THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



775 



Hyrachyus; yet from this type has been derived the 

 heavy graviportal manus of Brontops, ia which the 



FiGUKE 702. — Back view of the carpus 

 of a middle Eocene titanottiere 



Mesatirhinus petersoni, Princeton Mus. 10013. 

 Showing the lunar as the keystone of the proximal 

 row of carpals and of the magnmn and tinciform, 

 on which it rests. In front view it is largely 

 displaced on the unciform. Each bone of the 

 second row of carpals (for example, the magnum) 

 forms the keystone between one of the proximal 

 row and one or more of the metacarpals. 



lunar rests broadly on the magnum. Similarly, the 

 carpus of Lamhdotherium is of the cursorial type, all 



little is known of the primitive tapir (Sysfemodon) 

 shows a similar subcursorial structure, while the 

 primitive lophiodont {Heptodon) shows a markedly 

 cursorial carpus, in which the lunar has no contact 

 with the magnum in front. The adaptive mechanical 

 principles involved are explained by Osborn in his 

 "Evolution of the ungulate foot" (1890.51). In 

 Figures 722, 723 it is shown that as the animal passes 

 from the cursorial into the mediportal condition 

 {Tapirus, Limnohyops , Palaeosyops) the lunar broadens 

 and gains a facet on the broadening magnum. In 

 extreme graviportal forms (Brontotherium, Rhinoceros) 

 the carpus is very broad, the magnum is flattened and 

 supports half the weight of the lunar. 



Adaptation in the magnum carpi. — Each carpal, 

 like each tarsal, mirrors the primitive cursorial, medi- 

 portal, or graviportal locomotor stage of the limb. 

 As shown below, the magnum alone mirrors the entire 

 locomotor evolution of the titanotheres from Eotiianops 

 to Brontotherium. The cursorial adaption at the 

 back of the magnum is the deep hook forming the 

 long arm of a lever in flexing the carpus, well developed 

 in cursorial forms, also in the mediportal tapir, in 

 Mesatirhinus, and in the long-footed rhinoceroses. 

 This hook is progressively reduced as an adaption to 

 weight in the graviportal brontotheres. In the back 



Aoo/l- 



FiGTJEE 703. — Left magna of an Eocene titanotliere and two chalicotheres 

 A, Palaeosyops hidyi, a titanothere; B, Moropus sp., and C, Macrotherium sp., chalicotheres. Top row, front view; middle row, outer 

 side; lower row, inner side. In the chalicotheres the front face of the magnum becomes extremely high and compressed. The 

 magnum becomes very deep anteroposteriorly, and the posterior process becomes confluent with the dorsal facet. In spite of these 

 differences the ordinal kinship of titanotheres and chalicotheres is only partly disguised in the magnum. 



the elements high and narrow, the lunar resting by 

 very small contact on the small magnum. What 



view of the carpus of the middle Eocene titanothere 

 Mesatirhinus petersoni the magnum with^its long 



