CHAPTER VI 

 EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHEEES 



SECTION 1. REVIEW OF THE ENVIRONMENT, GEO- 

 LOGIC SUCCESSION, AND GEOGRAPHIC DISTRIBU- 

 TION OF THE LOWER OLIGOCENE TITANOTHERES 



In order to facilitate an understanding of the evolu- 

 tion of the skull and dentition of the Oligocene 

 titanotheres, a brief resume of the matter presented 

 in Chapter II is here given. The environment of the 

 lower Oligocene titanotheres, described in Chapter II, 

 was different from that of their Eocene ancestors. It 

 consisted mainly of the broad flood-plain region east 

 of the Kocky Mountains. The geographic range, 

 however, extended westward into the broad upland 

 plateaus west of the Rockies and northward over 

 British Columbia into Asia as far west as the eastern 

 part of Europe. 



The conditions in the 

 Great Plains region appear 

 to have been eminently 

 favorable to the existence 

 of the titanotheres, because 

 the members of all the 

 numerous branches into 

 which this great family was 

 divided show an increase 

 in size, which is especially 

 conspicuous in the males. 



Our ideas regarding the 

 geologic levels of the species 

 and the mutations of the 

 members of the four sub- 

 families are founded upon 

 the original observations of 

 Hatcher, who very care- 

 fully recorded the vertical 

 distribution of the types 



and other specimens in his great collection for the 

 National Museum, which are fully enumerated else- 

 where in this monograph. To the records of these 

 specimens have been added records obtained from 

 other museum collections. 



The faunistic subdivisions of the Titanotherium 

 zone, which forms part of the White Eiver deposits 

 and is of Chadron age (lower Oligocene), range in 

 thickness from 150 to 200 feet, as follows: 



Feet 



Chadron C (upper or Brontops robustus zone) 30-50 



Chadron B (middle or Brontops dispar zone) 70-90 



Chadron A (lower or Brontops brachycephalus zone) 50-60 



The lower Oligocene deposits of the Chadron forma- 

 tion of the Great Plains were laid down on the irregu- 



larly worn surface of the Pierre shale (Upper Cre- 

 taceous), which had been exposed to erosion for a long 

 time. Consequently the deposition of the Titanothe- 

 nwm-bearing beds was not uniform: it began at some 

 points earlier than at others, and the total thickness of 

 the Titanotherium zone accordingly ranges from 1 50 to 

 200 feet. There are also some discrepancies in the 

 records, which are doubtless due to irregularities of 

 deposition in the overflow and stream channel deposits. 



The known areas where deposits of the Titanothe- 

 rium zone are exposed and where fossils occur are shown 

 on the accompanying map (fig. 373). 



The change of form of the lower Oligocene titano- 

 theres was almost as great while this 200 feet of 



Former land areas Former migration areas 



Figure 373. — Map showing the areas in which remains of titanotheres have been found (solid 

 black) and areas in which, during Eocene and Oligocene time (oblique lines) titanotheres were 

 probably in migration 

 The general regions in which titanotheres have been found are the northwestern United States, the Gobi Desert (Mongolia), 

 Burma, and southeastern Europe. 



sediment was being deposited as that of the Eocene 

 titanotheres while 2,000 feet of sediment was being 

 deposited. We infer that the average deposit of 200 

 feet of sediment in so many localities entirely deceives 

 us as to the length of lower Oligocene time. These 

 sediments were being laid down probably not for hun- 

 dreds but for thousands of years. During this long 

 period the titanotheres were certainly very abundant 

 over the entire western plains. 



Without exception all the animals whose remains 

 are found at the base of the Titanotherium zone were 

 relatively small, and all had short and superficially 

 similar horns. The great increase in size observed be- 

 tween the animals of the beds of Chadron A and those 



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