444 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



of Chadron C is made evident by a comparison of the 

 outlines of the skulls (figs. 389-393) and lower jaws 

 (figs. 395-397). 



A2 



Figure 374. — Comparison of upper Eocene and lower Oligocene titanotheres, showing 

 similar early stages in the evolution of the horns 



Ai, A?, Reconstruction of the skull and jaw of an adult Protitanotherium emarginatum, Princeton Mus. 11242; upper 

 Eocene. Bi, Ba, Partly reconstructed skull ofa young Brontopsbrachycephalus, Nat. Mus. 4258; lower Oligocene. 

 One-eighth natural size. 



In order to comprehend the extraordinary number 

 and the extremely varied forms of the titanotheres 

 that inhabited South Dakota in early Oligocene time 



we must first imagine the existence of a vast conti- 

 nental region as the theater of evolution of these mi- 

 grating animals, a region far larger than any of the 

 comparatively small areas in which 

 the fossil remains have been col- 

 lected and which are shown by 

 the black areas on Figure 373. 



A vast level or undulating 

 country, consisting of great flat 

 plains traversed by slow meander- 

 ing streams, bounded on the west 

 ,,- — "V by mountain ranges, valleys, and 



\ plateaus interspersed with active 



/' volcanic peaks but allowing free 



y' migration to the east, north, and 



/A, south — such was the environment 



""' ' I \ of the Oligocene titanotheres. 



\^.'i; I'' J SECTION 2. INTRODUCTION TO 



[\ ■■"'' THE ANATOMY OF THE SKULL 



i AND THE DENTITION OF THE 



i OLIGOCENE TITANOTHERES 



I 



\ HORNS: TRANSFORMATION, ELON- 



1 GATION 



Length oj the horns. — The grad- 

 ual evolution of the horns in the 

 Eocene titanotheres was followed 

 by their more rapid evolution in 

 the Oligocene titanotheres, until 

 they became the dominant and 

 central feature of the skull, con- 

 ditioning its entire architectiu-e. 

 With the development of the 

 horns as powerful weapons are cor- 

 related changes in the structure of 

 the nasals, of the zygomatic arches, 

 of the cranial vertex, of the occi- 

 put, of the vertebral spines and in 

 the entire anatomy of the anterior 

 portion of the body. The primary 

 divisions of the Oligocene titano- 

 theres as determined by length of 

 horn are as follows: 



Menodontine group (short-horned) : 



Teleodus, Brontops, Diploclonus, 



Allops, Menodus. 

 Brontotheriine group (long-horned) : 



M egacerops {" Symborodon"), 



Brontotherium. 



First stage of development. — The 

 horns in the Oligocene titano- 

 theres, as in the Eocene Dolicho- 

 rhinus and Protitanotherium, arise 

 at the junction of the frontonasal 

 suture, slightly in front of the 

 orbits, overhanging the sides of 

 the face (fig. 374). The primitive 

 horn section is an anteroposterior oval. The longest 

 diameter of all the earliest horn tips is parallel with 

 the long axis of the skull. The anterior edge of the 



