116 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



SOURCES OF ERROR IM DETERMINING STRATIGEAPHIC LEVELS 



It should be borne in mind that owing to the great 

 difference in the thickness of the " Titanotherium 

 beds" in different locaUties and to the irregular topog- 

 raphy of the Pierre shale upon which the beds rest 

 it often happens that the base of these beds at one 

 point may correspond to the middle Titanotherium 

 zone at others, so that an exact stratigraphic subdi- 

 vision of the Chadron formation over wide areas is 



some 165 feet below their summit. Hence this skull 

 is assigned to Hatcher's level A 3, although its large 

 size and progressive structure would lead one to infer 

 that it came from the upper Titanotherium zone 

 (Chadron C). 



Notwithstanding these discrepancies we are able 

 to follow the evolution of five separate phyla of 

 titanotheres, from the small animals of the lower 

 Titanotherium zone (Chadron A), which have small 



Adaptive 



radiatiorL of the 



subfaTnilies of 



PerLssodcLctyls 



Haiiits OTui habitats 



AQUATIC Kv-3 MEDIPORTAL WpM GRAVI PORTAL ESS 



Figure 77. — The family tree of the Perissodactyla 



Adaptive radiation of the 9 families and 35 subfamilies. Their divergence in limb and foot structure into cursorial, forest-living, mediportal, 

 and graviportal types, and in tooth structure into browsing and grazing types, is indicated by respective symbols. 



not at all possible. In spite of such opportunity 

 for error only a few well-authenticated records (such 

 as that of the type of Brontops dispar) appear to con- 

 tradict Hatcher's statement that the titanotheres of 

 advanced structural development are confined to 

 the upper levels of the beds. One such striking 

 exception is recorded by Prof. R. S. Lull (1905.1), 

 who states that he found the type of Diploclonus 

 tyleri at a point only 35 feet above the Pierre shalei 

 at the base of the Titanotherimn beds, which was there 



horns, through intermediate types to the latest forms, 

 which have highly specialized skulls, from the top 

 of the " Titanotherium beds." This evolution was 

 rather even and regular in the phyla Brontotherium 

 and Menodus {= Titanotherium) , but in the Brontops 

 phylum it appears that some of the primitive types 

 of the lower zone survived with little change into the 

 middle zone (Chadron B), and that other primitive 

 types evolved gradually into the more specialized 

 species of the middle and upper zones. 



