MECHANICS OF LOCOMOTION 



733 



INDICES AND RATIOS OF LIMB SEGMENTS IN CUR- 

 SORIAL AND GRAVIPORTAL UNGULATES (ALLOMETRIC 

 ADAPTATIONS) 



RESULTS OF COMPARATIVE STUDIES 



The comparative studies pursued by Osborn and 

 Gregory, based upon limb segment measurements 

 taken by Gregory for this monograph in about sixty 

 quadrupeds, bring forth some new and significant laws 

 of limb proportion in addition to those already adum- 

 brated by various students of animal locomotion and 



types we have, in the fore limb, humerus long, radius 

 shorter, manus very short; whereas in cursorial, swift- 

 moving types we have humerus short, radius long, 

 manus very long. Similarly in the hind limb we have, 

 in graviportal types, femur long, tibia short, pes very 

 short, whereas in cursorial types we have femur short, 

 tibia long, pes very long. 



During the course of evolution small subcursorial 

 types have frequently diverged on the one hand into 

 gigantic graviportal types and on the other hand into 



Figure 669. — Angulation of the limb bones at the shoulder and hip joints in the standing 

 pose of cursorial (A) and graviportal (B, C) ungulates 



A', A^ Equus scotti, a Pleistocene horse; B', B^, Brontops robiistus, an Oligocene titanothere; C, C, Mastodon ameri- 

 canus, a Pleistocene proboscidean. In the cursorial tj'pe the humerus is nearly at right angles with the scapula 

 and the femur is nearly at right angles with the ilium. In the graviportal types all the elements become more 

 nearly vertical in position. 



of the evolution of quadrupeds. Valuable as have 

 been the previous studies of Marey, Osborn, Gaudry, 

 and others on miscellaneous tj^pes, it is only through 

 the measurement and comparison of all the ungulate 

 types in continuous phyletic series, such as are now 

 afforded through American paleontology, that these 

 laws can be positively formulated as fundamental and 

 universal principles of progressive evolution in the 

 limbs. 



Antithetic proportions. — In general these measure- 

 ments show that in extreme graviportal, slow-moving 



Weight allometry 

 Lengthen. 

 Shorten. 

 Shorten. 



true cursorial tj^pes. The divergent evolutionary 

 changes may be summarized thus: 



Speed allometry 



Humerus and femur Shorten. 



Radius and tibia Lengthen. 



Mtc III and Mts III Lengthen. 



Between these extremes we find many intermediate 

 forms. Taken together, the living and extinct quad- 

 rupeds may be arranged with reference to primitive 

 or ancestral proportions and to either graviportal or 

 cursorial adaptations into four general groups of types, 

 adapted for various speeds and paces as follows. 



