CENOZOIC ERA 205 



crowned teeth and the spreading, three-toed feet indicate that these 

 were forest horses Hving on soft vegetation — three-toed browsers hke their 

 ancestor, Hyracotheriitm . 



In Fig. 10.7 Miohippus is shown as an ancestor for several radiating 

 lines. Of these the line involving the least change led to Hypohippus, a 

 "forest horse" living in Miocene and early Pliocene times. This three-toed 

 browser was much like an enlarged version of Miohippus. 



Parahippus, another descendant of Miohippus, presented in its various 

 species a nearly complete spectrum of transitional stages between its 

 ancestor and Merychippus, i.e., between ""three-toed browsers" and 

 "three-toed grazers" (Fig. 10.7). The teeth were becoming high-crowned 

 grinders (hypsodont). The legs were becoming longer, and digit III was 

 becoming more predominant than it had been in the forest-dwelling 

 ancestors. 



As indicated in Fig. 10.7, Merychippus was a three-toed grazer adapted 

 for life on the western plains which arose in the Miocene as a result of 

 widespread continental elevation. The elongated legs, the predominance 

 of digit III (Fig. 10.5C), the elongation of the preorbital portion of the 

 skull (Fig. 10.3C), and the high-crowned molar teeth all point in this 

 direction. Interestingly enough, the milk teeth ("baby teeth") of Mery- 

 chippus were low-crowned like those of Oligocene horses, thus affording us 

 a prehistoric example of recapitulation (Chap. 4). As might be anticipated, 

 Merychippus showed an increased development of the ligaments men- 

 tioned above as forming a spring mechanism in the foot of the modern 

 horse. The relatively short side toes (II and IV) probably did not touch 

 the ground most of the time, serving as support only "when the foot was 

 under great pressure or sunk into sand or mud" (Camp and Smith, 1942). 

 There was probably no foot pad of the type found in Hyracotherium and 

 Mesohippus. 



It is also significant that during the course of the Miocene Merychippus 

 underwent striking evolution of the cerebral hemispheres of the brain. 

 Later specimens exhibited the fundamental pattern of fissures (convolu- 

 tions) which was to characterize later horses, e.g., Equus, Fig. 10.6 (Edin- 

 ger, 1948). 



Merychippus is shown (Fig. 10.7) as the center of another radiation. 

 Some descendants continued as three-toed grazers, e.g., Hipparion. On 

 the other hand, Merychippus was ancestral to horses which reduced the 

 number of digits on each foot to one — the line leading through Pliohippus 

 to Equus. This loss of the side digits was a most striking evolutionary 

 change. Why did it occur in the Equus line but not in the other lines? Did 



