586 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



DIVERGENCE AND CONVERGENCE IN THE SKELETON OF 

 POLYPHYIETIC SERIES 



DIVERSE ADAPTIVE TYPES OF LIMB STRUCTURE 



Our study of the locomotor skeleton of the titano- 

 theres strongly supports the polyphyletic theory that 

 we derived from the study of the skull and the teeth — 

 namely, that there were two larger branches or groups 

 of Eocene titanotheres, the palaeosyopine and the 

 Manteoceras-DoUchorhinus, each of which was divided 

 into two or more smaller branches or subfamilies. 



The interpretation of the structure of the feet and 

 limbs would be simple if the feet and limbs of all these 

 six or seven genera were invariably divergent, but the 

 law of convergence again comes into play, and long 

 after one subfamily has diverged from another there 

 remain or arise within each subfamily forms which, 

 through convergent adaptation and inheritance, par- 

 allel in foot and limb adaptation forms in the other 

 subfamilies. 



To make this double convergence clear we recall the 

 existence in Eocene time of five distinct groups or sub- 

 families of titanotheres with respect not only to geo- 

 logic level but to divergence as to speed and to weight, 

 namely: 



Palaeosyopine group: 

 Lower Eocene: 



Lambdotheriinae (cursorial types). 

 Eotitanopinae (subcursorial types). 

 Middle Eocene: 



Palaeosyopinae (mediportal and subgraviportal types) . 

 Manteoceras-DoUchorhinus group : 

 Middle and upper Eocene: 



Manteoceratinae (subgraviportal types). 

 Dolichorhininae (mediportal types). 



The Lambdotheriinae (fig. 486) are primitively 

 cursorial, like the early Eocene Equidae and Lophio- 

 dontidae. 



How far the Eotitanopinae may be directly ancestral 

 to the Palaeosyopinae or Manteoceratinae we do not 

 know. 



The known Eotitanopinae (figs. 484, 492) are light- 

 limbed, or subcursorial — that is, the feet are relatively 

 shorter (more mediportal) than those of the tapir and 

 less light in structure than those of the primitive 

 Equidae. 



In the various Palaeosyopinae we find a condition 

 somewhat transitional between the tapir and the 

 heavier types, like the rhinoceros. These animals 

 include more heavy-limbed (Palaeosyops) and light- 

 limbed (Limnohyops) types. 



Similarly within the Manfeoceras-DolicTiorJiinus 

 group there are two series of forms, mediportal {Mesa- 

 tirhinus) and subgraviportal (Manteoceras). Thus we 

 discover mediportal Palaeosyopinae (such as Limno- 

 hyops) and mediportal Dolichorhininae (such as Mesa- 

 tirMnus), which are somewhat alike in their limb 

 adaptations and can be separated only by careful 

 scrutiny of certain less conspicuous features of ances- 



tral separation, which bring out the subfamily char- 

 acters. There are also graviportal Palaeosyopinae 

 (such as Palaeosyops), which resemble graviportal 

 manteoceratines (such as Manteoceras) in some respects 

 but differ in others. 



It has taken a vast amount of study of the scattered 

 and often unassociated limb materials to determine 

 the I'eal subfamily relationships in the limbs of these 

 different forms, which are concealed by the veneer of 

 similar adaptation or ceno telle resemblance; but it 

 may now be demonstrated positively that in each 

 subfamily of the middle Eocene titanotheres there 

 were relatively light-limbed and relatively heavy- 

 limbed forms. The differences between these adaptive 

 extremes of relatively rapid-moving and slow-moving 

 forms may be observed and measured in every single 

 bone of the limbs, and especially in the small bones of 

 the carpus and tarsus. 



To illustrate how a single bone may be highly 

 distinctive, an outline of the central bone of the carpus, 

 the magnum, as it appears in different forms, is given 

 herewith (fig. 485). On the left is the magnum of the 

 subcursorial Eotitanops, relatively high and narrow; 

 on the right that of the subgraviportal Palaeosyops 

 rohustus, relatively broad and low. In general, high 

 and narrow proportions of the magnum characterize 

 cursorial forms and low and broad proportions char- 

 acterize graviportal forms. Thus the transformation 

 of this single bone reflects the kind of allometric change 

 which prevailed in all parts of the skeleton. 



Or, to take the foot as a whole (fig. 484), the manus 

 of MesatirJiinus represents the mediportal extreme 

 among the titanotheres analogous to that of Tapirus, 

 while the manus of Palaeosyops represents the sub- 

 graviportal extreme analogous to that oi Hippopotamus. 

 Both types of feet occur at the same geologic levels and 

 in the same geographic regions; they belong to con- 

 temporaneous titanotheres, one perhaps seeking forest 

 ground like the habitat of Tapirus, the other the bor- 

 ders and possibly the waters of rivers and lakes like 

 the habitat of Hippopotamus. 



There are thus, as shown in Figures 502, 503, 510, 

 515, 620, wide adaptive radiations among the Eocene 

 titanotheres in modes of locomotion on different kinds 

 of soil. 



All these adaptively diverse types of limb structure 

 appear to have been derived from a small and primitive 

 type, which was a subcursorial, light-limbed, slender- 

 footed, and relatively speedy animal, well adapted for 

 escape rather than combat. 



The adaptive transformation of the limbs of titano- 

 theres has not yet been traced so continuously as that 

 of the skull or of the teeth; the scattering of parts of 

 limbs has rendered the association and identification 

 of many separate bones exceptionally difficult. We 

 are still in doubt as to the limb structure in certain 

 phyla, especially in Telmatherium. Much remains to 

 be discovered through further exploration in the field. 



