EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



449 



All primitive members of the menodontine group 

 (such as Menodus or Brontops) have long, pointed 

 canines, whereas all primitive members of the bronto- 

 theriine group (such as Brontotherium leidyi) have short, 

 obtuse canines. Secondarily some progressive members 

 of the menodontine group (such as Brontops rohustus) 

 develop short, obtuse canines that are not readily 

 distinguished from the short, obtuse canines of the late 

 members of the brontotheriine group (such as Bronto- 

 therium gigas). This is an example of convergence. 



Within each of these two groups the canines are 

 differentiated. The extremely elongate and powerful 

 canines of Menodus are readily distinguished from the 

 smaller canines of Brontops, as well as from the antero- 

 posteriorly compressed canines of Allops. Again, in 

 the brontotheriine group the robust canines of Bronto- 

 therium are readily distinguished from the diminutive 

 canines of Megacerops. 



I Premolar evolution; retrogression, abbreviation. — In 

 i correlation with the abbreviation of the face (brachy- 

 i opy) seen in the titanotheres the premolars are sacri- 

 ficed to the evolution of the molars. This is observed 

 in the variability or loss of px, in the arrested molari- 

 zation of the premolars (or their failure to acquire the 

 complete molar pattern), and in the relative abbrevi- 

 ation of the premolar series as compared with the molar 

 series and as expressed in the premolar-molar index. 

 This evolution is just the reverse of that in the 

 dolichopic Equidae, in which the premolars evolve 

 more rapidly than the molars. 



In the Menodus phylum the face is relatively 

 elongate (dolichopic) and the premolar index (50-53) 

 remains more constant. In the Megacerops and Bronto- 

 therium phyla the face is relatively abbreviate and the 

 premolar-molar index (42-46) is low and retrogressive, 

 although the premolars increase greatly in width. 



Premolar-molar indices 



Vm'-mV 



Arrested molarization of 

 premolars. — In all Oligocene 

 titanotheres arrested molar- 

 ization is seen, first, in the 

 entire absence of a meso- 

 style on the ectoloph of the 

 permanent superior pre- 

 molars, although the meso- 

 style is present on the milk 

 premolars; second, in the 

 retarded development of 

 the tetartocones, especially 

 on p*. The retarded devel- 

 opment of the tetartocone 

 of p* may be adaptively cor- 

 related with the fact that 

 this tooth erupts much later 

 than p' or p^ (PL XXI, 

 figs. 405, 406; Carnegie Mus. 

 116). Nevertheless, p* is 

 nearly as broad as m\ 

 whereas in Eocene titano- 

 theres, except those of the 

 very highest levels, p^ is much narrower than m'. 



Figure 380. — Inferior as- 

 pect of chin in Manieoceras 



A fragment from the upper Bridger, 

 Am. Mus. 1746, probably Manieo- 

 ceras manieoceras, showing large size 

 of canine roots, length of roots of ia, 

 and shortness of roots of ii and h- 

 One-half natural size. 



The differential rate of molarization of the premolars is 

 one of the most characteristicdistinctions between phyla. 

 Each phylum has its own rate of molarization. In Bron- 

 tops the premolars transform very slowly. In Diploclo- 

 nus, Allops, and Menodus, respectively, they transform 

 with increasing rapidity. In Megacerops and Brontothe- 

 rium the premolars transform very rapidly. Thus in the 

 two extremes the retarded premolars of Brontops brachy- 

 cephalus with incipient tetartocones are readily distin- 

 guished from the progressive premolars of Brontotherium 

 leidyi with strongly developed tetartocones, although 

 both animals belong to the same geologic level. 



Loss oj p\ with age. — The presence or absence of px, 

 which was much cited as a specific character by Marsh, 

 is rather an age character. This tooth is present in 

 many young skulls and absent in many old skulls, as 

 has been observed in specimens of B. brachycephalus, 

 B. dispar, B. robustus, Allops crassicornis, Menodus 

 giganteus, Brontotherium gigas. It lacks a firm hold 

 in its socket, and its root impinges against the obliquely 

 placed roots of the canines. This tooth comes into 

 use very early in Brontops, Menodus, and Bronto- 

 therium and tends to drop out early because all the 

 teeth protrude from their sockets as wear on them 



