44 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



All estimates orgeologic time are highly provisional, 

 because they involve two unknown quantities — the 

 amount of overlap and the relative rate of deposition. 

 The rate of the deposition of sediments varies enor- 

 mously. For example, certain Fort Union sediments 

 of Montana, aggregating 6,000 feet in thickness, are 

 at present considered contemporaneous with Torre- 



Lambdotheruum popoaoicum £otUanoDS orinceps 



Figure 33.- 



-Outlincs of the bodies of titanotheres at different stages 

 of evolution 



jon sediments of New Mexico, which aggregate only 

 385 feet. It would therefore appear that sedimenta- 

 tion in Montana was more than thirteen times as 

 rapid as in New Mexico. The only sedimentary 

 stage which appears fairly uniform in several geo- 

 graphic localities is the Wasatch, which exhibits beds 

 of approximately the same thickness in many different 

 regions. 



If an average rate of deposition of a foot in a century 

 is assumed, the period from basal Eocene to lower 

 Oligocene time, inclusive, is estimated as not exceeding 

 1,100,000 years, a moderate estimate considering the 

 great biologic changes that took place in the titano- 

 theres and other groups during this period. The 

 epoch of the titanotheres is roughly estimated at 

 500,000 years or more, during which they steadily 

 increased in size, from the geologically earliest 

 animals, which are no larger than a sheep, to some 

 of the latest members of the race, which exceeded 

 in size the largest rhinoceroses, standing over SJ^ 

 feet at the shoulders. 



The recorded history of the titanotheres is 

 nearly unbroken, but there have been two evolu- 

 tionary gaps, one between the lower and the mid- 

 dle Eocene, which was filled in 1918 by explora- 

 tions of the Huerfano (Osborn, 1919.494), and one 

 between the upper Eocene and the lower Oligo- 

 cene, which will be filled by the exploration of 

 the upper part of the Uinta formation (theoretic 

 faunal zone 16, still unknown). The record also 

 shows sudden transitions caused by invasions of 

 animals from other regions. 



The geographic range of the titanotheres was 

 probably continent wide in America and also ex- 

 tended across Asia into the Balkan region of south- 

 eastern Europe. In the relatively small Rocky 

 Mountain and western plains region, where most 

 of the fossil remains have been discovered, we 

 observe the successive invasion of new kinds of 

 titanotheres, which had apparently evolved pre- 

 viously in other regions, probably in areas to the 

 north and east. 



The geologic age of the little-known European 

 titanotheres is somewhat uncertain. The type 

 and only known specimen of Brachydiastemaihe- 

 rium, an animal about the size of Diplacodon, is 

 recorded fi'om a formation in eastern Hungary 

 that was originally assigned to the lower Eocene, 

 but this animal is in a stage of evolution corre- 

 sponding to that of the uppermost Eocene titano- 

 theres of America, and the same European forma- 

 tion has yielded remains of a primitive rhinoce- 

 ros (ProTiyracodon) of upper Eocene or even lower 

 Oligocene type. Brachydiastematlierium is there- 

 fore probably not of lower Eocene age. The 

 animals described as Menodus rumelicus and 

 Titanotherium hohemicum are in all respects sim- 

 ilar to American titanotheres of lower Oligocene 

 age, but as the localities and horizons from which 

 these fragmentary specimens were obtained are in 

 doubt they may be imported American fossils to which 

 a European origin has been erroneously imputed. 



The correlation of the chief geographic, geologic, 

 climatic, and faunistic events during the Tertiary 

 period in the Eocky Mountain region with those in 

 western Europe has been studied by the author con- 

 tinuously during the last 20 years, with the coopera- 



