774 



TIT.\NOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



cuboid. The cursorial, mediportal, and graviportal 

 proportions and articulations of the astragalus consti- 

 tute an invariable feature of the adaptive radiation 

 of the perissodactyls into these respective types of 

 locomotion. They are correlated with the cursorial 

 types of limb, pelvis, and shoulder girdle structure, 

 which have been fully described in Chapter IX in the 

 summary of cm'sorial and graviportal proportions of 

 limb segments in imgulates (p. 739). 



In his studies on the evolution of the ungulate 

 foot Osborn (1890.51) pointed out that the shape of 

 the facets between the astragalus, calcaneum, and 

 cuboid are distinctive in each of the perissodactyl 

 families. The original distinctions, however, tend 

 to be obscured by the convergent influence of the 

 respective adoption of the cursorial, the mediportal, 

 or the graviportal gait. The primitive arrangement 

 of these facets in the Perissodactyla is possibly 

 derived from the arrangement seen in the Condylarthra 

 (Phenacodus), which, in turn, is an inheritance from 

 clawed ancestors in which (a) the digits spread apart, 

 (6) the astragalus and calcaneum diverge distally, 

 and (c) there is no contact of the astragalus upon the 

 cuboid and no astragalocuboid facet (W. K. Gregory, 

 1905, MS.). In the primitive Perissodactyla the 

 progressive development of cursorial habits caused 

 the long axes of the calcaneum and astragalus to 

 become more parallel, so that these bones articulated 

 distally as well as proximally. 



The subsequent evolution of these astragalocal- 

 caneal facets depends upon the following conditions: 

 (1) Reduction of lateral digits in the monodactyl 

 Eqims makes for immobility between the astragalus 

 and calcaneum; (2) the tibio-astragalar joint is more 

 mobile than in the succeeding mediportal and gravi- 

 portal forms; (3) the extent of the astragalocalcaneal 

 facets depends partly upon the shape of the tibia and 

 fibula above and of the supporting elements, the 

 cuboid and navicular, below; (4) the size and direc- 

 tion of pull of the ligaments of the limb and foot; (5) 

 the shape and arrangement of the astragalocalcaneal 

 facets and adjacent parts condition the degree of 

 rotary, vertical, and horizontal motion between the 

 astragalus and calcaneum; (6) two general types are 

 observed — (a) the astragalus with oblique distal ex- 

 tremity, correlated with movabdity, (6) the astragalus 

 with truncate distal end, correlated with less mova- 

 bility. 



In general the perissodactyl astragalus is much 

 more movable upon the calcaneum than the artio- 

 dactyl astragalus. The horse alone has developed a 

 comparatively rigid astragalocalcaneal union, by a 

 system of interlocking joints and by the arrangement 

 of the facets in three planes, more or less at right 

 angles. This rigid astragalus is adapted to the elon- 

 gate cannon bone (as in Artiodactyla), also to the 

 reduction of the lateral digits. 



MECHANICS OF THE PERISSODACTYL MANUS 



Steps in evolution of the manus. — In the stem 

 Perissodactyla we observe that (1) elevation from 

 the plantigrade to the digitigrade position of the 

 manus has already taken place in the ancestral 

 stages, and there remains the transformation into the 

 extreme unguligrade position shown in the Equidae; 



(2) atrophy or transposition of the muscles of rotation, 

 of pronation, and of supination has also taken place, 

 the muscles involved in fore-and-aft movement again 

 reaching their highest development in the Equidae; 



(3) extreme reduction of the pollex has already taken 

 place, for not a vestige of Mtc I has yet been dis- 

 covered, although there is some evidence that it may 

 have existed; (4) reduction of the trapezium follows, 

 although this element is still retained in all primitive 

 perissodactyls; (5) coalescence of the centrale with 

 the scaphoid has already taken place, followed by a 

 reduction of the centrale process of the scaphoid; (6) 

 no "displacement" or slipping of the first and second 

 row of carpals takes place, as Cope believed; (7) 

 there is either growth and enlargement or reduction 

 of several elements of the carpus, correlated with 

 the expansion of the radius and the reduction of the 

 ulna above and with the distribution of the weight 

 through a diminishing number of the metacarpals 

 below; (8) at each stage of reduction the functional 

 tridactyl, isotridactyl, and monodactyl condition has 

 its distinctive type of correlated carpal structure; (9) 

 displacement of Mtc II against the magnum, of Mtc 

 III against the unciform, the "alternating type" of 

 displacement of Osborn, is a universal feature of the 

 perissodactyl carpus. 



The mechanics of reduction and displacement, as 

 worked out by Ryder (1877.1), Cope (1887.1), and 

 Osborn (1890.51), require complete restudy in view 

 of the fact that the primitive perissodactyl carpus is 

 of the displaced type. 



The law enunciated by Osborn in 1890 (op. cit., p. 

 568) is, however, probably in the main correct, 

 namely : 



The direction and degree of intercarpal displace- 

 ments are adapted to the gradual alteration of the 

 major axes in the bones of the forearm and of the 

 metapodimn, respectively, as brought about by en- 

 largement and reduction, and tend to maintain these 

 proximal and distal axes in the same vertical line. 



Carpus of cursorial, mediportal, graviportal types. — 

 Like the bones of the tarsus, every bone of the peris- 

 sodactyl carpus has its peculiar form, adapted to 

 graviportal, mediportal, subcursorial, or cursorial 

 locomotion. It is notable that the ancestors of each 

 of the lower Eocene families have the cursorial type 

 of carpus, in which the lunar rests mainly upon the 

 unciform, with small, lateral contact on the magnum. 

 Thus in Eotitanops we observe an arrangement of the 

 carpals somewhat similar to that in the subcursorial 



