INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



33 



EVOIUTION OF THE LIMBS AND FEET OF THE 

 TITANOTHERES 



The feet of the titanotheres, like their skulls, pass 

 through a lower Eocene tapir-like phase, which is 

 followed by a middle and upper Eocene rhinoceros- 

 like phase and finally they attain a structure similar 

 to that of the rhinoceroses, as shown in Figure 26, 

 except that all the titanotheres, like the existing 

 tapirs, retained four distinct and functional digits 

 in the fore foot. 



The fore foot of the tapir resembles the fore foot of 

 the lower Eocene titanothere except that in the latter 

 D. II, III, IV, V were all of nearly equal size, as 

 shown in the diagram (B). This is known as the 

 mediportal stage, for it is adapted to carrying a 

 moderate amount of weight. The 

 foot of the rhinoceros (C, C, C) 

 is like that of the upper Eocene 

 and lower Oligocene titanotheres 

 except that in these there were 

 four weight-bearing digits instead 

 of three. This is known as the 

 graviportal type of foot, in which 

 a large cushion pad is developed 

 at the back to relieve the shock 

 of impact, and the end phalanges 

 of the digits are incased in the 

 horny sheath in front. In the 

 tapir and rhinoceros the main 

 weight passes directly through the 

 center of the median phalanx 

 (D. Ill), but in the tetradactyl 

 titanotheres the main weight 

 passes between D. Ill and D. IV. 

 The concentration of the weight 

 on the central digit of the horse 

 and its resultant monodactylism, 

 correlated with the expansion of the horny hoof and 

 the contraction of the pad, is part of the evolution 

 of a cursorial type of foot, which presents the widest 

 contrast to the graviportal type. 



In addition to comparing the head structure it was 

 found necessary to compare the foot and limb struc- 

 ture of the titanotheres with that of all the other 

 perissodactyls — not only the bony parts but the 

 musculature. The work done on the musculature led 

 to an exhaustive study of all that is known of the 

 muscular anatomy of the members of the three exist- 

 ing families of perissodactyls. This study, which was 

 directed by William K. Gregory, formed the basis of 

 the restoration of the muscular anatomy of the giant 

 Brontops rolustus presented in Chapter VIII (pp. 722, 

 723). This restoration of an extinct animal is the first 

 that has been based upon exact comparative study. It 



presents the titanothere as a superb example of the 

 graviportal type of musculature and skeleton, sur- 

 passed only by the existing elephants. 



The study of the structure of the foot led to a special 

 investigation of the proportions of the limb bones in 

 the ungulates. This investigation, directed by Osborn 

 and cooperated in by Gregory, resulted in the striking 

 discovery that the proportions of the upper and lower 

 segments of the limbs and of the feet are invariably 

 adjusted, first, to the weight that the limb must carry, 

 and second, to speed of locomotion. These propor- 

 tions are evolved, quite irrespective of ancestry, in 

 adaptation to different modes of progression. Thus 

 similar proportions of limb segments are observed not 

 only in all mammals but in reptiles as well. A study, 



'^^-^Ga^;^^' I 



Figure 25. — Lower jaws of the first and the last of the titanotheres 



One-sixth natural size. A, EotUanops gregoryi, a small-jawed species from the Wind River formation (lower 

 Eocene); B, BrontotTierium medium, from Chadron C level of Chadron formation (lower Oligocene). 



therefore, which was designed to disclose the habits of 

 the titanotheres led to a thorough investigation of the 

 principles of limb evolution in all the hoofed mam- 

 mals in adaptation to various modes of locomotion 

 and to various loads. This special study forms the 

 subject of Chapter IX, in which acknowledgment is 

 made to previous investigators. 



Not only the proportions of the upper and lower 

 segments of the limbs but all the bones of the shoulder 

 and pelvic girdles are gradually transformed from the 

 subcursorial stages of Lambdoiherium and Eotitanops 

 through the mediportal tapir-like stages to the gravi- 

 portal stages of the ponderous Oligocene titanotheres. 

 This transformation is continuous, not sudden; it is 

 brought about gradually by the simultaneous and 

 correlated modification of all the bones and muscles 

 involved in locomotion. Function (habit) is evi- 



