EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHBRES 



465 



CHAEACTERS AND EEIATIONS OF THE SUBFAMILIES 



Each of the major groups described above divides 

 into two separate branches to which Osborn gives the 

 rank of subfamilies, as follows : 



Menodontine group (short-horned) : 



Brontopinae : Mesaticephalic (in females) to brachy- 

 cephalic. Incisor teeth persistent. 



Menodontinae : Mesaticephalic, stenocephalic. In- 

 cisor teeth reduced or wanting. 

 Brontotheriine group (long-horned) : 



Megaceropinae : Brachycephaho. Incisor teeth re- 

 duced or wanting. 



Brontotheriinae : Mesaticephalic to brachycephalic. 

 Incisor teeth persistent. 



Each of these subfamilies includes one or more 

 phyla, which in turn may embrace one or more genera. 



PolypTiyly. — The fact that the Ohgocene titano- 

 theres separated into four subfamilies before the be- 

 ginning of the Ohgocene epoch has been shown both 

 by the distinct connection of two of these subfamilies 

 with different Eocene ancestors and by the marked 

 differences between titanothere remains that are 

 found at the very lowest levels of the Titanotherium 

 zone. Here the ancestors of the Menodontinae, of 

 the Brontopinae, and of the Brontotheriinae are found 

 to be quite separate and distinct. No Megacero- 

 pinae have yet been found at these low geologic levels 

 of the Oligocene. 



Summary of distinctions. — The means of distin- 

 guishing the numerous branches and sub-branches of 

 the great family differ somewhat from those em- 

 ployed to distinguish the Eocene branches from one 

 another. The proportions of the head and of the 

 zygomatic arch, whether dolichocephalic or brachy- 

 cephalic, still remain a distinguishing characteristic. 

 Owing to the buccal expansion of the zygomatic 

 arches the males of Menodus are technically mesati- 

 cephaUc or even sub-br achy cephalic rather than 

 "dolichocephalic," in spite of the fact that the skuU 

 in general appearance and in form of the teeth is long 

 and narrow, especially in contrast with the wide skull 

 and grinding teeth of Brontops, Megacerops, and 

 Brontotherium. The narrower Oligocene skulls may 

 therefore be described as stenocephalic. 



As the horns become the dominant feature of the 

 skull the main line of division first arises between the 

 short-horned and long-horned titanotheres. The typi- 

 cal shape of the horns, whether triangular, rounded, 

 oval, or flattened, also becomes of very great value. 

 With these weapons of offense are developed the 

 gigantic swellings of the zygomatic arches by which 

 the breadth of the skull as a whole is measured and 

 cephalic indices are determined. A further differen- 

 tiation is found in the presence or absence of incisor 

 teeth and in the shape, size, and offensive character 

 of the canine tusks. Wide divergence is seen also in 

 the process begun in upper Eocene time — namely, the 

 molarization of the premolars, or their transforma- 



tion into the molar pattern. In some lines of descent 

 this process is accelerated, and in others it is retarded. 

 Another distinction is in the development of the 

 cingulum on the grinders. 



FiGtTBB 395. — Lower jaws of the Brontotherium phylum 



A, Brontotherium leidyi, Carnegie Mus. 93 (paratype); B, B. leidyi. Am. Mus.516; 

 C, B. hatcheri. Am. Mus. 1070; D, B. gigas, Yale Mus. 12009 (type); E, B. gigas 

 elatum, Yale Mus. 12061 (type of Titanops elatus); F, B. medium. Am. Mus. 1051. 

 All one-twelfth natural size. 



