THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



785 



Eocene browsing skull -phase. — In the Eocene phase 

 we conapare the primitive titanothere skull, as seen 

 above in the synoptic plate, with the skulls of other 

 Eocene and recent browsing perissodactyls, having 

 short-crowned, brachyodont grinding teeth, in which 

 there is no special modification of the nares for a pre- 

 hensile upper lip. The general resemblance 

 in proportions is quite close; the face and 

 cranium are of equal length, the orbit is placed 

 above the last superior grinder. 



The changes that took place are profoundly 

 disharmonic. They show progressive brachy- 

 cephaly, mesaticephaly, and dolichocephaly, 

 affecting more or less similarly all other trans- 

 formations, and are as follows: 



1. The cranium is greatly elongated — 

 opisthopic dolichocephaly, dolichocrany. 



2. The face is relatively abbreviated— - 

 proopic brachycephaly, brachyopy. 



3. The orbits are shifted forward (dis- 

 placed) from above m^ to above p^. 



4. The horns, which originate directly 

 above the orbits, are shifted forward to a point 

 above the canines, in the long-horned forms 

 especially. 



I 5. As the horns shift forward in the long- 

 horned forms the nasals are absorbed and 

 relatively abbreviated. 



6. The cranium is flattened and elongated 

 both in the long-horned and short-horned 

 dolichocephalic and mesaticephalic genera. 



7. The occiput is broadened as the cranium 

 is flattened. 



8. To fill the spaces of the enlarging skull 

 the frontal ethmoid and olfactory sinuses are 

 greatly expanded. 



9. The zygomatic arches are broadly ex- 

 panded — buccal brachycephaly. 



10. The auditory meatus is closed inferiorly 

 by the approximation of the postglenoid and 

 post-tympanic processes. 



11. The dental series, expanding in upper 

 Eocene time, is relatively constant throughout 

 the lower Oligocene, taking up about half the 

 length of the skull. 



The above eleven changes, which are begun 

 in the upper Eocene phyla of titanotheres, 

 culminate in all the Oligocene phyla. In the 

 Oligocene they represent the second phase of 

 titanothere skull evolution, in which there 

 is a double coadaptation to the functions of the 

 great grinding teeth and of the horns, respectively. 

 In coadaptation to the greater or less dominance 

 of the grinding teeth or of the horns we find a large 

 number of correlated and compensatory characters. 

 For example, in Menodus, in which the grinding teeth 

 are superbly developed and which show some tendency 



even to hj'psodonty, the horns remain of moderate 

 length, the canine tusks in compensation become 

 enlarged and relatively effective weapons, the nasals 

 are moderately abbreviated, and the skull as a whole 

 is mesaticephalic, the zygomatic arches being moder- 

 ately expanded. 



Orbit (ant. border)' 



'Uwfmesostyle) 



Brojiiotheriujiv 



Figure 710. — Relative proportions of the skulls of Manteoceras and 

 Brontothei'iutn 



Contrast in the proportions of the facial and cranial parts of the skull in Eocene and OUgocene- 

 titanotheres. Middle stage (Manteoceras) in dots, lower Oligocene stage (BrontotJierium) in 

 heavy black line. Both figures are reduced to the same absolute length from premasillaries to 

 occipital condyles. In A the premaxillaries and the condyles, respectively, of the two skulls 

 are superposed, showing the forward shifting of the horns and orbits and the lengthening of 

 the cranial region in the Oligocene type. In B the anterior rim of the orbits of the two skulls 

 is made to coincide, emphasizing the shortening of the face in the Oligocene type. 



Brontotherium presents an extreme contrast. The 

 horns, which are greatly elongated, are the dominant 

 character of the skull; the nasals are absorbed and 

 abbreviated; the canine teeth are blunt and less effec- 

 tive as weapons; the grinding teeth evolve slowly and 

 do not become subhypsodont; the buccal brachy- 

 cephaly is very extreme. 



