EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



335 



Cranial evolution. — The breadth of the skull, even 

 in the earlier stages (P. major), is the most conspicuous 

 feature of the quantitative and differential evolution. 

 The comparative measurements in the three succes- 

 sive stages are as follows : 



Measurements of skull in species of Palaeosyops, in millimeters 



P. major (Am. Mus. 12182) 



P. major (Am. Mus. 13116) 



P. leidyi (Am. Mus. 1544) 



P. robustus (Am. Mus. 11678)__ 



Assuming that these measurements are fairly repre- 

 sentative, between P. major and P. rohustus we note 

 little if any rise in the cranial index — that is, relative 

 increase in breadth over length of skull. 



In the same period the grinding series (p--m') has 

 increased in length from 10 to 15 per cent over that 

 of P. major, or about as rapidly as the cranial length. 



Prominent features of the aged skull (Am. Mus. 

 1580) are (1) the width and power of the zygomatic 

 arch, including the deepening of the forward extension 

 of the masseteric insertion, which now has a depth of 

 62 millimeters below the orbits; (2) pterygoid wings of 

 the alisphenoid are very heavy, for the insertion of the 

 external pterygoid muscles opposing the temporals and 

 masseters; (3) sagittal crest, while largely broken away, 

 apparently broadened, as seen in the aged skull; in the 

 younger skull (Am. Mus. 1554) still narrow (11 mm.); 



(4) occiput apparently broad and low, not confluent 

 superiorly with the vertex of the cranium, as in the 

 type of P. leidyi, resembling rather that of P. major, 

 with the broadly flaring pillars above the condyles; 



(5) similarly paroccipital and post-tympanic processes 

 suturally separate, as in P. major, and not closely 

 conjoined, as in P. leidyi. Viewed from below the 

 basioccipitals are sharply keeled, the keel bifurcating 

 posteriorly into the occipital condyles, as in P. major, 

 and dissimilar from the same region in the type of 

 P. leidyi. While these advances upon P. leidyi are 

 bridged over by several other skulls, yet they are all 

 metatrophic and thus significant. 



These characters (assuming them to be specific and 

 not merely individual) would seem at first to indicate 

 that P. rohustus is to be regarded as a successor of 

 P. major rather than of P. leidyi, and that in the 

 intermediate levels (Bridger C) we should look for the 

 species contemporaneous with P. leidyi but directly 

 intermediate between P. major and P. robustus. In 

 this connection, however, we should bear in mind the 

 apparent variability in metatrophic characters which is 

 displayed in the skulls referred to P. leidyi (see also 

 remarks under "Mutations, " above). 



Nasals. — Skull Am. Mus. 1510 exhibits the nasals 

 (fig. 288) robust and tapering anteriorly, strongly 

 arching from side to side and anteriorly, extremely 

 solid in section. 



Palaeosyops granger! Osborn 



Plates LIX, LXII; text figures 119, 284 B 

 [For original description and type references see p. 181] 



Type locality and geologic horizon. — Bridger forma- 

 tion, UinfatJierium- Manteoceras- MesatirJiinus zone, 

 level Bridger C 1, is recorded as the geologic horizon 

 of this species. The type is from Twin Buttes, 

 Bridger Basin, Wyo., 200 feet below the "red 

 stratum. " 



Specific characters. — Exceeding P. rohustus in certain 

 dental proportions; p^-m^, 165 millimeters; fourth 



Figure 288. — Nasals of Palaeosyops 

 rohustus 



One-fourth natural size. A, Yale Mus. 11122 (type). 

 Bridger Basin, Wyo.; upper (?) part of Bridger 

 formation. Bi, Am. Mus. 1510; Bridger Basin, 

 top view. B2, The same, side view. 



superior premolar enlarged; molars with extremely 

 prominent parastyles and oblique ectolophs. 



This species was named in honor of Walter Granger, 

 associate curator of fossil mammals in the American 

 Museum of Natural History, whose explorations have 

 done so much to advance our knowledge of the Bridger 

 titanotheres and of Bridger stratigraphy. The ani- 

 mal seems to be a collateral rather than a main-line 

 form, distinguished by several peculiarities of its 

 grinding teeth. 



Materials. — The only specimen known is the type, 

 consisting of a palate and grinding teeth, with por- 

 tions of the jaw and skull (Am. Mus. 12189). 



General specific characters. — The species appears to 

 be collateral to the stage represented by P. rohustus. 

 It comes from Bridger C and was found 200 feet 

 below the "red stratum"; there is no exposure of 

 Bridger D at Twin Buttes. It is more progressive 

 than P. rohustus, chiefly in its enormous size, for the 

 premolars (p^~^) are quite backward in development, 

 in both the ectoloph and internal border. 



