numerous fossils showing the transitional stages of the 

 changes in body structure required to transform the ancestor into its 

 modern descendant. Space limitations prevent more than a brief glance at 

 a few of the main stages, but the interested reader may obtain more detailed 

 information by consulting references at the end of the chapter. 



Fig. 10.7 summarizes some of the events in this evolutionary history. The 

 diagram is designed to emphasize the point that at various times in the 

 history of horses evolutionary radiations occurred, several or many forms 

 arising from an ancestor. Thus Hyracotherium in the Eocene gave rise to 

 several somewhat differing forms, among them the line which led to 

 Mesohippus in the Oligocene. 



Mesohippus was of the size of a small dog, different species varying from 

 18 to 24 inches high at the shoulder. Each foot had three toes, but digit 

 III was the largest and strongest of the three (as in Miohippus, Fig. 

 10.5B). A small nodule of bone representing metacarpal V remained in 

 the forefoot. The metacarpals and metatarsals of the three functional 

 digits (II, III, IV) were elongated, as compared to the corresponding 

 bones of Hyracotherium, an indication that the legs were beginning to 

 lengthen. This point is not well shown in Fig. 10.5, where all the feet are 

 shown reduced to the same absolute length instead of being drawn to 

 scale. As compared to Hyracotherium, Mesohippus had a slightly greater 

 development of the ligaments that were to develop into a spring mecha- 

 nism in its descendants, but like Hyracotherium (Fig. 10.2) it doubtless 

 depended upon a pad under the toes for support of the body's weight. 



The preorbital portion of the skull of Mesohippus had begun to 

 elongate, a larger diastema being present than was present in Hyraco- 

 therium (Fig. 10. 3B). The molar teeth were low crowned, but the pre- 

 molars, except for the first one, already resembled molars in structure. 

 From this point on in horse evolution the premolars and molars combined 

 to form the dental battery. 



As indicated in Fig. 10.6, the brain of Mesohippus was markedly differ- 

 ent from that of Hyracotherium. The opossumlike configuration had been 

 lost. The cerebral hemispheres had enlarged and become convoluted so that 

 the brain assumed much the appearance of a small horse brain. 



Miohippus was much hke Mesohippus but of larger size. The low- 



