CENOZOIC ERA: AGE OF MAMMALS 605 



man. They belong to the odd-toed division of mammals (perissi- 

 dactyls), with four toes on the fore and three on the hind foot, the 

 larger middle toe of the fore foot showing that, like the living tapir, 

 the titanotheres belonged to the odd-toed division of mammals. 



No sooner had the titanotheres reached the climax of their evolu- 

 tion than, with apparent suddenness, they became extinct. This 

 is well shown in the Oligocene deposits of the Bad Lands of South 

 Dakota (p. 588), where they are magnificently represented and 

 undergo their entire final evolution and extinction during the 

 time taken in the deposition of the 200 feet of sediments in which 

 their remains are embedded. (Osborn.) The bulk and specialization 

 of the animals rendered them more liable to extinction, since they 

 were at a disadvantage with the smaller and more active true rhinoc- 

 eroses of similar food habits which were, perhaps, able to make 

 longer journeys between water and feeding grounds. To this should 

 be added a growing scarcity of food, emphasized by drought at 

 certain seasons. It is not improbable that the competition with 

 the camels and other swift-moving forms with teeth better adapted 

 to the conditions may have been an important factor in causing the 

 scarcity of food which was fatal to the huge titanotheres, although 

 not so to the less bulky rhinoceroses. 



From the Oligocene on, the swifter, grazing forms tended to replace 

 the slow-moving, browsing (feeding on the leaves of shrubs and trees) 

 forms, although some, such as the rhinoceros, have survived to the 

 present. 



REFERENCES FOR TITANOTHERES 



Hutchinson, H. N., — Extinct Monsters and Creatures of Other Days, (Brontops), 



p. 261. 

 Osborn, H. F., — Age of Mammals, pp. 134, 239-240. 

 Scott, W. B., — A History of Land Mammals in the Western Hemisphere, pp. 308-319. 



Rhinoceroses. — The history of this great, odd-toed, hoofed 

 (perissidactyl) family illustrates two points to which attention 

 will be directed in the discussion of other families : (1) the presence 

 in abundance in North America of members of a family that has 

 long since been extinct in the western hemisphere but is still living 

 elsewhere, and (2) the evolution of a number of side branches, dif- 

 fering widely in structure and habits. The rhinoceros family is 

 now confined to Africa, southern Asia, and a few of the large islands 

 of the Indian Ocean, but in the Oligocene and Miocene not only 



