THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



783 



the bulk of the animal as a whole. Such an increase 

 might have been expected, for it can not be said that 

 the grinding teeth of Brontotherium gigas are greatly 

 superior mechanically to those of Eotitanops iorealis. 

 If these grinders of Brontotherium gigas had been 

 perfected through hypsodonty and plication of the 

 enamel, as in the hypsodont horses, rhinoceroses 

 (Elasmotheriujn) , and elephants, it is obvious that the 

 bulk of the grinding teeth need not have been so 

 great. In other words, BrontotJierium provided for 

 the greatly increased size of its body by increasing the 

 size of its grinding teeth rather than by mechanically 

 improving them. 



The above series of comparative measurements 

 shows that in the Oligocene titanotheres the true 

 molars (m^ to mf) were, relatively to the skull. 



of the disharmonic evolution which is in progress in all 

 parts of the skull and teeth. The above comparisons 

 include changes which distinguish the two generic 

 extremes in the evolution of the family, namely, 

 Eotitanops and BrontotJierium; but the following table 

 shows that similar changes distinguish each of the 

 three principal phyla of the lower Oligocene, namely, 

 Brontops, Menodus, and Brontotherium. 



Each genus has its distinctive velocity in the 

 evolution of each cranial and dental character. The 

 researches made for this monograph have shown that 

 the most significant diagnostic feature of a genus is 

 the relative rate of evolution of the separate parts of 

 which the skull and teeth are composed. For ex- 

 ample, we observe that in Brontops the increases in 

 different parts of the skull and teeth are more nearly 



Figure 708. — Disharmonic evolution, progressive and retrogressive, shown in eight characters of the skull 



and teeth of Brontops, Menodus, and Brontotherium 



Range of species from the lower Titanotherium zone (Chadron A) through the upper Titanotherium zone (Chadron C) . 



twice as long as in the Eocene Eotitanops. For 

 example, in Brontotherium gigas the true molars are 

 29 per cent of the skull length, in Eotitanops horealis 

 only 17 per cent; yet in Menodus the molars were 

 even proportionately larger, namely, 32 per cent of 

 the skull length. In the grinders as a whole the 

 relative gain in the true molar series is, however, offset 

 by the loss in the premolar series, for the ratio of the 

 premolars to molars drops from 63 per cent in Eoti- 

 tanops to 42 per cent in Brontotherium curtuin. Yet 

 the gain in the molar series more than offsets the rela- 

 tive loss in the premolar series. 



DISHARMONIC EVOLUTION IN LENGTH AND BREADTH OF 

 SKULL 



The above disharmonic evolution of the true molars 

 as compared with the skull and of the premolars as 

 compared with the true molars is a very simple example 



harmonic, whereas in the related short-horned Meno- 

 dus they are most widely disharmonic. This con- 

 trast may be summarized as follows: 



Percentages of disharmonic evolution in eight characters in Bron- 

 tops, Menodus, and Brontotherium 



