EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 



365 



Manteoceras manteoceras Hay 



[Telmalotherium manteoceras (Osborn MS.) ; Telmatotherium 

 vallidens Osborn, not Cope] 



Plates XVI, XVII, XXIX, XLVI, LI, LIII, LV, LXIII, 

 LXVII; text figures 27, 29, 33, 87, 113, 215, 219, 255, 302-311, 

 323, 324, 380, 406, 408, 483, 508, 512-514, 516, 517, 521, 

 551-554, 556, 557, 566, 646-649, 661, 686, 701, 709, 712, 721, 

 723, 724, 745 



[For original descriptions and type references see p. 177. For skeletal characters 

 see p. 630] 



Type locality and geologic horizon. — Washakie Basin, 

 Wyo.; Uintatherium- Manteoceras- Mesatirhinus zone 

 (Washakie A). The most abundant material is from 

 the Bridger Basin, Wyo., some from Bridger C but 

 more from Bridger D. 



Specific characters. — Skull of medium size, basilar 

 length, 447-500+ millimeters; cephalic indices, 58 to 

 68, face relatively short; zygomata stout; horn swelling 

 prominent; p'-m', 176-186+ millimeters; postcanine 

 diastema short; i^ much larger than i^; p^ with deutero- 

 cone and tritocone poorly developed; p^ (ap. by tr.), 

 19 by 26 to 22 by 29 millimeters; m^ 28 by 29 to 32 

 by 33; m^ 35 by 37 to 41 by 40; m^, 36 by 39 to 39 

 by 43. 



The sTceleton. — The skeleton of Manteoceras is by 

 no means so fully known as that of Palaeosyops. The 

 feet are more slender than those of Palaeosyops leidyi 

 but much more robust than those of Mesatirhinus. 

 Intermediate proportions are seen throughout between 

 those characteristic of Palaeosyops, the extremely 

 broad-headed titanotheres, and of Dolichorhinus, the 

 extremely long-headed titanotheres. In Manteoceras, 

 therefore, the moderate breadth of the skulls (mesa- 

 ticephaly, PI. LIII) is associated with moderate 

 breadth of the feet (mesatipody). 



Progressive and specific characters: (1) Several muta- 

 tions, subspecies, or substages from several different 

 levels may be represented in the 20 or more specimens 

 that have been referred to M. manteoceras; (2) as 

 shown above, there is a considerable range in size 

 between the smallest specimen (Am. Mus. 12683) and 

 the largest; (3) some skulls have rather small grind- 

 ing teeth in transverse measurement and are thus 

 microdont; others have large grinding teeth and are 

 thus macrodont, and this is not a sexual character; 

 (4) in some the canines are more slender (figs. 308, 

 309), in others more robust, the form typical of the 

 species being represented in Figure 311; but it is cer- 

 tain that the canines in some lines become progres- 

 sively obtuse and posteriorly cingulate and thus ap- 

 proach the Oligocene types. 



From the detailed list of the materials given above 

 it is seen that the known individuals from the upper 

 levels of the Bridger Basin and the lower levels of the 

 Washakie Basin are from a single geologic horizon — 

 the Vintatherium- Manteoceras- Mesatirhinus zone. This 

 horizon, however, represents a long period of time, 

 but, owing partly to the slow rate at which the pre- 



molars in Manteoceras were evolving, the known 

 specimens, although probably representing several 

 different levels, do not present very marked progres- 

 sive differences, except that Am. Mus. 12683, from 

 Bridger D, is less advanced in the condition of the 

 deuterocone of p^. 



Horns. — As shown in the carefully drawn detailed 

 figures (Pis. XVI, XVII), the rudimentary horn con- 

 vexity is borne chiefly upon an anterior spur of the 

 frontals; it thus presents exactly the same relations 

 as those observed in Protitanotherium emarginatum 

 (figs. 318, 319, 374). In D. hyognathus (PI. XVII) the 

 maximum horn convexity is on the posterior spur of 

 the nasals, and the same is the case in the very much 

 more rudimentary horn of Mesatirhinus petersoni. In 

 another M. manteoceras skull (Am. Mus. 1545) the 

 swelling and rugosity is shared partly by the nasals. 

 In this stage of evolution, therefore, the osseous horn 

 is, strictly speaking, a frontonasal horn. Some of the 

 more aged specimens (especially Am. Mus. 1569) show 

 a very faintly rugose condition of the surface of the 

 bone on these horn bases. 



Proportions. — The width of these skulls (see table 

 of measurements) is increased by the great out- 

 ward arching of the zygomata posteriorly, the propor- 

 tions, as presented in Am. Mus. 1569, being, length, 

 condyles to incisive border, 492 millimeters, width 

 310. In other words, the zygomatic breadth is nearly 

 three-fifths of the skull length, whereas in Mesati- 

 rhinus petersoni the breadth is a little less than one- 

 half the length, and in Dolichorhinus hyognathus the 

 breadth is only a little more than one-third the length. 



Additional specific characters. — The other chief fea- 

 tures of the cranium are as follows : (1) The rudimen- 

 tary frontonasal horn swellings above described; (2) 

 the widening of the nasals posteriorly; (3) the pos- 

 terior spreading of the frontoparietal region; (4) the 

 deep parietofrontal pit between the posterior portion 

 of the supratemporal ridges, which have now almost 

 replaced the sagittal crest; (5) the relatively broad, 

 low occiput. 



In many details of structure, enumerated below, 

 this skull unmistakably exhibits subfamily afHnity 

 with Mesatirhinus petersoni, yet it differs from that 

 species in many important features — namely, (1) the 

 infraorbital ridge is incipient but not prominent, (2) 

 the zygomata are stout and the zygomatic width of 

 the skull is much greater than in Mesatirhinus, (3) 

 the. basioccipital region is relatively broader and less 

 elongate, and the same is true of the palate. 



The sTcull. — The superior view of the skull (fig. 304) 

 shows several characters which are prophetic of the 

 Oligocene Brontops: 



1. The nasals are slightly expanded at the anterior 

 extremities, measuring 63 millimeters (Am. Mus. 

 1569), then contracting slightly to 60 millimeters 

 and again steadily expanding to 112 millimeters at 



