EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 



Groups and subfamilies of Oligocene titanotJieres 



467 



[See fig. 697, p. 



Menodontine group: Persistently short-homed titanotheres; 

 teeth with persistent cingula 



i broad; grinding Brontotheriine group: Originally short-homed, progressively long-homed titano- 

 theres; nasals pointed, reduced; grinding teeth with cingula reduced 



SUBFAMILY BBONTOPINAE 



Descendants of the Eocene Manteoceras and Protitanotherium. 



Subbrachy cephalic to hyperbrachy cephalic. 

 Horns suboval, subcircular, transversely oval. 

 One to two pairs of persistent incisor teeth. 

 Premolar transformation retarded. 

 Genera: Teleodus, Brontops, Diploclonus. 



SUBFAMILY MENODONTINAE 



Descendants of unknown Eocene ancestors, possibly related to 



Tehnatherium. 

 Mesaticephalic to dolichocephalic and to brachycephalic. 

 Horns typically subtriangular. 

 Incisor teeth vestigial or reduced to one pair. 

 Premolar transformation not retarded. 

 Genera: Allops, Menodus. 



SUBFAMILY MEGACEROPINAE 



Descendants of unknown Eocene forms, possibly of Rhadinorhi- 



nus. 

 Brachycephalic to hyperbrachycephalic. 

 Horns rounded and without connecting crest. 

 Incisor teeth reduced or vestigial. 

 Premolar transformation accelerated. 

 Genus: Megacerops. 



SUBFAMILY BEONTOTHBRIINAE 



Descendants of unknown Eocene forms. 



Mesaticephalic to brachycephalic. 



Horns transversely oval and progressively flattened. 



Incisor teeth persistent. 



Premolar transformation accelerated. 



Genus: Brontotherium. 



The members of these subfamilies are more or less 

 clearly connected with ancestral forms in the Eocene, 

 as shown in Figures 402-409. Further exploration, 



however, is needed to bridge over securely these 

 lines of descent. Our present knowledge is about as 

 follows : 



Genetic relations of Oligocene to Eocene titanotheres 



Eocene titanotheres 



OUgoceue titanotheres 



Subfamily Palaeosyopinae. Becoming entirely extinct in 

 middle Eocene time. 



Subfamily TelmatheriLnae. Exhibiting some resemblances 

 to the Oligocene Menodontinae but differing widely in the 

 presence of large incisor teeth. 



Subfamily Manteoceratinae. Including two stages, Manteo- 

 ceras and Protitanotherium, which exhibit _ many resem- 

 blances to the Brontopinae but differ in the possession of very 

 broad nasal bones. 



Subfamily Dolichorhininae. Composed chiefly of generic forms 

 that became extinct in the upper Eocene but early gave off one 

 branch. 



Subfaraily Rhadinorhininae, which shows some resemblance to 

 Megacerops. 



Subfamily Diplacodontinae. Including the genera Diplacodon 

 and Eotitanotherium, of uncertain relationships with the 

 lower Oligocene Menodontinae and Brontotheriinae. 



No known descendants of the Palaeosyopinae. 



Subfamily Menodontinae. Exhibiting some resemblances to the 

 Eocene Telmatheriinae; no known connecting forms. 



Subfamily Brontopinae. Including Teleodus, Brontops, and 

 Diploclonus, which show many resemblances to the Eocene 

 Manteoceratinae. 



Subfamily Megaceropinae. Including Megacerops, the "Sym- 

 borodon"of Cope, which shows certain resemblances to Rhadi- 

 norhinus and other very strong resemblances to Bronto- 

 therium. 



Subfamily Brontotheriinae. Showing resemblances to the 

 Eocene Diplacodon, Eotitanotherium, Rhadinorhinus. Direct 

 Eocene ancestors unknown. 



Brontopinae. — The more heavily built titanotheres 

 of the genus Brontops exhibit close resemblances to 

 the Eocene Manteoceras and Protitanotherium. They 

 pass through the lower Oligocene species of Teleodus 

 and exhibit a wide adaptive radiation into the genera 

 Brontops and Diploclonus. In general they have 

 short, robust limbs, very broad skulls, and short, 

 rounded or pointed horns and are provided with one 

 or two pairs of cropping teeth. The incisors have 



rounded crowns; the canines are roundly pointed; 

 the premolars are very slow in developing the second 

 internal cones. Short limbs and broad, spreading 

 feet give them a singularly graviportal and brachy- 

 podal character, whtch is analogous to what is known 

 of the limb structure of Manteoceras, of the Eocene. 



Menodontinae. — The long-limbed relatively cursorial 

 animals typified by the genus Menodus agree with the 

 Telmatheriinae in the very robust development of the 



