256 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



CHANGING PROPORTIONS OF THE CRANIUM AND FACE 



The back of the eye socket, or orbit, is the dividing 

 line between the face, which lies in front of it, and the 

 cranium, which lies behind it, as indicated by the 

 shaded and unshaded parts of the skulls in Figure 212, 

 which shows that in the oldest true titanothere 

 (Eotitanops) of lower Eocene time, as in the oldest true 

 horse (Eohippus) of the same period, the face (shaded) 

 and the cranium (outline) are equally long — in fact, in 

 Eotitanops the face is a little longer than the cranium. 

 The imiversal allometric character of titanotheres is 

 abbreviation of the face (proopic region) and elonga- 

 tion of the cranium (postopic region) — that is, brachy- 

 opy and dolichocrany. Thus in all middle and upper 

 Eocene titanotheres the cranium is longer than the 

 face. This disparity keeps increasing until in the 

 Oligocene titanotheres, such as Brontotherium (fig. 

 212), the face is greatly abbreviated and the cranium 

 greatly elongated. In the horses (Equus) this allome- 

 try is just reversed: the face becomes very long (fig. 



E7otita7iop5 



Figure 212. — Unequal elongation of face and cranium in 

 titanotheres and horses 



Eoliianops and Eohippus, primitive perissodactyl type; face and cranium sub- 

 equal in length. BTontoiheTium, titanothere type; face abbreviated, cranium 

 elongated. B^uus, Equidae type; face elongated, cranium abbreviated. 



212), but the cranium remains very short 

 (dolichopy and brachycrany). These differ- 

 ences are expressed in the so-called facio- 

 cephalic index, which is obtained as follows: 



length of face including orbits X 100 

 basilar length of skull 



The relative faciocephalic indices in titano- 

 theres and horses are as follows: 



author (Osborn, 1912.382); the term "cyptocephaly" 

 is correct. 



In primitive ungulate skulls and in the fetal skull 

 the anteroposterior planes of the face and palate and 

 of the basicranial axis are more nearly in parallel 

 lines — that is, the skulls are "orthocephalic." 



In certain specialized ungulates there is either an 

 upward or a downward deflection of the face on the 



Figure 213. — Faciocranial flexure, or cypto- 

 cephaly 



In the reindeer {RoTigifeT) the face is much less bent upon the 

 cranium than in the hartebeest (Bubalis). A similar but less 

 pronounced contrast is seen in the Eocene titanotheres Palaco- 

 syops and Solichorhinus. 



cranium, which appear respectively to be adapted to 

 different forms of feeding, as follows: (1) Horizontal 

 and upward flexure of the face is characteristic of 

 certain browsing types, such as Alces and Rangifer; 

 (2) downward flexure of the face and palate on the 

 basicranial axis is characteristic of certain grazing 

 types, such as the hartebeest (Bubalis) and other 

 grazing antelopes (see fig. 213); (3) in the young of 

 certain species of Equidae, Bovidae, and Cervidae 

 the palatal line makes an angle of 19° to 25° with the 



Titanotheres : 



Brontotherium, 33. 

 Eotitanops, 56. 



Horses: 



Equus, 70. 

 Eohippus, 53. 



In the titanotheres the imiversal tendency 

 of facial abbreviation and cranial elongation 

 distinguishes all the branches alike, but since 

 the allometric movement takes place at un- 

 equal rates each genus or phylum has its YiGvnn 214.— Faciocranial flexure in Palaeosyops (A), orthocephalic, and 



Dolichorhinus (B), cyptocephalic 



distinctive faciocephalic index 



CYPTOCEPHALY, OR FACIOCRANIAL FLEXURE 



The upward or downward flexure of the facial and 

 palatal parts of the skull upon the basicranial axis 

 was first erroneously termed "cytocephaly" by the 



basicranial line, which may increase to 53° in the 

 adults of extremely deflected types. 



Among the many authors who have more or less 

 directly contributed to this subject are Riitimeyer 



