264 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



SECTION 2. INTRODUCTION TO THE ANATOMY OF 

 THE SKULL AND TEETH OF THE EOCENE TITANO- 

 THERES 



TYPES OF SKULL OF EOCENE TITANOTHERES 



For reasons that are fully set forth in Chapter VIII, 

 on the origm and descent of the titanotheres, we regard 

 the skull of Eotitanops horealis (figs. 250, 251), from 

 the lower Eocene, as the ancestral type from which 

 all the highly modified Eocene skulls were derived. 



The structure of the middle Eocene skulls is corre- 

 lated with certain feeding habits and exhibits a marked 

 contrast to that of the Ohgocene skulls. In middle 

 Eocene time the horns had not yet become weapons of. 

 offense and defense. 



The forms of the skulls of the following Eocene 

 titanotheres are noteworthy: 



1. LimnoJiyops priscus, a primitive hornless titano- 

 there, had a moderately brachycephalic skull and 

 primitive low-crowned grinding teeth. 



Figure 218. — Contrasting forms of upper teeth in Eocene titanotheres 



Brachycephalic (A) and dolichocephalic (B) types of upper premolar-molar series. One-half natural 

 size. A, Pa!(icos!;op5Zei(iv!,Am.Mus.l544(t5'pe); B, DoUcliorhinusliyogmthus, Am. Mas. ISbl. 



2. Palaeosyops leidyi (figs. 275-278) was entirely 

 hornless and represents the extreme brachycephaUc 

 and brachyodont type. 



3. The skull of TelmatJierium ultimum (figs. 294-296) 

 may be regarded as an elongated or mesaticephalic 

 modification of the primitive Limnohyops type. The 

 horn rudiments are retarded, and the crowns of the 

 teeth are more elongated than in Palaeosyops. In 

 many respects this skull resembles that of the suc- 

 ceeding type, Manteoceras. 



4. The skull of Manteoceras manteoceras (figs. SOS- 

 SOS) differs from that of TelmatJierium ultimum in the 

 vigorous development of the very precocious horn rudi- 

 ments, which are seen in profile above and in front of 

 the eyes. It is also mesaticephaUc, and the molars 

 are more brachyodont than those of TelmatJierium. 



5. The skull of MesatirJiinus petersoni has passed 

 from mesaticephaly into dolichocephaly. It resembles 

 an elongated skull of Manteoceras in having similar 

 rudiments of horns above the eyes, and the conforma- 



tion of the zygomatic arch is similar to that of Manteo- 

 ceras and very distinct from that of Palaeosyops and 

 TelmatJierium. 



6. The skuU of DolicJiorJiinus JiyognatJius (figs. 347- 

 349) is a decidedly long-headed derivative of Mesati- 

 rJiinus petersoni (figs. 327-329). The horn rudiments 

 are much more prominent and show some progressive 

 characters, such as the flattening of the top of the 

 cranium, which is analogous even to the cranium of 

 the Oligocene titanotheres. It is also decidedly 

 cyptocephalic, the face being strongly bent down on 

 the cranium. This is perhaps a river-loving type. 



7. The skull of MetarJiinus earlei (fig. 361) presents 

 a striking contrast to that of MesatirJiinus petersoni. 

 It is less dolichocephalic and shows a marked recession 

 of the narial openings and very prominent orbits, 

 indicative, perhaps, of semiaquatic habits. 



FEEDING HABITS OF BROAD-HEADED AND LONG-HEADED 

 TITANOTHERES 



Peculiar forms of tJie teetJi. — Tlue mode of 

 feeding and the food of the titanotheres can 

 not be inferred with certainty, because their 

 dentition differs considerably from that of 

 any modern mammal. In middle Eocene 

 titanotheres the grinding teeth were per- 

 fectly adapted to a combination of cutting 

 and crushing the food, as noted below. 

 This adaptation implies a choice of succu- 

 lent food consisting of relatively coarse 

 leaves, grasses, buds, twigs, roots, and 

 tubers such as would be found in forest 

 and stream habitats. In later Eocene and 

 Ohgocene titanotheres the shearing action 

 of the teeth was more perfect and the food 

 may have included smaller objects of tougher 

 fiber. 

 Although the structure of the grinding teeth of the 

 titanotheres is very different from that of the grinding 

 teeth of members of related families — the tapirs, 

 rhinoceroses, and horses — the titanotheres neverthe- 

 less present certain analogies in the form of the head, 

 from which we may infer that analogies existed also 

 in the feeding habits. 



Again, a survey of the feeding habits of the existing 

 Perissodactyla reveals a certain family likeness running 

 throughout the families of this order, which was prob- 

 ably manifest also among the extinct Perissodactyla. 

 Primitive types. — The primitive form of head and 

 tooth of Eotitanops is analogous to that of the primitive 

 paleotheres and horses, in which the proportions of 

 the cranium and face and the structure of the grinding 

 teeth are again similar. We may infer that all these 

 animals had a marked similarity of diet, from which 

 the Eocene titanotheres diverged in two directions, 

 developing into the extremely brachycephalic Palaeo- 

 syops and into the extremely dolichocephalic Doli- 

 cJiorJiinus. 



