EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



405 



Dolichorhinus (Mesatirhinus) superior Riggs 



Plates LXXV-LXXVII; text figures 137, 339, 340 

 [For original description and type references see p. 190] 



Type locality and geologic Tiorizon. — Uinta Basin, 

 Utah, MetarMnus zone, top of the " Metarhinus sand- 

 stones," summit of Uinta B 1. D. superior comes 

 from a horizon 200 to 400 feet above that at which D. 

 longiceps is commonly found. The type was found 

 in the same ledge and associated with one of the more 

 advanced stages of MetarMnus {M. earlei). Thus D. 

 superior is contemporary with a more advanced stage 

 of development of DolicliorTiinus and with the last of 

 the Metarhinus phylum (Riggs). 



Specific characters. — "Skull 485 by 255 millimeters, 

 molar series 182 millimeters, nasals free to a point over 

 last premolar, infraorbital process present, arches 

 slender anteriorly, nasals infolded at margins, sagittal 

 area expanded, canines small, p^ and p^ oblique to 

 axis of series. Molars relatively small, strong hypo- 

 cone on m^, posterior nares opening opposite the 

 anterior margin of last molar." (Riggs, 1912.1, p. 26.) 



Materials. — The only specimen known is the type 

 skull in the Field Museum (No. 12188), described 

 below. This important form is transitional between 

 Mesatirhinus and Dolichorhinus. On the whole it 

 appears to be a primitive species of the genus Doli- 

 chorhinus. The original description by Riggs is as 

 follows : 



This genus [Mesatirhinus], reported for the first time from 

 the Uinta formations, is apparently indigenous to the Bridger 

 and Washakie Basins. It is represented in the Field Museum 

 collections by a single specimen — an incomplete skull collected 

 by Mr. J. B. Abbott from the top of the Metarhinus sandstones 

 near gilsonite vein No. 2. The right arch is wanting, together 

 with the basioccipital and condyles. The dentition is anatomi- 

 cally complete excepting the incisors. 



The skull presents striking similarities with the earlier 

 representatives of Dolichorhinus. From the dorsal view, the 

 nasals, facial, and supracranial regions appear very similar, 

 though the cranial region does not have the pronounced down- 

 ward curve characteristic of Dolichorhinus. In the palatal 

 view more marked differences are noticeable. The premolars 

 are more primitive, the molars smaller, and the posterior narial 

 opening is unmodified. In these characteristics the specimen 

 in hand resembles D. heterodon ^' from upper Uinta B more 

 closely. However, it differs from that species in having a 

 strong hypocone on the last molar and in the whole facial 

 profile. In our present knowledge of these many closely 

 related forms, this species may be regarded as the largest and 

 most highly specialized representative of Mesatirhinus. 



This animal occurs geologically at the very summit 

 of Uinta B 1 (upper A of Riggs), fully 300 feet above 

 the first occurrence of Dolichorhinus longiceps. This 

 fact is important, because otherwise it would certainly 

 be considered the direct ancestor of Dolichorhinus , 

 since it affords a complete structural transition to this 

 genus, as shown in the comparative outlines displayed 

 in Figure 339. This is another very interesting in- 



'8 Douglass, Earl, Carnegie Mus. Annals, vol. 6, p. 310, 1910. 



stance of the survival of a primitive stage side by side 

 with a progressive stage. We have an analogy in 

 existing nature in the survival of the hippopotami 

 of Liberia and the Nile regions of Africa, namely, 

 H. liheriensis and H. amphihius, the former extremely 

 primitive, the latter rather progressive. 



Although the profile and the top views (figs. 339, 

 340) of the cranium of D. superior are closely similar 

 to those of D. longiceps, the palatal view is less similar 

 because of the entire lack of the secondary palate, 

 which in its various stages of development is so 

 characteristic of Dolichorhinus. In D. superior, 

 moreover, the horn cores are even more rudimentary 

 than in D. longiceps. There is a wide orbital-nasal 

 area, and a sharp downward curve of the nasals. 

 The species is also related to M. petersoni in its cephalic 

 index, which is 52 as compared with 47 in D. longi- 

 ceps — in other words, the skull is less dolichocephalic 

 than that of the typical Dolichorhinus. 



The opening of the posterior nares is opposite the 

 margin of the second molar tooth, or in the same posi- 

 tion as the primary nares of Dolichorhinus. The 

 crowns of the molar teeth are somewhat shorter or 

 more brachyodont than in Dolichorhinus. The molar 

 cephalic index, or ratio of the length of the grinding 

 series to basilar length of skull, is estimated as 38, the 

 same as in D. hyognathus. 



Dolichorhinus intermedius Osborn 



Plate LXXIII; text figures 125, 342, 343 

 [For original description and type references see p. 184] 



Type locality and geologic horizon. — Uinta Basin, 

 Utah; Eohasileus-Dolichorhinus zone (Uinta B 2). 



Specific characters. — As compared with D. hyo- 

 gnathus, of inferior size; p'-m^ 179 millimeters; m'"', 

 109; length, premaxillaries to condyles, 462; trans- 

 verse zygomata, 190 (estimated); cephalic index of 

 type 41, of paratype 45; faciocephalic index 49. 

 Secondary palate present btit less developed than in 

 D. hyognathus; infraorbital shelf of malar relatively 

 narrow; premolars less progressive with subconic 

 deuterocones; all cingula less robust; nasals more 

 pointed or less expanded distally. 



This species when described in 1908 was regarded 

 by Osborn as a structural ancestral stage, or ascending 

 mutation toward the typical D. hyognathus. It now 

 appears to be a dwarfed and somewhat more primi- 

 tive form, which thus coincides in some of its char- 

 acters with D. longiceps (the true ancestor of D. 

 hyognathus) except that the horn bases appear to be 

 more distinct. It might perhaps be regarded as a side 

 or dwarfed phylum related to or identical with the 

 D. heterodon of Riggs. 



Materials. — The type is the skull Am. Mus. 1837, 

 representing the main characters of this species. 

 Another skull (Am. Mus. 2001) is somewhat less 

 typical. These skulls are recorded from Uinta B 2. 



