CHORD ATA — VERTEBRATA — MAMMALS 



375 



specialization in whatever direction, such as an exceptional 

 development of organs of combat (e.g. horns) . Growing aridity, 

 with the consequent drying up of water courses, is another 

 powerful factor in extinc- 

 tion. For example, the 

 growing aridity of the 

 Pliocene in the Rocky 

 Mountain region and the 

 Great Plains region to the 

 east, seen also in the sandy 

 nature of the stratigraphic 

 deposits as well as in the 

 peculiar fauna and flora, 

 was most probably the 

 cause of the extinction of 

 the rhinoceroses as well as 

 of some of the browsing 

 types of horses and camels. 

 During early and mid- 

 Pleistocene time North 

 America was covered with 

 herds of mastodons, many 

 varieties of elephants, 

 large llamas, camels and 

 enormous numbers of 

 horses ; of the last there 

 were at least ten species. 

 Prong-horn antelopes, 

 white-tailed deer, pec- 

 caries, giant sloths and 

 glyptodonts were also 

 abundant. (The moose, 

 bison, mountain goat, musk-ox, red-deer, bear and reindeer did 

 not arrive from Eurasia until near the close of the Pleistocene.) 

 Preying upon these were the saber-tooth tigers as well as large, 



Fig. 162. — A comparison of relative size of 

 brains of ungulate mammals from the 

 Eocene to the present. A, outline of skull 

 of the Eocene U intathtriiim mirabile Marsh 

 with cast of brain in position, (x xe-) 

 B, same of Brontothermm ingots Marsh 

 (X 2V) from the Miocene; C, same of the 

 modern horse, Equus caballus Linn. ( X i\.) 

 (From Marsh.) 



