A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 497 



brain. Primates will be considered more fully in connection with the 

 evolution of man (Chapter 36). 



Carnivores and Whales. As mammals increased in number and 

 diversity, the opportunity arose for them to feed upon one another. 

 Certain ones became specialized for a carnivorous mode of life. The 

 living members of the order Carnivora are the weasels, dogs, raccoons, 

 bears and cats (Fig. 24.17). The shift from an insectivorous to a flesh- 

 eating diet was not difficult. An improvement in the stabbing and 

 shearing action of the teeth, and the evolution of a foot structure that 

 enabled them to run fast enough to catch their prey, was about all that 

 was necessary. Speed has been increased in most by the development of 

 a longer foot, and by standing upon their toes (though not their toe tips) 

 with the rest of the foot raised off the ground in the manner of a 

 sprinter. This digitigrade foot posture (Fig. 24.18) is in contrast to the 

 primitive plantigrade posture, in which the entire foot is placed squarely 

 upon the ground or tree branch. 



Most carnivores are semiarboreal or terrestrial, but one branch of 

 the order, which includes the seals, sea lions and walruses, early spe- 

 cialized for exploiting the resources of the sea. In addition to their 

 adaptations as carnivores, which include the large canine tusks of the 

 walrus used in gathering shell fish, these species evolved flippers and 

 other aquatic modifications. \Vhen they swim, the large pehic flippers 

 are turned posteriorly and are mo\ed from side to side like the tail of 

 a fish. 



Whales, dolphins and porpoises, of the order Cetacea, are more 

 highly specialized marine mammals that also may have evolved from 

 primitive, terrestrial carnivores. They have a fish-shaped body, pectoral 

 flippers for steering and balancing, no pelvic flippers, and horizontal 

 flukes on a powerful tail that is moved up and down to propel the 

 animal through the water. Some species have even reevolved a dorsal fin. 



Figure 24.18. Lateral and anterior views of the skeleton of the left hind foot of 

 representative mammals. A, The primitive plantigrade foot of a lemur: B, the digiti- 

 grade foot of a cat; C and D, the unguligrade foot of a pig, an even-toed ungulate; E, 

 the unguligrade foot of a horse, an odd-toed ungulate. The digits are indicated by 

 Roman numerals, the metatarsals are black and the tarsals are stippled. 



