EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 



567 



specimen. There are large, distinct tetartocones on 

 the superior premolars. Cingula on all the grinding 

 teeth are wanting. The nasals are of medium length 

 and thickness, contracting anteriorly and decurved 

 slightly at the tip. The horn pillars are stout, con- 

 tracting the anterior nares, but to a less extent than 

 in B. dolicJioceras. The postglenoid and post-tympanic 

 processes are widely conjoined. These characters 

 concur with the position of the horns, the relative 

 shortness of the nasals, the breadth of the zygomata, 

 as indicating that this species belongs on the upper 

 levels of the TitanotJierium zone. 



Measurements of Brontotherium tichoceras 



Millimeters 



Molar-premolar series 330 



Free length of nasals 65 



Free breadth of nasals 130 



Tips of nasals to connecting crest 130 



Occipital condyles to tips of nasals 800? 



Transitional skull (Nat.Mus. 8313). — Measurements 

 of this skull will be found under Megacerops (p. 542). 



Additional observations on the measurements of Bron- 

 totherium tichoceras. — The relationships of the type 

 and only known skull of this species are somewhat 

 doubtful. It possibly belongs near B. gigas and B. 

 hatcheri, although the detailed dimensions of its grind- 

 ing teeth do not support this view but suggest, on 

 the contrary, possible relationship with Megacerops, 

 as follows: 



Comparative measurements of teeth of Brontotherium tichoceras, 

 in millimeters 



The deep basal section of the horns is also consistent 

 with Megacerops affinities. 



Brontotherium gigas Marsh, 1873 



(Titanops elatus Marsh, 1887; "Brontotherium gigas" ,OsboTn, 

 1902) 



Plates XX, LXXXII, CXXXIII, CLXX, CLXXI, CLXXIII- 

 CLXXV, CLXXXIV, CXC, CCXXX-CCXXXV; text fig- 

 ures 29, 165, 229, 377, 378, 382, 390, 394, 395, 405, 467, 458, 

 460, 468, 470-472, 516, 622, 623, 625-631, 633, 634, 639, 

 662, 662, 690, 719, 721, 724, 744 



[For original description and type reterences see p. 209. For skeletal characters see 

 pp. 690-694] 



Geologic horizon. — Upper Titanotherium zone, lower 

 to upper levels. 



101959— 29— VOL 1 39 



Specific characters. — P'-m' 330-353 millimeters; 

 m'-m^ 216-241; dental index typically 42, rising to 46. 

 Basilar length of skull S (typical) 830 millimeters, 9 

 760, zygomatic breadth c? 740, index 89 (brachy- 

 cephalic). Nasals rather short, free length (? 87 milli- 

 meters, free breadth c? 115. Horns very large, typi- 

 cal <? 330-365 millimeters, 9 180 but with high 

 connecting crest as in cf , basal section in males trans- 

 versely oval but not flattened, more flattened at top 

 but less so than in succeeding species, horns much in 

 front of orbits and above premaxillaries. Buccal 

 expansion of zygoma very 

 broad in males, slight or 

 moderate in females. Ca- 

 nines, incisors, premolar 

 tetartocones, and cingula 

 much as in B. leidyi and 

 other brontotheres. 



The characters of Bron- 

 totherium gigas elatum as 

 determined from the type 

 jaw of B. gigas and the 

 type sknU and jaws of B. 

 {Titanops) elatum represent 

 the next stage beyond B. 

 hatcheri. This advance is 

 seen not in the dimensions 

 of the grinding teeth, which 

 are no greater than in B. 

 hatcheri (see table), but in 

 the more advanced evolu- 

 tion of the horns and con- 

 necting crest and in the 

 further reduction of the 

 nasals (figs. 468, 470). 



General characters of 

 Brontotherium gigas.— y^ith 

 the species B. gigas, the 

 "giant thunder beast," we 

 pass to the long-horned tita- 

 notheres of the upper Tita- 

 notherium zone, recorded 

 by Hatcher from the lower, 

 middle, and upper levels. 

 The imposing animal B. 

 gigas was apparently in 

 the line of ascent to the 

 still more remarkable B. curtum, B. ramosum, and B. 

 platyceras of the summit. The largest bulls of B. gigas 

 (Am. Mus. 492) greatly exceeded those of Brontops 

 rohustus in size, the skull measuring 32.6 inches 

 (830 mm.) from the premaxillaries to the occipital 

 condyles by 29.1 inches (740 mm.) across the zygo- 

 matic arches, as compared with 765 by 667 milli- 

 meters in the aged B. rohustus skull. The expansion 

 of the zygomata renders the brachycephaly, as mea- 

 sured on the palatal surface of the skull, still more 

 marked, the length exceeding the breadth by only 3.5 



Figure 469. — Sections and 

 contours of skull of Bron- 

 totherium? tichoceras 



Harvard Mus. (type of Menodus 

 iichoceras). One-ninth natural size. 

 The extremities of the horns are un- 

 fortunately missing but must have 

 been massive. The basal section is 

 very large; the external face is flat- 

 tened, and the internal and posterior 

 faces are well rounded. Connecting 

 crest low, nasals massive and wide, 

 zygomata much expanded, parietal 

 crest wide and convex. The generic 

 reference is uncertain; although these 

 sections suggest those of Megacerops 

 bitcco, the animal may be related to 

 Brontoihenum hatcheri. 



