342 



THE AGE OF MAMMALS 



It is important to note that Matthew and Merriam have compared the 

 'Snake Creek' and 'Virgin Valley' faunas with that of Pikermi, or the 

 Upper Miocene of Greece (p. 267). It is obviously premature to attempt 

 to correlate these subdivisions with the Pliocene stages of Europe, yet there 

 seems to be a broad correspondence in America with the divisions of the 

 'Older Pliocene Fauna' and 'Newer Pliocene Fauna.' of the Old World. 



As in all other epochs, subdivisions will be finally made with clearness 

 and exactness through the successive extinctions of older forms and the 

 successive arrivals of newer forms. Pending this more exact research of the 

 future, the following provisional subdivision may be offered : 



Provisional Subdivision of American Pliocene Life 



Lower Pliocene 

 Rhinoceroses 



acerathcrine, teleocerine 

 ' Giraffe ' or browsing camels 

 True or grazing camels 

 Earliest Cavicornia 

 Browsing horses 

 Grazing horses 

 Long-jawed mastodons 

 Tapirs 



Middle Pliocene 

 Rhinoceroses extinct 

 Browsing horses extinct 

 Grazing horses of Proto- 



hippus, Pliohippus, and 



Hipparion type 

 Gigantic browsing camels 

 Grazing camels 

 Short-jawed mastodons 



Upper Pliocene 



Elephants 

 (Elephas) 



Grazing and monodactyl 

 horses (Equus) 



Browsing camels extinct 



True grazing camels and 

 llamas only 



Mastodons disappear in 

 the western plains re- 

 gion 



Tapirs disappear in the 

 western plains region 



Climatic and Physiographic Conditions 



Great Plains. — There is evidence, both in the sandy nature of the 

 deposits in the Great Plains region, in the extinction of browsing tj^pes of 

 horses and camels, and in the survival of grazing types in the same fam- 

 ilies, of increasing aridity in the Western plains and mountain region. This 

 was probably accompanied by more widely prevailing summer droughts 

 and by the contraction of the streams during the dry season. It is cer- 

 tainly significant that the rhinoceroses, brachyodont or browsing horses, and 

 giraffe or browsing camels successively disappear. In the earlj^ Pliocene or 

 in the close of the Miocene we find proofs (Sternberg ^) of the existence of 

 great herds of large land tortoises moving slowly across the plains. Their 

 presence in such large numbers is in itself proof of arid conditions, and it is 

 an interesting bit of collateral testimony from paleobotany that seeds, 

 found within a fossil skull of one of these animals, belong to a species of 

 plant (Tithynialus willistoni) which, according to Cockerell,' indicates an 

 open, relatively arid, although not strictly desert country. 



' Sternberg, C, Letter. - Cockerell, T. D. A., Letter to the author, April, 1909. 



