330 THE AGE OF MAMMALS 



Evolution of the Proboscidea 



The true Proboscidea exhibit a great range of evolution, from the con- 

 servative phylum of trilophodont mastodons (related to the Mastodon ameri- 

 canus of the American Pleistocene) through intermediate stages in which 

 the number of transverse crests on the grinders steadily increases, until 

 we reach the Stegodon stage, in which there are from six to twelve low 

 transverse crests on the grinding teeth; finally into elephants which em- 

 brace both (1) the African, or Loxodonia type {E. planifro7is) with narrow 

 grinding teeth having comparatively short crowns and few transverse 

 crests, and (2) the Indian elephant, or Elephds type {E. hysudricus) , which 

 is said to be closely related to the southern mammoth {E. meridionalis) of 

 the Upper Pliocene and Pleistocene of Europe. 



The evolution of these remarkable mammals is evidently polyphyletic 

 and involves a great number of contemporaneous changes in the skull, 

 tusks, and grinders, among which the following are the most important: 

 (1) abbreviation or increasing brachycephaly of the skull; (2) abbreviation 

 of the lower jaws and loss of the lower tusks; (3) loss of the enamel of the 

 upper and lower tusks; (4) addition of transverse crests on the inter- 

 mediate and posterior grinding teeth in both jaws; (5) transition from a 

 short-crowned, brachyodont browsing adaptation of the grinding teeth of 

 the true mastodon type into the long-crowned hypsodont grazing adapta- 

 tion of the elephant type ; (6) addition of a ' cemen t ' coating to the grinders. 



Arranged in ascending order of complexity,^ as observed in the older, 

 intermediate, and newer Miocene and Pliocene formations of Asia, these 

 animals may be listed as follows: 



Euelephas hysudricus, a primitive hypsodont elephant of the Indian 



type, related to E. meridionalis of Europe. 

 E. (Loxodonta) planifrons, a hypsodont elephant of the African type, 



related to E. antiquus of the Pleistocene of Europe. 

 Stegodon insignis { = S. ganesa), sub-hypsodont, with as many as 



thirteen transverse crests, found also in China, Japan, and Java, 



Upper Pliocene and Lower Pleistocene. 

 Stegodon bombifrons, brachyodont, with as many as nine transverse 



crests, found also in China. 

 Stegodon cliftii, brachyodont, closest to Mastodon, found also in China 



and Japan. 

 Mastodon (Tetralophodon) sivalensis, molars with four to five trans- 

 verse crests and cement. 

 Mastodon (Tetralophodon) perimensis, molars with four transverse 



crests and some cement. 

 Mastodon (Tetralophodon) punjabiensis, molars with four transverse 



crests. 



^ In each case the word ' molars ' refers to the ' intermediate molars.* 



