202 INTRODUCTION TO EVOLUTION 



was in the middle, measuring from front to rear (Fig. 10. 3A). The molar 

 teeth were not high-crowned, continuously growing grinders. In fact they 

 were much like human molar teeth (Fig. 10.4). They had low crowns, 

 developed pronged roots, and had surfaces covered by rounded tubercles 

 or cusps much as do our own molar teeth. 



Another most unhorselike characteristic of Hyracotherium was its brain. 

 Studies of casts of the interiors of skulls (endocranial casts) have revealed 

 that the cerebral hemispheres were small and smooth; they did not cover 

 the olfactory bulbs anteriorly or the midbrain posteriorly as did the cerebra 

 of later horses (Edinger, 1948). In fact, as Fig. 10.6 shows, among brains 



Olfactory bulbs underneath 



Olfactory bulb 



Cerebrum 

 Midbrain 



Cerebellum 



Opossum Hyracotherium 

 (for comparison) 



Mesohippus 



Equus 



FIG. 10.6. Horse brain evolution. Comparison of the brain of Hyracotherium with 

 that of the opossum and with the brains of Mesohippus and Equus. The horse 

 brains, based on endocranial casts, are drawn to scale; Hyracofherium and opos- 

 sum brains are of about the same size. (After Edinger, Evolufion of the Horse 

 Brain, Memoir 25, Geological Society of America, 1948.) 



of living mammals the one most similar to the Hyracotherium brain is that 

 of the opossum, a marsupial which has retained an almost reptilian brain 

 configuration. Evidently, in the early stages of horse evolution brain 

 development lagged behind evolution of the limbs. We shall see that 

 this also appears to have been true of human evolution (Chap. 11). 



Hyracotherium was a forest dweller, a browser subsisting on soft 

 vegetation quite unlike the food of its plains-dwelling descendants. Its 

 spreading toes formed better support on the soft forest floor than does the 

 single hoof of its modern descendant. It probably escaped its enemies 

 by hiding, as do most forest-dwelling, herbivorous animals, instead of by 

 running away, as must inhabitants of treeless plains. 



