798 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



entirely, causing the vertebral artery to pass beneath 

 the pleurapophysis without perforating it. Thus in 

 certain Oligocene titanotheres the vertebrarterial 

 canal is reduced or wanting. 



By comparison with the tapirs, rhinoceroses, 

 paleotheres, horses, and other perissodactyls we have 

 found (1) that the locomotor skeleton of the titano- 

 theres exhibits certain distinctive titanothere family 



Figure 720. — Atlas and axis of titanotheres and other perissodactyls, showing probable course of 



the arteries through atlas 



Based on comparison with the horse and rhinoceros, Gregory and Christman, 1920. One-fourth natural size. 



A, Manteoceras, Am. Mus. 12204. The transverse process of the atlas was pierced posteriorly by the vertebral artery, which prob- 



ably then united (below the transverse process) with the ascending trunk of the occipital artery, which in turn ran dorsally and 

 passed through the atlanta Igroove and foramen, again turning sharply downward and finally turning forward and entering 

 the cranial cavity along the floor of the spinal canal. The suboccipital nerve (N. cervicalis I) passed outward and downward 

 through the atlantal foramen. These are the conditions in typical placental mammals, and the positions of the foramina and 

 grooves of the atlas indicate that they were retained by Eocene titanotheres from Eotitanops onward. 



B, Brontoihenum leidyi, Carnegie Mus. 114. The vertebral artery did not pierce the transverse process of the atlas posteriorly 

 but probably passed immediately below it. The transition from the primitive conditions shown in A apparently took place 

 in Eothanoiherium osborni, of the upper Eocene, for Mr. Peterson records the fact that in the type atlas the base of the trans- 

 verse process was pierced by a small foramen, whereas in the paratype of the same species there is no evidence of the foramen. 



C, Taphus americanus. The atlantal groove is bridged over by the anterior border of the transverse process. 



D, Opskeros bicornis, black rhinoceros. The vertebral artery enters the base of the transverse process on the upper surface, but 

 the course of the tuimel is otherwise normal. The atlantal groove is bridged, as in C. 



E, Equus caballus. The vertebral artery pierces the transverse process on its upper surface and then joins the occipital artery. The 



atlantal groove is bridged. 



PROGRESSIVE ADAPTATIONS OF THE PECTORAL AND 

 PELVIC ARCHES, THE LIMBS, AND THE FEET OF THE 

 TITANOTHERES 



From the chief adaptations to speed and weight in 

 the limbs of the larger ungulates (Chap. VIII, sec. 3) 

 we may deduce from the bony limb structure of the 

 titanotheres some knowledge of their relative size — 

 that is, their height and weight combined — as well as 

 of the speed and general mode of locomotion of the 

 different Eocene and Oligocene forms. 



features; (2) that in its cursorial, mediportal, and 

 graviportal adaptations it closely parallels the peris- 

 sodactyls of other families, living and extinct; (3) that 

 through divergence from each other in modes of 

 feeding and locomotion there evolved a great variety 

 of titanotheres, adapted to the varied feeding and 

 soil conditions of the mountain-basin and plains 

 regions. Our inquiry has been directed to three 

 determinations — proportions of the feet and limbs; 

 proportions of the head and body of the animal as a 

 whole; weight of the animal as a whole. 



