784 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



These percentages of increase and decrease are 

 quite distinct from relative size. For example, the 

 grinding series of Alenodus is relatively much longer 

 than that of Brontotherium, yet the percentage of 

 increase is less during lower Oligocene time; the horns 

 of Menodus are much shorter than those of Bronto- 

 therium, yet the percentage of increase in Menodus is 

 greater. 



In Menodus the skull gains 37 per cent in length and 

 7 per cent in breadth; it lengthens 30 per cent faster 

 than it broadens. 



EVOLUTION OF THE SKULL IN CORRELATION (COADAP- 

 TATION) WITH THAT OF THE TEETH AND HORNS 



The skull evolution of the titanotheres passes 

 through two great phases. The first, in the lower and 

 middle Eocene, is in coadaptation to the grinding 



Figure 709. — Evolution of the skull in the titanotheres 



Top and palatal views. One-twelttb natural size. A, Eiiitanops princeps: B, Manteoeeras manteoceras: C, Brontotherium leidyi; 

 D, Brontotherium curtum. Note the lengthening of the cranial portion of the skull, the origin of the horn swellings and 

 their forward displacement in front of the orbits, the widening of the occipital crests, and the spreading of the buccal process 

 of the zygomatic arches. The palatal view reveals the progressive widening of the molar teeth, the partial molarization of the 

 premolars, and the relatively conservative (nonprogressive) character of the base of the cranium. 



The most interesting contrast is in the skulls, 

 namely, between those which are progressively brachy- 

 cephalic, like Brontops, and those which are progres- 

 sively dolichocephalic, like Menodus. In Brontops the 

 skuU gains 31 per cent in length and 39 per cent in 

 breadth; it broadens 8 per cent faster than it lengthens. 



teeth; the second, in the upper Eocene and lower 

 Oligocene, is in coadaptation to the grinding teeth and 

 to the support of the increasingly powerful horns. 

 Both these phases are affected in all their characters 

 by progressive brachycephaly or dolichocephaly, pro- 

 ceeding from an original mesaticephalic condition. 



