472 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



tJierium phylum. The free portion of the nasal bones 

 is correspondingly elongate; the nasals contract in 

 width anteriorly. The canine tusks, especially in 

 males, are elongate, pointed, and slightly recurved. 

 It appears that all lower Oligocene titanotheres also 

 have elongate nasals. The extremely primitive char- 

 acter of the nasals and of the horns in the Teleodus- 

 Brontops-Diploclonus group is correlated with a re- 

 tarded stage in the evolution of the premolar teeth, a 

 very characteristic feature which sharply distinguishes 

 members of this group from members of the Megace- 

 rops and Brontotherium phyla. The premolars (fig. 

 406) are even more retarded or simpler than those of 



iherium zone we note that the skulls referred to B. 

 hrachycephalus progress in size and in general evolution. 

 The horns shift forward somewhat on the face and 

 become elongate; the base of the horn becomes longer 

 in transverse diameter rather than in anteropos- 

 terior diameter. The free portion of the nasals 

 becomes shorter and spreads out distally. The pre- 

 molar grinding teeth gradually become somewhat 

 more complex in these stages of mutation, of change 

 of proportion, and of rectigradation, which are pro- 

 phetic of the next higher phase of evolution. It must 

 be remembered that many of these skulls are crushed 

 and distorted and represent differences in age and sex 



Figure 403. — Skulls of Rhadinorhinus and Brontotherium 



Top view. A, RJtadinorUnua abioiti. Field Mus. 12179 (type); Uinta B 1; two-nintlis natural size. B, Brontotherium 

 leidyi, Nat. Mus. 4249 (type); Chadron formation; one-sixth natural size. 



Diplacodon elatus from the upper Eocene Uinta forma- 

 tion, a proof that D. elatus was certainly not the an- 

 cestor of Brontops. On the inner side of the superior 

 premolar crowns we see a large anterior cusp (deutero- 

 cone) followed by a low posterior ridge or small 

 rudimentary posterior cusp (tetartocone). The muta- 

 tions of B. IrachycepTialus in the lower beds were 

 therefore very characteristic and clearly separable 

 both from the upper Eocene forms and from succeed- 

 ing Oligocene forms. 



Ascending mutations. — As we pass upward into the 

 middle A and upper A levels of the lower Titano- 



as well as a number of progressive stages of evolution. 

 Among the animals specifically classed as B. hracTiy- 

 cephalus it is quite possible that ancestors of more 

 than one subsequent phylum may be foimd, such as 

 that leading to Diploclonus. 



Prophetic characters among the aged individuals of 

 B. hracTiycepTialus point toward the much more robust 

 titanotheres of the middle beds to which Marsh gave 

 the names Brontops dispar and Brontops validus, the 

 former name having the priority. As the following 

 tables show, the Hatcher collection in the National 

 Museum is so rich in forms that it includes a series 



