36 



TITANOTHBEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



portal stage of locomotion. In doing so they acquired 

 an entirely new set of proportions, which are shown 

 in detail in Chapter IX. 



The teeth form the readiest means of distinguishing 

 different branches and subbranches of the Perisso- 

 dactyla from one another. The ancestral pattern, 

 whether bunoselenodont or lophodont, is so marked 

 and persistent that it is only partly modified through 



their evolution, and these give off one mediportal, 

 forest-living branch, HypoMppus. The horses are 

 paralleled by cursorial or subcursorial titanotheres, 

 such as LamhdotJierium, by cursorial paleotheres 

 {Palaeotherium and Paloplotherium) , mistakenly sup- 

 posed by Huxley to be the ancestors of the horses, by 

 two cursorial branches of the lophiodonts (the helale- 

 tids and the chasmotheres), and by two cursorial 



Figure 28. — Evolution of the skeleton of the titanotheres 



A, First stage (subcursorial), lower Eocene, Lambdotlierium popoagicum; B, second stage (subcursorial), lower Eocene, Eotitanops 

 horealis; O, intermediate stage (mediportal), middle Eocene, Palaeosyops leidyi; D, final stage (graviportal), lower Oligocene, Brontops 

 Tobustus. From one twenty-eighth to one-thirtieth natural size. 



branches of the rhinoceroses (the triplopodines and the 

 hyracodonts) . It is shown elsewhere (see Chap. IX) 

 how the cursorial habit, independently assumed in 

 each of these subfamilies, modified not only the limbs 

 but the skull and the entire skeleton into analogous 

 forms that simulate real affinity. In Figure 32 all 

 these cursorial branches, independently evolving in 



analogous adaptation. The manner in which the 

 skeleton and limbs similarly became adapted inde- 

 pendently to various modes of locomotion and thus 

 assumed analogous forms and proportions is no less 

 remarkable than the independent adaptation of the 

 teeth to similar kinds of food. 



Of the nine typical perissodactyl families the horses 

 alone are cursorial through the entire period of 



