EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



477 



Out of a multiplicity of specimens we are able to 

 select evidences of two or three directly successive 

 phyla, as follows: 



I. Teleodus avus, an ancestral form to Broniops, from the lowest 

 geologic levels. 



Teleodus primitivus, of doubtful relationship, preserving 

 three lower incisors. 



II. Brontops phylum, occiput progressively prolonged back- 

 ward behind zygomata; rounded horns vertically placed: 



C. Upper beds: B. dispar Marsh. 



B. Middle beds: B. dispar Marsh. 



A. Lower beds: B. brachycephalus (Osborn). 



Ila. Brontops phylum, less directly successive; occiput less 

 extended backward, horns laterally overhanging maxiUaries, 

 obliquely to transversely oval in basal section: 



C. Upper beds: B. rohustus Marsh. 



III. Collateral phylum, less directly successive; occiput ex- 

 tended very far back, nasals narrow, progressively reduced, horns 

 with internal hornlets. Levels largely undetermined: 



C. Upper beds: Diploclonus amplus Marsh. 



B. Middle beds: D. tyleri (LuU). 



B. Middle beds: D. bicornutus (Osborn). 





§.3 



UPPER 

 EOCENE 



M. giganteus 



ALLOPS MENODUS 



' / 



B. brachycephalus ' A. walcotti M. heloceras 

 \ t I ^ 



Teleodus avus ^^^ "^ *^ 



Protitanotherium 



nasals progressively broadening and abbreviating; 

 horns moderately elongate, rounded, or transversely 

 oval; facial region progressively abbreviated. 



Teleodus, a primitive stage. 



Brontops, extreme brachycephaly, single-horned. 

 Diploclonus, brachycephaly to extreme brachycephaly, 

 duplicate-horned. 



Figure 410. — Evolution of the horns in the Brontops phylum 

 a, Brontops tmchycephalus, Nat. Mus. 4261 (type), Chadron A 1; b, B. brachy- 

 cephalus, Nat. Mus. 1214, Chadron A 3; c, B. dispar, Nat. Mus. 4703, Chadron B 2; 

 d, B. dispar, Nat. Mus. 4941 (type), Chadron B 2; e, B. robustus, Nat. Mus. 

 4696, Chadron C 2. All one-fourth natural size. These outlines show progres- 

 sive increase in height and thickness of the horns; their gradual displacement 

 forward, in front of the orbits; progressive thickening of the nasals and the 

 confluence of their posterior upper border with the anterior border of the horns. 



Figure 409. — Phyla of the Brontopinae and Menodontinae, 

 titanotheres of the short-horned group 



CONSPECTUS OF CHARACTERS OF THE SUBFAMTLT BRONTOPINAE 



Summary of general characters. — Titanotheres ex- 

 tending through lower Oligocene time; incisor teeth 

 2-1, the crowns rounded; canines of medium size, 

 pointed; premolars and molars cingulate; premolar 

 transformation retarded; skuU progressively brachy- 

 cephalic to hyperbrachycephalic, indices 71-91; 



FiQUHE 41L — Basal section of the horns in the Broniops 

 phylum 

 A, Broniops brachycephalus, Nat. Mus. 4261 (type), Chadron A 1; B, B. brachy- 

 cephalus, Nat. Mus. 1214, Chadron A 3; C, B. brachycephalus, Nat. Mus. 4259, 

 Chadron B 1; D, B. dispar, Nat. Mus. 4703, Chadron B 1; £, B. dispar, Nat. 

 Mus. 4696, Chadron C 2; F, B. robustus, Yale Mus. 12048 (type), Chadron C 3. 

 All one-fourth natural size. These outlines show a progressive change in the basal 

 section of the horns from the obliquely oval section in B. brachycephalus through 

 the rounded trihedral section of B. dispar to the transversely oval section of B. 

 robustus. 



Summary of special cTiaracters. — If^. Third upper 

 and second lower incisors the largest; second (or 

 median) upper incisor frequently shed in adult; circu- 



