A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 



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artiodactyls had four toes (the second, third, fourth and fifth). Pigs and 

 their alUes, which live in a soft ground habitat, retain tliese four toes, 

 though the second and fifth are reduced in size. Vestiges of the second 

 and fifth toes, the dew claws, are present in some deer, but camels, 

 giraffes, antelope, sheep and cattle retain only the third and fourth 

 toes. Artiodactyls, then, are even-toed ungulates. It is probable that these 

 two orders have had a separate evolutionary origin, and owe their points 

 of similarity to parallel evolution. 



Subungufafes. Subungulates are a group of plant-eating mammals 

 that have certain incipient ungulate tendencies. Elephants (order Pro- 

 boscidea, Fig. 24.19 A), for example, have five toes, each ending in a 

 hooflike nail. They also walk to some extent upon their toe tips, but 

 a pad of elastic tissue posterior to the digits supports most of the body 

 weight. Elephants are noted for their enormous size, which must ap- 

 proach the maximum for a completely terrestrial animal. Though large 

 mammals have a relatively lower metabolic rate than small mammals, 

 the huge mass of elephants necessitates their obtaining large quantities 

 of food. The trunk, which rei^resents the drawn out upper lip and nose, 

 is an effective food-gathering organ. Elephants have a unique dentition 

 in which all of the front teeth are lost except for one pair of incisors, 

 which are modified as tusks. Their premolars, which have come to re- 

 semble molars, and their molars are very effective organs for grinding up 

 large quantities of rather coarse plant food. They are high-crowned and 

 so large that there is room for only one in each side of the upper 

 and lower jaws at a time. When it is worn down, a new one replaces it. 

 Mammals, unlike reptiles and other lower vertebrates in which there is a 

 continuous replacement of worn-out teeth, have a limited replacement 

 of teeth. Deciduous incisors, canines and premolars are present in young 

 individuals and these are replaced later in life by permanent ones. The 

 molars, which do not develop until after infancy, are not replaced. 

 Elephants, by using up their premolars and molars one at a time, have 

 evolved an interesting way of prolonging total tooth life. 



Living elephants are restricted to Africa and tropical Asia, and are 

 only a small remnant of a once world-wide and varied proboscidean 



Figure 24.19. Subungulates. The elephant, A, and the manatee, B, are believed 

 to have had a common ancestry. (Courtesy of the American Museum of Natural His- 

 tory.) 



