EVOLUTION OF THE SKULL AND DENTITION OB^ OLIGOCENE TiTANOTHERES 



515 



On a higher plane but still in an intermediate stage 

 of development is the female type skull of A. marsJii 

 (Am. Mus. 501). The Carnegie Museum skull No. 

 123, formerly referred to this species, is more probably 

 a Brontops dispar (Am. Mus. 1445). The paratype 

 is the largest and most progressive form known. 

 This range of progressive evolution in the materials 

 at hand afi'ords strong additional proof of continuity. 



Characters oj the lower jaw. — The characters of the 

 lower jaw of this species are illustrated in Figure 432, 

 A. The chin is gently convex, not angulate; the angle 

 is but little produced downward; the lower border 

 of the ramus is nearly horizontal. 



Allops serotinus Marsh 



("Allops serotinus" Osborii, 1902) 



Plate.s XCIII, CXVII-CXX, CXXXII; text figures 184, 375, 

 387, 391, 393, 394, 399, 409, 433, 434 



[For original description and typs reterencas see p. 225] 



Geologic horizon. — Titanotherium zone of South 

 Dakota; summit (?) of middle beds (Chadron B). 



Specific characters. — Skull mesaticephalic. Length 

 705 millimeters (c?), breadth 525. Zygomatic index 

 74. P'-m^, 330 millimeters. In males nasals broad 

 (133 mm.) and short (81 mm.). Horns subtriangular 

 elongate, widely divergent. Incisors 2-1, external 

 incisor large, median incisor reduced or wanting. 

 Superior canines, males, 41 millimeters. Premolars 

 with deuterocones forming main internal portion of 

 crown. Tetartocones much smaller, especially on p''. 

 External cingula defined on premolars, more or less 

 continuous on molars. 



Measurements of Allops crassicornis and A. sero- 

 tinus. — The skulls referred to Allops serotinus and 

 Allops crassicornis form an ascending series, in which 

 p'-m' rises from 330 to 370 millimeters. The three 

 skulls referred to A. crassicornis differ from those re- 

 ferred to A. serotinus, especially in the greater length of 

 the premolar and of the molar series. The contrasts 

 in measurements with Brontojjs dispar and with Meno- 

 dus trigonoceras are shown in the accompanying table. 



Measurements of Allops crassicornis, A. serotinus, Brontops dis. 

 par, and Menodus trigonoceras, in millimeters 



[All specimens male] 



P'-m3 



P'-p* 



iVIi-ms 



P*, ap. by tr_ 



355-370 



131-150 



210-220 



46X68 



1S43, ap. by tr 80X82 



Pmx to condyles 



Zygomatic index 



Nasal length 



Nasal breadth 



330 



13.3-140 



195-206 



43X59 



68X72 



750i 705-739 



76^ 72-78 



72[ 68-81 



146 106-137 



320-345 



122-145 



200-215 



40X60 



72X75 



643-665 

 78-87 

 85-94 



102-120 



335-360 



127-136 



220-225 



43X51 



72X70 



to 82X79 



670-770 



66-79? 



115-140 



120-135 



From this table it will be seen that Allops crassi- 

 cornis and A. serotinus are distinguished by the great 

 extent of p'-p'', by the relatively great width of p* and 

 m^ by their high zygomatic index as compared with 

 that of Menodus trigonoceras, and by their relatively 

 short, wide nasals. In general the skulls of Allops 

 serotinus and A. crassicornis combine the character- 

 istics of Brontops and of Menodus. 



General characters. — This species includes crania that 

 immediately suggest Menodus giganteus, but on close 

 examination they are seen to be less powerful and less 

 robust. It is noteworthy that the supraoccipital crest 

 is less rugose and was probably provided with less 

 powerful muscles. 



Materials. — The type skull (Nat. Mus. 4251) was 

 originally recorded by Hatcher from the top of the 

 Titanotherium zone of South Dakota. This record, 

 however, was subsequently revised by Hatcher as 

 possibly due to secondary erosion and deposition of 

 the overlying Brule formation {Oreodon zone) below 

 the true upper level. Thus we may provisionally re- 

 gard Allops serotinus as typically of the uppermost 

 levels of the middle beds. It is noteworthy that the 

 matrix is a fine clay, and the skull has a brilliant 

 yellowish-white surface. 



Another skull (Nat! Mus. 2151) is recorded as 

 foimd by Hatcher is the same whitish clay matrix, 

 about 15 feet away from the type, and entered as 

 80 feet above the Cretaceous Pierre shale. This fur- 

 ther confirms the view that these animals belong in 

 the upper portion of the middle beds, or B, and not 

 in the upper Titanotherium zone (C). 



Two other skulls are known, namely, Nat. Mus. 

 4938 and Am. Mus. 520. 



Description of the type sTcull. — In addition to the 

 general characters noted above in which this skull 

 differs from that of the typical Menodus or the typical 

 Brontops, it is important to record the following 

 detailed characters: 



The type skull (Pis. CXVII, CXIX) is vertically 

 crushed, and owing to this the lateral spread of the 

 horns is unnaturally increased. It is a male speci- 

 men. Although in the seventh stage of growth, the 

 skull is decidedly delicate in its structure and pro- 

 portions. It represents an animal about a fourth 

 smaller than the majority of the specimens referred 

 to the great animal Menodus giganteus. The inferior 

 aspect of the skull is well represented in Plate 

 CXVIII, Am. Mus. 520. The nasals of the type 

 skull (Nat. Mus. 4251) are much shorter (81 mm.) 

 than in Menodus giganteus and relatively as broad 

 (133 mm.). The rugose expansion of the tips is an 

 age character. The horns are long (240 mm.), ex- 

 panding directly outward at the sides, with a low 

 but broad connecting crest; the basal section (fig. 

 433) is peculiar in the incurvation of the outer border 

 between the nasal and the malar ridges. The gen- 

 eral form is similar to that in M. giganteus. Seen 



