EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



417 



Comparative measurements of Dolichorhinus, in millimeters 



Skull: 



Pmx to condyles 



End of nasals to middle top 



of occiput 



Face, anteroposterior (pmx 



to postorbital frontal) 



Cranium, anteroposterior 

 (postorbital frontal to con- 

 dyles) 



Transverse zygomata 



Dentition: 



Pi-m' 



P'-p* 



Mi-m3 



P', ap. by tr 



P^, ap. by tr 



P', ap. by tr 



P*, ap. by tr 



Ml,, ap. bytr 



M2, ap. by tr 



MS, ap. by tr 



heterodon, 



Carnegie 



II us. 2340 



(type) 



487 

 492 

 245 



240 



189 

 76 

 114 



17X17 

 20X20 

 22X25 

 33X32 

 42X41 

 39X38 



D. intermedius 



Am. Mus. Am. Mus. 

 2001 1837 (type) 



"485 

 473 

 247 



250 



"225 



177 

 72 



105 

 12X10 

 16X17 

 19X20 

 21X ? 

 31X ? 

 37X39 

 38X38 



463 

 476 

 227 



236 

 190 



180 



72 

 109 



16X16 

 17X20 

 19X21 

 32X ? 

 40X38 

 35X37 



Dolichorhinus fluminalis Riggs, 1912 



Plates LXXV-LXXVII; text figure 140 

 [For original description and type references see p. 191] 



Type locality and geologic horizon. — Uinta Basin, 

 Utah; "Amynodon sandstones," summit of Eohasileus- 

 Dolichorhinus zone (Uinta B 2). 



Specific characters. — Extreme backward extension of 

 secondary palate. Skull rather small, 520 by 233 

 millimeters, cephalic index 45; faciocephalic index 48; 

 molar-premolar series 171 millimeters; molar cephalic 

 index 36. Nasals narrow and slightly tapering; 

 posterior nares open between hamular processes; 

 postorbital processes of jugal back of the last molar; 

 incipient horn cores in the form of narrow ridges. 

 Molar series relatively short; canines short and 

 recurved. 



Materials. — The only specimen known is the type 

 skull in the Field Museum (No. 12205). 



Specific relations. — The high geologic level, namely, 

 the summit of Uinta B 2, is to be especially noted — 

 that is, this animal occurs contemporaneously with the 

 most advanced specimen of D. hyognathus. Like D. 

 heterodon it appears to be related as a much more pro- 

 gressive form to D. intermedius, with which it agrees 

 in the angulate form of the narial recess (which is 

 rounded in D. hyognathus), in the tapering nasals, in 

 the position of the postorbital processes of the jugal 

 behind m^ (which process is in front of m^ in D. 

 hyognathus). The face is relatively short, the facio- 

 cephalic index being 48; in i*. hyognathus it is 53 to 51. 

 The molar series is relatively short, the index being 



36, while in D. hyognathus it is 38. Moreover, the 

 skull and dentition is smaller than in D. hyognathus and 

 is highly specialized in the extension of the secondary 

 palate, the broad occipital vertex, straight tooth row, 

 and extreme dolichocephaly of the basicranium. 



In his original description Riggs observes (1912.1, 

 p. 36): 



D. fluminalis displays a high degree of specialization in the 

 postnarial characters. The nares are bridged over so as to oblit- 

 erate almost all evidence of their primary position. In this 

 process the opening has receded to a point back of the hamular 

 processes. This recession, noted in less degree in other species, 

 is evidence of a secondary adaptation to aquatic habits of 

 feeding. Other characters of the skull in this species would 

 not indicate that this animal was aquatic in its general habits. 

 Like many other terrestrial mammals it probably fed upon sub- 

 merged plants. The slenderness and delicate modeling of 

 the skull would suggest an animal lighter of limb and more 

 active than other species of this genus. In the development of 

 horns the type of this species is more advanced than the type 

 specimen of D. cornutus [hyognathus]. Its narrower sagittal 

 area, its strongly recurved canines, and much smaller molars 

 readily distinguish it from that species. 



Sphenocoelus Osborn 



Text figures 111, 354 

 [For original description and type references see p. 174] 



Sphenocoelus appears to be referable to the sub- 

 family Dolichorhininae. This animal may be an 

 aberrant derivative of Mesatirhinus, but it is clearly 

 distinguished from Dolichorhinus. The type skull of 

 Sphenocoelus, so far as preserved, resembles that of 

 Metarhinus riparius Riggs in general form and in 

 many details. Sphenocoelus may therefore be closely 

 allied to that type. 



Geologic horizon. — Uinta B 1. 



Characters. — Of extreme dolichocephalic type. Base 

 of cranium with the basisphenoid laterally compressed 

 to afford space for a pair of bony pits in the roof of the 

 pharynx; a sessile sagittal crest; occiput low; occipital 

 condyles excessively broad; glenoid facets oblique as 

 in Dolichorhinus; external auditory meatus widely 

 open inferiorly. 



Geologic distribution. — The geologic level of the 

 only loiown specimen (the type) is the middle portion 

 of Uinta B 1 or "upper Metarhinus zone" of Riggs, 

 where remains of this animal are found associated with 

 Dolichorhinus longiceps but especially with the Meta- 

 rhinus fluviatilis type. The type specimen (Am. Mus. 

 1501), consisting of the posterior part of the skull 

 only, affords further proof of the wide adaptive radia- 

 tion of the titanotheres. Although our present 

 knowledge is confined to the posterior half of the type 

 skull, it appears that this animal, although aberrant, 

 probably belongs not far from the Mesatirhinus- 

 Dolichorhinus phylum. 



The name Sphenocoelus was applied to this form by 

 Osborn because of the presence of two cavities in the 

 alisphenoid bones on each side of the basisphenoid, 



