EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 



257 



(Cervidae, 1882.1), Flower (1885.1, pp. 185-201), Lan- 

 kester (Giraffidae, 1902.1), Ewart (Equidae, 1907.1). 

 Riitimeyer pointed out that the Cervidae, as brows- 

 ing animals, have an extremely horizontal axis of the 

 skull, in contrast with that of the Bovinae, grazing 

 animals, in which the face is strongly deflected. 

 Lankester pointed out that in the hornless Olcapia, 

 a forest animal that browses on the leaves of trees, the 

 facial and cranial regions are orthocephalic, or nearly 

 in the same horizontal plane; in the related Giraffa, 

 however, which also browses on high trees but possesses 

 horns, the face is deflected on the cranium almost as 

 much as in the grazing sheep (Ovis), which feeds upon 

 the ground. Lankester consequently attributed the 



Osborn's examination of the horses (1912.382, p. 96) 

 shows that in the adult domesticated horse the 

 palatocranial angle ranges from 10° to 17°; in the 

 Burchell zebra the palatocranial angle increases with 

 age from 19° at three years to 25° at six years. In 

 the domesticated horses it varies from 20° to 23°. In 

 the Burchell zebras it increases from 15° at birth to 

 25° at the fourth year. Since the asses are more 

 given to browsing than the horses or zebras the 

 slight difference in flexure may be attributed to the 

 prevailing browsing habit. (See p. 259.) 



In titanotheres the faciocranial angle is measured as 

 shown in Figure 214. In a comparison of the brachy- 

 cephalic Palaeosyops, presumably a browsing type with 



Figure 215. — Cranial proportions of Eocene titanotheres — Palaeosyops, Manteoceras, and Dolichorhinus 



A, Brachycephalic (Palaeosyops major), zygomatic-cephalio index 77. B, Mesaticephalic (J/orjieocfrasTnaHfeoccras), zygomatio-cephalio index 

 63-68. C, Dolichocephalic (DoUchorMnus Jiyognathus) , zygomatic-cephalic index 43^6. h, Eudiments (rectigradations) of the horns. 



deflection of the face to the possession of horns. 

 Ewart applied cyptocephaly as a means of distinguish- 

 ing the various phyla of horses and, like Riitimeyer, 

 explained the flexures as adaptations to a prevailing 

 browsing or grazing habit, respectively. He pointed 

 out that Alces and Ovis illustrate the two extreme 

 types of skull: (1) the elk (Alces) is a short-necked, 

 forest form adapted to feeding on shrubs and trees — 

 that is, to holding the head in a nearly horizontal 

 position, (2) whereas the sheep {Ovis) grazes or 

 browses on the ground and is adapted to holding the 

 head when feeding in a nearly vertical position. 



There are excellent reasons for believing that a bent 

 skull facilitates grazing on short herbage. 



short-crowned teeth, the lines of the face and the 

 cranium are more nearly parallel, the angle being 

 154°. In the extremely dolichocephalic DolicJiorTiinus , 

 which has more hypsodont teeth and presumably 

 subgrazing habits, the face is sharply bent down on 

 the cranium, forming an angle of 135°. (See fig. 214.) 



Cyptocephaly, whatever its adaptive significance, 

 is certainly one of the important progressive characters 

 in the transformation of the ungulate skull and is 

 decidedly marked in certain titanotheres. 

 DOIICHGCEPHAIY, BRACHYCEPHAIY, AND CORRELATION 



In 1902, when the measurements of Oligocene 

 titanotheres were brought together, the conclusion 

 was again reached that dolichocephaly and brachy- 



