EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



439 



Diplacodon Marsh 



Plates LXXXI, LXXXII; text figures 99, 368-371, 406, 594, 



595, 597, 723 



[For original description and type references see p. 166] 



Geologic Jiorizon. — True Uinta formation (Uinta C). 



History. — The original and only known remains of 

 Diplacodon elatus, an animal that has played a very 

 large part in titanothere literatm-e, consist of a 

 crushed palate with a full series of grinding teeth, 

 found by the well-known prospector Samuel Smith on 

 White River, Utah, August' 24, 1874. The geologic 

 level is now known to be the lower part of Uinta C, 

 a level higher than that of the species Eotitanotherium 

 osborni, which we have just been considering. The 

 specimen was described by Marsh in February, 1875, 

 as representing a new genus, which he named Diplaco- 

 don in reference to the double internal cones on the 

 superior premolar teeth. Marsh immediately recog- 

 nized the importance of this animal as a structiu-al 

 connecting link between "Palaeosyops" and "Bronto- 

 therium." He subsequently observed: "In the upper 

 Eocene, both [Limnohyops and Palaeosyops] have 

 left the field, and the genus Diplacodon, a very near 

 relative, holds the supremacy." (Marsh, 1877.1, 

 p. 31.) This animal was also regarded by Earle^ 

 Scott, and Osborn as a direct connecting link between 

 the Eocene and Oligocene titanotheres. 



Generic cTiaraciers. — Skull mesaticephalic to dolicho- 

 cephalic; zygomatic arches slender. Superior premo- 

 lars with flattened ectolophs and double convexities; 

 p^~* progressive, quadritubercular — that is, with 

 tetartocones; molars of elongate or dolichocephalic 

 type — that is, laterally compressed. 



Horns. — In his original description Marsh stated 

 that this animal was distinguished from the Oligocene 

 titanotheres "by the absence of horns." As the 

 skull in the type and only known specimen of D. 

 elatus is badly crushed it is difficult to determine 

 whether or not the type animal possessed rudimentary 

 horns; if it had they were certainly not so large as 

 those of the Oligocene titanotheres. In the related 

 type of Eotitanotherium (Diploceras) oshorni Peterson 

 there are well-developed horns with elongate oval 

 bases. 



Progressive molar characters. — The type skull is too 

 imperfectly known for us to distinguish all its con- 

 servative and progressive characters. In the dentition 

 the following progressive characters are observed: 

 (1) Premolars with well-developed tetartocones; (2) 

 complete internal cingula; (3) rudimentary fossettes 

 on the crown surface; (4) premolar tritocones large 

 and subequal with protocones; (5) p^ much more 

 progressive than in Telmatherium ultimum or Manteo- 

 ceras uintensis, with large, centrally placed deutero- 

 cones and subquadrate rather than triangular contour; 

 (6) molars with very distinct fossettes or pits on the 

 crown surface near the ectoloph. 

 101959— 29— VOL 1 31 



All these molar teeth characters point toward the 

 Oligocene stages of the teeth of Menodus rather than 

 of Brontotherium; on the other hand, the structure of 

 the canines and incisors points rather toward Bronto- 

 therium than Menodus. 



Diplacodon elatus Marsh 



Plates LXXXI, LXXXII; text figures 99, 368-371, 406, 597 

 [For original description and type references see p. 166] 



Type locality and geologic horizon. — Uinta Basin, 

 Utah; Diplacodon-Protitanotherium-Epihippus zone, 

 Uinta C 1, the true Uinta formation. 



Figure 368. — Type skull of Diplacodon elatus 



Palatal view. One-fourth natural size. Yale Mus. 10320; Uinta O I. Partial 



reconstruction of the under surface of the slcuU based on the type materials. 



Specific characters. — Skull mesaticephalic to doli- 

 chocephalic. P'--m', 242 millimeters; molars elongate 

 anteroposteriorly and subhypsodont; premolars short 

 and broad; tetartocone rudiment on p^, tetartocones 

 increasing in distinctness on p^ and p"*; m^ without 

 hypocone; canines small in females. 



The type specimen (Yale Mus. 10320) is a female, 

 as indicated by its small, rounded, recurved canines. 

 The postcanine diastema is considerable, measuring 

 24 millimeters. The grinding teeth are laterally 



