786 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



ARE THE PKOPOETIONS OF SKULL AND TEETH 

 ADAPTIVE? 



BracTiycepTiahj, mesatkephaly , dolichocephahj. — Be- 

 sides the dominance of the development of the grind- 

 ing teeth and horns there is repeatedly manifested in 

 the Eocene and Oligocene titanothere crania progres- 

 sive brachycephaly, persistent mesaticephaly, and 

 progressive dolichocephaly. The adaptive nature of 

 these profound changes of proportion, which affect 

 the skull as a whole, is less obvious than the domi- 

 nance of the grinding teeth and horns, because in the 

 Eocene titanotheres Palaeosyops and DolicliorMnus 

 extreme brachycephaly and dolichocephaly appear to 

 be expressed in general changes of proportion which 

 affect all the characters of the skull and teeth and 

 seem to proceed to inadaptive extremes. Similarly, 



Figure 711. — Proportions of skulls of Palaeosyops, Dolichorhinus, and 

 Eotitanops 



Midsection of the crania of Palaeosyops (A) and Dolichorhinus (B), superposed, showing the 

 straight (orthocephalic) and the flexed (cyptocephalic) condition of the faciocranial axes 

 C, Hypothetic outline of the skull of Eotitanops, drawn to the same scale. 



in the comparison above of Brontops and Menodus we 

 observe that the Brontops skulls are increasing in 

 buccal brachycephaly; moreover, that this brachyce- 

 phalic tendency is manifest in all the measurements 

 of the teeth as compared with those of the dolichoce- 

 phalic Menodus. Thus we can immediately distin- 

 guish a grinding tooth of Menodus because it is longer 

 than it is broad, in contrast to a grinder of Brontops, 

 which is broader than it is long. 



Conclusion. — It is not demonstrated that all propor- 

 tional changes are adaptive; some appear to be 

 relatively inadaptive. 



The above is an epitome of the entire evolution of 

 the titanothere skull and of the main tendencies of 

 titanothere skull evolution, which are altogether dif- 

 ferent from those in any other line of perissodactyls. 

 Each of the Eocene branches, as described in detail 

 in Chapter V, exhibits a distinctive line of cranial 



and dental evolution in which various extremes of 

 brachycephaly, dolichocephaly, and cyptocephalj^ 

 evolve more or less rapidly. 



RADIATION AND DIVERGENCE IN EOCENE SKULLS 



The divergences of the skull from the primitive type 

 in the various Eocene branches are doubtless adapta- 

 tions to the browsing and grazing habits in different 

 habitats. While in each subfamily branch the 11 main 

 tendencies of titanothere skull evolution, enumerated 

 above, are more or less clearly manifested, there ap- 

 pear certain special generic tendencies distinctive of 

 each line. Some of these changes are prophetic of 

 those that occur in the Oligocene, although they may 

 appear in lines that become extinct in the Eocene and 

 do not lead into the Oligocene genera. They are 



therefore parallel or convergent and not truly 



ancestral characters. 



EVOLUTION OF THE SKULL IN EOCENE TIME 

 PARTLY PROPHETIC OF THAT IN OLIGOCENE 

 TIME 



Ahhreviation of the jace. — The abbreviation 

 of the muzzle and premaxillary borders in 

 Palaeosyops and Metarhinus, the abbreviation 

 of the premolar series and elongation of the 

 molar series in all the Eocene phyla is pro- 

 phetic of Oligocene evolution. 



Shifting and reduction of orhits. — The for- 

 ward shifting and reduction in the size of the 

 orbits occur independently in .several of the 

 Eocene phyla, such as Palaeosyops, Manteo- 

 ceras, and Metarhinus. 



Flattening of the cranium. — The spreading 

 of the supratemporal crests, the flattening 

 of the top of the cranium, the closure of the 

 superior cranial and facial sutures occur inde- 

 pendently in Palaeosyops and Dolichorhinus. 

 In the latter the flattening of the cranium 

 proceeds more rapidly than in Manteoceras, 

 which is ancestral to certain of the Oligocene titano- 

 theres (Brontops). This feature misled Osborn (1908. 

 318) to suppose that Dolichorhinus was an ancestor 

 of the Oligocene genera, an error corrected by Hatcher 

 in his description of Protitanotherium. 



Massive and slender zygomatic arches. — The massive, 

 wide, arching zygomata of Palaeosyops present an 

 independent parallel to those of the basal Oligocene 

 titanotheres. The zygomata of Telmatotherium are 

 deep but not spreading; the zygomata of Manteoceras 

 are prophetic of the Oligocene Brontops; the slender 

 zygomata of Diplacodon are analogous to those of 

 Dolichorhinus. 



INDEPENDENT EVOLUTION OF THE SKULL IN EOCENE 

 TIME NONPROPHETIC OF THAT IN OLIGOCENE TIME 



The features of the evolution of the Eocene skull 

 that are quite independent of those of the evolution 

 of the Oligocene skull are principally the following: 



