THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



801 



six facets, as follows: Magnum-trapezoid, magnum- 

 scaphoid, magnum-lunar, magnum-unciform, mag- 

 num-Mtc III, magnum-Mtc II. The most significant 

 change is that in the passage from the vertical to the 



EVOIUTION OF THE ASTRAGALUS 



The astragalus of the tarsus displays similarly a 

 complete transformation in its facets in passing from 

 the cursorial through the mediportal to the^ gravi- 



FiGUHE 722.- 



g -w ^ c 



-Three stages in the evolution of the manus in titanotheres 



A, Subcursorial (Eotiianops); B, mediportal (Limnohyops): C, graviportal (Brontotherium). Not drawn to scale. In all three figures the 

 height from the top of the lunar to the bottom of the third metacarpal is arbitrarily made equal. In this way the progressive relative 

 widening of the metacarpals, the shortening of the phalanges, and the great widening of the carpals are revealed. 



Figure 723. — Six stages in the evolution of the manus in titanotheres 



Lambdotherium, lower Eocene; B, Eotitanops, lower Eocene: C, Limnohyops mortoconus, middle Eocene; D, Manteoceras manteoceras, middle Eocene; E, "Biplacodon," 

 upper Eocene; E, Brontotherium, lower Oligocene. The earlier members of this series retain the more primitive perissodactyl cursorial pattern of the manus. Th& 

 carpus is of the interlocking, displaced type, and the third digit is predominant. In the mediportal and graviportal members the whole manu? becomes very 

 wide, digits III and IV become subequal, and the lunar and magnum broaden. 





Figure 724. — Evolution of the magnum in titanotheres 



Cursorial, mediportal, and graviportal types. This series shows the progressive widening of the magnum (the increase of the 

 horizontal compared with the vertical diameter) in correlation with the widening of the whole foot, a graviportal adaptation. 

 The facets for scaphoid, lunar, and Mtc III are greatly widened. A, Eotitanops borealis, Am. Mus. 296, lower Eocene; 

 B, Mesatirliinus petersoni, Princeton Mus. 10013, middle Eocene; C, Manieoceras manteoceras, Am. Mus. 12204, middle 

 Eocene; T>, Palaeosyops copei (?referred), Am. Mus. 12205, middle Eocene; E, Palacosyops roiusius, Am. Mus. 1581, middle 

 Eocene; F, Dolichorhinus longiceps?, Carnegie Mus. 2865, upper Eocene; G, BronMlierium gigas, Am. Mus. M43, lower 

 Oligocene. 



lateral expansion, the magnum passes beneath the 

 lunar (as described above in this chapter) and the 

 lunar also spreads over the upper surface of the 

 magnum. Thus, as was first pointed out by Osborn 

 (1890.51, sec. 4), the growth of the magnum and 

 lunar is reciprocal. 



portal type, in the course of which it parallels other 

 perissodactyls in their similar transformation, while 

 it also displays certain peculiar titanothere family 

 features. In the diagram of Figure 701 we observe 

 a series of stages corresponding with those observed 

 in the manus and in the magnum, namely, the trans- 



