THE MIOCENE OF EUROPE, ASIA, AND NORTH AMERICA 297 



Most remarkable of all the rodents are the short-headed mylagaulids, 

 which may be regarded, together with the sewellels or haplodontids and 

 Eocene and Oligocene ischyromyids, as a primitive division of the sciuro- 

 morph rodents. Beside the typical genus Mylagaulus, there has been dis- 

 covered the extraordinary Ceratogaulus rhinocerus, which has a special horn- 

 bearing ])one on the nasals, which was undoubtedly capped with a prominent 

 pointed dermal horn. 



Upper Miocene 



Hippario7i and Procamelus Zone 



The appearance in great numbers of herljivores with long-crowned or 

 grazing teeth such as the horses (Protohippus and Hipparion) and the camels 

 (Procamelus) establishes at once a broad evolution parallel with the Upper 

 Miocene of Pikermi, of Eppelsheim, and of Mont Leberon. The teeth and 

 feet of these animals demonstrate beyond question the spread in America 

 as in Europe at the close of the Miocene of great, dry, grassy plains, of 

 droughts or arid seasons, of long distances between the water pools at cer- 

 tain seasons of the year; in short, of East African and plateau conditions 

 of life. 



Wliile no new mammals especially distinctive of western Europe appear 

 in America at this time, it is evident that in Protohippus and Hipparion 

 there was an invasion of progressive types from the north, either from 

 British Columbia or from northern Asia. It is a striking fact that while 

 Protohippus and Hipparion may have been descended from certain stages 

 of the highly characteristic Middle Miocene Merychippus, horses of the 

 genus Merychippus as well as of the primitive Parahippus and of the forest 

 horse Hypohippus still persist in the Upper Miocene side by side with the 

 highly specialized and hypsodont Protohippus and Hipparion. This is in 

 wide contrast with the Old World, in which only the Hipparion type is 

 known. Browsing horses (Hypohippus) occur with the hipparions in China. 



This Upper Miocene fauna was very widespread geographically, very 

 rich in specific forms, highly varied in character, and represented by num- 

 bers of complete skeletons. In other words, the American, like the European 

 Miocene, closes wdth a great and famous mammalian fauna. 



Geologic formations. Plains region. — This is the typical ' Loup Fork ' 

 of all the early literature of Hayden, Leidy, Marsh, Cope, also of Scott and 

 Osborn in part. But th(^ deposits to which the names 'Loup Fork' and 

 'Loup River' were originally applied lie in eastern Nebraska and are of 

 Upper Pliocene or Lower Pleistocene age; the term Loup Fork was also 

 stretched to cover Middle Miocene formations, and gradually lost all definite 

 meaning. Tj^Dical Upper Miocene deposits of western Nebraska were 

 named the 'Nebraska Formation' by Scott in 1894;^ officially, however, 



1 Scott, Bull. Geol. Soc. Amer., Vol. V, 1894, p. 595. 



