CENOZOIC ERA 



201 



knowledge: Hyracotherium (also called Eohippus). This animal lived in 

 North America in Eocene times, migrating to Europe during that period. 

 Although Hyracotherium was definitely horselike in many ways it differed 

 greatly from our modern horse. In the first place, it was small, about the 

 size of a fox terrier dog. Its legs were short and had four toes on the front 

 feet, three on the hind (Fig. 10.5A). We note, however, that digit III 

 already showed incipient signs of predominating. Through study of the 

 scars left on the foot bones by attachments of ligaments and tendons, 



FIG. 10.5. Forefeet (left) and hind feet (right) of four horses. A, Hyracotherium. B, 

 Miohippus. C, Merychippus. D, Equus. Not drawn to scale. (A, after Cope; B and C, 

 after Osborn; from Romer, Vertebrate Paleontology, University of Chicago Press, 1945, 

 p. 422.) 



Camp and Smith (1942) came to the conclusion that Hyracotherium did 

 not have the springing mechanism characteristic of the modern horse (see 

 above). Instead the foot must have been supported by a pad (Fig. 

 10.2) as in many forest-dwelling animals, including the tapir, a distant 

 relative of the horse. It will be noted from the figure that the interosseous 

 tendon is shown arising from a muscle instead of attaching directly to the 

 back of the cannon bone as it does in the modern horse. Most mammals, 

 including other hoofed forms than Equus. have such a muscle. Indeed, oc- 

 casional horses have muscle tissue in this tendon. Apparently reduction 

 of fleshy fibers, virtually changing the tendon to a ligament, has been one of 

 the evolutionary changes in the evolution of the springing mechanism. 

 "Muscular tissue, by weakening the tendon, would tend to enfeeble this 

 [springing] action" (Camp and Smith, 1942). 



The preorbital portion of the skull was not elongated; the orbit of the eye 



