792 



TIT.USrOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



DIVERGENCE IN THE FORM OF HORNS OF THE 

 OLIGOCENE TITANOTHERES 



The wide divergence in the form of the Oligocene 

 horns is followed in great detail in Chapter VI and 

 need not be recapitulated here. In the short-horned 

 titanotheres there were doubtless very considerable 

 differences in the manner in which the horns were 

 used, both in combats between bulls for the possession 

 of females and as weapons of offense and defense in 

 the combats between the titanotheres and their 

 carnivorous enemies. It is probable that the short- 

 horned titanotheres used a side thrust of the head, 

 while the long-horned types used also a vertical, 

 tossing motion. 



ing would explain the absence of sharp points at the 

 tips of the horns (as in the ruminants and rhinoceroses) 

 as well as the absence of the pointed horny epidermal 

 sheath, which is the conspicuous offensive and defen- 

 sive weapon in the rhinoceroses. 



CORRELATION (COADAPTATION) OF HORNS WITH 

 CRANIA! AND DENTAI CHARACTERS 



There is some reason to believe that the extreme 

 development of the horns in Brontotherium platyceras 

 is correlated with extreme development of the buccal 

 plates on the zygomatic arches, which may have 

 served as defensive structures for the side of the skull 

 and face. These gigantic buccal plates are most 

 strongly developed on the skulls having the largest 



A 



Figure 713. — "Brain easts" (intracranial casts) of titanotheres (A, B, C) compared with the brain of a 



recent rhinoceros (D) 



A, Palaeosyops leidyi, Am. Mus. 1544; B, Mesatirhinus petersoni, Princeton Mus. 10041; C, Menodus giganteus (type of Broniothermm 

 ingens), after Marsh; D, Rhinoceros sondaicus, after Beddard and Treves. In the earliest stage (.Palaeosyops) the olfactory lobes 

 are relatively large and the cerebral hemispheres are small and do not overlap the cerebellum. The brain cast of Mesatirhinus (B) 

 is perhaps somewhat distorted because the cerebra appear to be abnormally wide across the frontal lobes. It is evidently a more 

 progressive type, however, than Palaeosyops. In Menodus giganteus (C) the cerebral portion appears to be considerably enlarged 

 so as to partly overlap the cerebellum. The anterior lobes, however, are narrower than in the rhinoceros, and the surface was 

 probably much less convoluted. The brain as a whole is also much smaller in proportion to the bulk of the animal. 



The connecting crest between the horns is strongly 

 developed in both the short-horned and the long- 

 horned genera (with the exception of Megacerops) 

 and is a very conspicuous feature of the evolution of 

 the horns in Bj-ontotherium. This connecting crest 

 doubtless served an important mechanical function 

 in withstanding the strain of a lateral thrust. In 

 Megacerops, on the contrary, there is little or no con- 

 necting crest, indicating the absence of lateral thrust 

 and the use of the horns principally in the tossing up 

 and down motion. In Brontotherium platyceras the 

 horns attain supreme development, and in head to 

 head combats the connecting crest of one animal 

 might engage the malar ridges at the side of the horn 

 of the opponent. This general use of the horns for 

 lateral striking and butting as well as for vertical toss- 



horns; and that they are not correlated with the 

 masseter temporalis and other muscles of mastication 

 is proved by the fact that the dental and masticating 

 structures in Brontotherium are decidedly inferior to 

 the same organs in the short-horned Menodus. These 

 buccal plates are also most strongly developed in the 

 large males and do not appear in the small-horned 

 females. They are thus secondary sexual characters, 

 correlated and coadaptive with the powerful develop- 

 ment of the horn, as are also the entire osseous struc- 

 ture of the anterior part of the skull, the inclosure and 

 reduced size of the orbits, the expansion and rugosities 

 of the occiput, the powerful ligamentum nuchae at- 

 tached to the spines of the anterior dorsal vertebrae — 

 in short, the entire bony and muscular structure of 

 the anterior part of the body. 



