EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



451 



Correlation of dolichocephaly and brachycephaly with propor- 

 tions of teeth 

 [MeasuremeDts in millimeters] 



Brontops 

 robustos, Yale! 

 Mtis. 12048 I Field Mus. 

 (type) P 6927 



Brontotherium 

 gigas. Am. 

 Mus. 492 



Pmx to condyles 



Width of skull across 



zygomatic arches 



Zygomatic index 



Pi-m3 



Dental index 



Pi-p* 



M>-m3 



P*, ap. by tr 



Sum of anteroposterior 

 measurements of m'-m^ 

 compared with sum of 

 transverse measurements- 



765 



667 

 87 



350 

 45 



137 



220 

 40X65 



220 X 255 



825 



515 



62 



425 



51 



150 



270 



■ 50 X 68 



» 249 X 246 



830 



740 

 89 



353 

 42 



130 



241 

 47X72 



241X277 



• Measurement taken from Am. Mus. 605. 



Lower molars. — The lower grinding teeth also 

 indicate either the brachycephalic or the dolichoce- 

 phalic proportions of the skull. This is especially 

 witnessed in the third lower molar (PI. XXII), in 

 which, for example, the relatively long, narrow form 

 in Menodus contrasts with the relatively broad, 

 robust form in Brontotherium. M3 is further dis- 

 tinguished clearly in the different phyla by the form 

 of the hypoconulid, or third lobe, which is more 

 lophoid in the Menodus group, more crescentic in the 

 Brontotherium group. Similarly the main crescents 

 are somewhat more open in dolichocephalic molars 

 and more closed or acute in brachycephalic molars. 



The cingulum is strongly developed on the lower 

 grinders in members of the menodontine group and 

 feebly developed or obsolete in members of the 

 brontotheriine group. 



The molarization of the lower premolars proceeds 

 step by step with the molarization of the upper 

 premolars. Thus the premolars acquire the molar 

 pattern slowly in the menodontine group and more 

 rapidly in the brontotheriine group. 



The internal wall, especially of the third lower 

 molars, develops sharp crests (metacristid, entocristid^ 

 fig. 383), which are similar in form and position to 

 those of certain other early perissodactyls, especially 

 the paleotheres and chalicotheres; but, with the 

 exception of Lamhdotherium, the titanotheres do not 

 develop the metastylid and entostylid, cusps which in 

 other perissodactyls arise by fissure of the metaconid 

 and entoconid respectively. 



DEVELOPMENT OF THE SKUIL AND DENTITION 



Stage 1. — The earliest known stage (PL XXIII) is 

 represented by a specimen in the Yale Museum, 

 which is a lower jaw containing the deciduous teeth 

 of a newly born animal. Three alveoli of deciduous 

 incisors and two deciduous premolars (dp2, dp3)are 



in place; the latter are unworn. The third and last 

 deciduous premolar had not yet erupted and much of 

 it is buried in the jaw. The titanotheres, like many 

 other ungulates, apparently had but three deciduous 

 premolars on each side in the upper and the lower jaws. 



The position of the incisor alveoli was the same as 

 in Teleodus avus (PL XIX, D) and suggests their 

 identification as dii, di2, dis. The opposite incisors 

 were separated in the midline, and dii lies much 

 below the plane of di2. Perhaps this indicates a 

 protrusile tongue. The first permanent premolar, 

 Pi, is just emerging. Possibly the deciduous canine 

 had been shed at an earlier stage. The deciduous 

 premolars (dpa, dps) have heavy external cingula. 

 The horizontal ramus of the jaw is very shallow; the 

 ascending ramus relatively very heavy. 



Stage 2.- — Stage 2 is represented by a "calf" jaw 

 with alveoli for three deciduous incisors and for the 

 deciduous canines (Am. Mus. 510; PL XXIV, A), 

 which is provisionally referred to Menodus giganteus. 



PROTOCONID 



HYPOCONUL 



•D ^ENTOCONID I METACONID PARACONID 



\ I 



METAC^RISTID | 



Figure 383. — Third left lower molar of Bronto- 

 therium leidyi, showing the metacristid and 

 entocristid 



Carnegie Mus. 93. One-half natural size 



It includes the alveolus of permanent pi of the left side. 

 The deciduous premolars (dp2-dp4) are in place and 

 slightly worn. They are more molariform than the 

 permanent premolars that succeed them. 



Stage S. — Stage 3 is represented by a "calf" jaw 

 of Menodus giganteus (Am. Mus. 509; PL XXIV, B). 

 The root of i2 (?) is in place; the remaining front teeth 

 are not preserved; the tip of the permanent canine is 

 embedded in the jaw, and behind it is a root that may 

 belong to pi; dp2-dp4 are in place; dp2 and dps are 

 considerably worn, but not dp4. Permanent pi is in 

 horizontal line with dp2, and although it is a very 

 small tooth it is probably the one that is present in 

 adult titanotheres. Mi lies nearly ready to cut the 

 gum. 



Stage 4- — One of the yoimgest Imown titanothere 

 skulls (fig. 384) is in the Musemn of the University 

 of Wyoming (No. 4). It was collected by Mr. W. H. 

 Reed in HeU's Half Acre, Natrona County, Wyo., 

 from a low level in the Titanotherium zone. 



