180 



THE AGE OF MAMMALS 



Fig. 75. — Amyiiodonts, aquatic rhinoceroses of Europe and America. The Lower OUgo- 

 cene M etamynodon of South Dakota. After original by Charles R. Knight in the American 

 Museum of Natural History. 



The especially characteristic hoofed mammals, common to this great 

 holarctic region and dominating in the two countries, which attain their 

 maximum evolution and then disappear, are the following: 



Supposed Origin 

 North America 



Types 

 Diceratheres, pair-horned rhinoceroses. 

 Amynodonts, amphibious rhinoceroses with canine tusks " " 



Entelodonts, giant pigs with elongate skulls and stilted limbs Eurasia 



Anthracotheres, buno-selenodont artiodactyls, varied and attain- " 



ing giant size in Europe only. 



These animals are all descendants of Upper Eocene ancestors. Among 

 artiodactyl ungulates we discover partly descendants of Eocene families, 

 partly new invading forms, the latter especially seen in Europe. The pre- 

 vailing artiodactyls common to both countries exhibit five-cusped, brachy- 

 odont, buno-selenodont molar teeth (anthracotheres); bunodont teeth are 

 more rare (suillines and entelodonts). True four-cusped selenodont molars 

 of modern type are observed in the oreodonts and hypertragulids and, in 

 more specialized form, in the smaller pro-ruminants, or gelocids, newl}^ 

 arriving; also in the true ruminants, or cervulines, arriving in Europe in the 

 Middle Oligocene, both probably of south Asiatic origin. It is noteworthy 

 that all these primitive Oligocene ruminants of Europe, like their selenodont 

 contemporaries, the hypertragulids in America, are hornless but usually 

 provided with defensive tusks. 



