THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



769 



STRUCTURE OF THE FOOT IN THE TITANOTHERES AND 

 IN OTHER PERISSODACTYLS 



Fundamental characters. — The structure of the fore 

 foot of the titanotheres is highly distinctive. It differs 

 from that of the fore foot of all other Perissodactyla 



existingfamUies of perissodactyls, the tapirs, rhinocer- 

 oses, and horses, the main weight passes directly through 

 the center of D. Ill; they are mesaxonic. As shown in 

 Figure 699 the mesaxonic extreme is the foot of the 

 horse (Eqtms) , while the paraxonic extreme is exempli- 



FiGUKE 697. — Family tree of the titanotheres 



Showing the relation between the branches (phyla) , subfamilies, and genera known to science in 1919. The shaded areas show connections 



that are well established; the dotted lines show gaps that remain to be filled by future discovery, especially in the Uinta formation, Utah. 



except Amynodon in its paraxonic character, owing to 

 secondary tetradactylism and to the enlargement of the 

 functional fifth digit (D. V). Thus the main axis of 

 weight, indicated by arrows, passes partly between D. 

 Ill and D. IV, as in the Artiodactyla, whereas in all the 



fied in the deer (Cermis). In the tridactyl pes of the 

 titanotheres the mesaxonic condition is exactly like 

 that of the tapirs and rhinoceroses, presenting a uniform 

 contrast to the paraxonic condition of the pes in all 

 the Artiodactyla {Oreodon, Sus, Dorcatherium, Cervus). 



