CENOZOIC ERA 197 



types of locomotion. In general, descendants of these ancestors attained 

 larger size and became more or less highly specialized in leg and tooth 

 structure. As samples of such evolutionary histories we shall summarize 

 those of horses and elephants (proboscideans). We choose these in 

 preference to others partly because of general interest in the end products 

 of the evolution, partly because of the fact that the fossil record is more 

 complete for them than it is for many other familiar mammals. 



EVOLUTION OF TH E HORSE 

 Adaptations of the Modern Horse 



Horses belong to the order of odd-toed ungulates, Perissodactyla. In 

 Chapter 3 we noted the hmb adaptation of horses for rapid running on 

 hard ground (pp. 23-28). We recall that digit III is greatly enlarged and 

 elongated, its "fingernail" having become the hoof. The other digits have 

 disappeared, except for the splint bones representing rudiments of 

 metacarpals (or metatarsals) of digits II and IV. The metacarpal (or 

 metatarsal) of digit III has become the powerful cannon bone of the 

 slender lower leg of the horse. Since the muscles are concentrated in the 

 proximal (attached) region of the limb, being connected to the bones 

 they move by slender tendons, the entire structure forms a light, rapidly 

 swinging pendulum, admirably adapted for swift movement. 



One of the most striking adaptations of the horse's foot for rapid running 

 consists of a set of spring ligaments on the posterior surface of the foot. 

 Fig. 10.2 shows the arrangement of some of the principal ligaments of the 

 forefoot. It will be noted that a large interosseous tendon (ligament) 

 arises from the posterior surface of the cannon bone and ends in movable 

 sesamoid bones which form a pulley arrangement back of the fetlock joint. 

 As shown, various ligaments connect these sesamoid bones to the 

 phalanges of the hoof (sesamoid ligaments). The whole arrangement is 

 elastic and may be compared to a powerful rubber band. When the 

 weight of the horse is placed upon the foot the toe is bent upward and 

 the ligaments are stretched. The tension so developed tends to spring the 

 foot back into its original position (to flex it), and so to propel the horse 

 forward. Thus the impact of the foot upon hard ground is translated into 

 upward and forward propulsion as from a springboard. Camp and Smith 

 (1942), to whom we owe much of our knowledge in this matter, state that 

 "the action resembles that of a boy jumping on a pogo-stick; the harder 

 the impact, the higher the bounce — up to the capacity of the apparatus." 



