EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



47L 



especially in males, progressively expanding into con- 

 vex buccal processes. Includes origiual contempora- 

 neous phyla known as Teleodus, Brontops, and 

 Diploclonus. 



These short-horned, ; broad-headed titanotheres, 

 which have persistent incisor teeth, are very abundant 

 in the lower, middle, and upper levels of the upper 

 TitanotJierium zone. They present certain character- 

 istics which seem to be shared most nearly by the 

 animal that Marsh called Brontops dispar. This 

 animal is represented by a large nmnber of specimens, 

 which grade below into other specifically distinct forms, 

 such as Brontops irachycepTialus, and above into 

 larger forms, such as Brontops rohustus. It appears 

 wise to retain as generic or subgeneric names the names 



The horns attain no very great length and are 

 usually circular in section at the summit. At the 

 base they are trihedral, rounded, or transversely oval 

 in section. The nasals, unlike those of Menodus and 

 Protitanotherium, are rounded anteriorly, progressively 

 shortened in ascending mutations, and in most old 

 individuals they expand at the extremities. 



At the very base of the Titanotherium zone Hatcher 

 found a number of small, broad-skulled titanotheres 

 that certainly belong to the Brontops phylum and 

 that have been termed Brontops hrachycepTialus by 

 Osborn, because they are surprisingly broad-skulled. 

 Although they are perhaps not directly descended from 

 any known upper Eocene form, such as Protitano- 

 therium emarginatum, they have one striking feature 



FiGUKE 402. — Skulls of Bhadinorhinus and Brontotherium 

 Side view. A, RhadiTwrhinus diploconus, Am. Mus. 1863 (type); White River, Uinta Basin, Utali; Uinta B 1; 

 two-ninths natural size. B, Brontotherium leidyi, Carnegie Mus. 93 (paratype); Chadron formation; one- 

 sixth natural size. 



proposed by Marsh to distinguish members of the two 

 chief' phyla — namely, Brontops rohustus and Diploclo- 

 nus arnplus. 



SUBFAIMILY CHARACTERS OF TELEODUS, BRONTOPS, AND 

 DIPLOCLONUS 



COMPARISONS AND CONTRASTS 



Very precise observation of the animals ranged 



under the phyla Teleodus, Brontops, and Diploclonus 



shows that they possess a large number of characters 



in common which distinguish them more or less 



clearly from the members of the far more readily 



defined phyla Menodus, Brontotherium, and Mega- 



cerops. 



101959— 29— VOL 1 33 



in common — the short or rudimentary horns are 

 placed directly above or slightly in front of the orbits 

 and exhibit an elongate oval section at the base, pre- 

 cisely like those of P. emarginatum. Such horns were 

 adapted to a lateral butting motion of the head, and 

 it is notable that they are always blunt, or have 

 elongate oval tips. 



On the same low geologic level was also found a 

 lower jaw containing three incisor teeth (an Eocene 

 character), described by Marsh as Teleodus avus, 

 which may belong to the same group as B. irachy- 

 cephalus. 



Features of the horns. — Horns thWt are transversely 

 oval at the tips belong to members of the Bronto- 



