EVOLUTION OF THE SKULL AND TEETH OF EOCENE TXTANOTHERES 



Characters oj 2?^ in Manteoceras, Mesatirhinus, and DolichorMnus 



361 



In general p^ in DolichorMnus is in a much more 

 advanced stage than in M. manteoceras. In p', p* 

 these differences become more pronounced. The 

 premolars were thus evolving along divergent lines in 

 Manteoceras on the one hand and in Mesatirhinus and 

 Dolichorhinus on the other. The general subfamily 

 kinship of M. manteoceras with Mesatirhinus and 

 Dolichorhinus is shown especially in the comparison of 

 p^, p*, in M. washaJciensis and Mesatirhinus petersoni, 

 but the generic differences are still evident. 



Jaw structure. — The jaws are prophetic of the Oligo- 

 cene type, especially in the posterior region, with an 

 elevated coronoid, and with the border sharply de- 

 pressed below the angle (fig. 310); the chin, however, 

 is weaker and the coronoid relatively much larger. 



Sex characters. — Differences in sex are indicated by 

 the smaller size of the canines in the females, as 

 observed in M. manteoceras. It is difficult to deter- 

 mine positively whether the horns are also less promi- 

 nent in the females than in the males. One well- 

 preserved, very old Manteoceras skull (Am. Mus. 

 12678) from Bridger C 5 has small canines and appears 

 to be a female. In it the horns are hardly less promi- 

 nent than in the type male. The type of M. washa- 

 Tciensis has very minute horn swellings and might be 

 taken for a female, but its canines are of intermediate 

 size. 



Mesaticephalic slcull proportions. — The skulls are in- 

 termediate in proportion, or decidedly broader than 

 those of Mesatirhinus and Dolichorhinus and much 

 longer and narrower than those of Palaeosyops, the 

 breadth being about three-fifths the length, and they 

 may thus be described as mesaticephalic. In the ear- 

 lier forms of M. manteoceras of the middle Eocene the 

 zygomatic arches are rather stout and well arched. 

 In the much later M. uintensis they are more slender 

 than in Telmatherium ultimum but diverge widely, 

 forming a decided angle with the glenoid region. 

 There is only a rudiment of the infraorbital shelf 

 that is so characteristic of most species of Mesatirhinus 

 and Dolichorhinus. 



Detailed features. — Characteristic detailed features, 

 some of which trend progressively toward the Oligocene 

 titanotheres, clearly distinguish these animals from 

 Palaeosyops and in a less degree from Mesatirhinus: 

 (1) The premaxillary symphysis is long and firm as 

 compared with that of Palaeosyops but shorter than in 

 Dolichorhinus; (2) the nasals are very characteristic, 

 being relatively short and stout, decidedly truncate, 

 distally somewhat spreading and laterally much 

 recurved; (3) in the sagittal line of the skull the suture 

 between the frontals becomes obliterated in adults, as 

 in many other ungulates with large diploe; (4) the 

 occiput is low and broad (fig. 306), very distinct in 

 form from that of Palaeosyops, and in the more 

 advanced specimens {Manteoceras washaJciensis) it 

 exhibits the lateral pillars which are so characteristic 

 of the Oligocene titanotheres. 



Summary of progressive characters of Manteoceras 

 toward Brontops and other Oligocene titanotheres. — 

 Hatcher, at the time of the discovery and description of 

 the animal now called Protitanotherium emarginatum of 

 Uinta C, pointed out the fact that Manteoceras is in or 

 near the main ancestral line of the Oligocene titano- 

 theres rather than Dolichorhinus, which Osborn had 

 supposed also in that ancestral line. 



The progressive characters of Manteoceras toward 

 the Oligocene forms are naturally somewhat more 

 marked in old than in young specimens. The follow- 

 ing items relate chiefly to the species M. manteoceras, 

 which is the most fully known and seems to lead espe- 

 cially toward the Oligocene Brontops: (1) Middle 

 part of the skuU elongate, face never very long, elon- 

 gation becoming very pronounced in the Oligocene 

 titanotheres; (2) rudimentary frontal-nasal horns ap- 

 parently increasing in size with age and probably more 

 pronounced and more rugose in the males, as in all the 

 Oligocene forms; (3) concavities in front of the orbits, 

 causing the rudimentary horns to overhang the sides of 

 the face (very prominent in the later Oligocene gen- 

 era); (4) nasals broad distally, shorter than in Doli- 

 chorhinus and in M. uintensis, suggesting the Brontops 



