EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENB TITANOTHERES 



591 



the adjoining digits. This enlargement of D. Ill 

 is indicated not only by its diameter but by the 

 enlargement of its proximal phalanx, which is much 

 broader than the proximal phalanges of D. 11 or D. 

 IV. This indicates a rather pronounced tendency 

 to mesaxonic structure, whereas the middle Eocene 

 titanotheres exhibit either a primary or secondry 

 paraxonic and finally isotetradactyl condition. D. 

 V is somewhat shorter and more slender. 



The structure of the astragalus of this specimen 

 (Am. Mus. 4880), as inferred from the distal end of 

 the tibia which is preserved, indicates that the keels 

 of the astragalus were quite sharp. 



The tarsus is compared with that of the less cur- 

 sorial Eotitanops and the more cursorial Eohippus in 

 Figure 502. An astragalus associated with Am. Mus. 

 2991 exhibits a high, long neck and a high, narrow 

 sustentacular facet, showing that the astragalo- 

 cuboidal facet was very narrow (figs. 491, 503). 



Figure 486. — Reconstructed skeleton and restoration 

 of Lambdotherium popoagicum Cope 



Made by E. S. Christman under the direction of W. K. Gregory. 

 One-tenth natural size. This provisional reconstruction is based on 

 the following American Museum specimens from the Wind River 

 Basin, Wyo.: 14007 (Alkali Creek, Wolton), maxilla, malar, top of 

 cranium; 14903 (Alkali Creek, Buck Spring), squamosal, basicranial 

 region; 14899 (Alkali Creek, Buck Spring), lower jaw; 4880, distal end 

 of scapula, fragments of humerus, radius, ulna, and manus; 14921 

 (Wolton), astragalus and calcaneum. Hemaining parts conjecturally 

 modified from Eohippus and other contemporary perissodactyls. 



SUBFAMILY EOTITANOPINAE 



Eotitanops 



Our knowledge of the skeleton of the species of 

 Wind River Eotitanops is relatively small; we know 

 that there were gradations of size from smaller and 

 101959— 29— VOL 1 41 



fac.ar±.post 



lighter to larger and heavier, subcursorial forms, the 

 general evolution being in the direction of the laiown 

 middle Eocene titanotheres. The pes especially gives 

 us valuable records of this progressive increase in 

 size and weight. 



As compared with the middle Eocene titanotheres 

 of the Bridger formation these lower Eocene forms 

 were small and light limbed 

 and had high, narrow ankle 

 and wrist joints, resembling B 

 most nearly the light-footed -' 

 members of middle Eocene age, 

 such as Mesatirhinus . In the 

 skeleton, as in the skull, there 

 are general primitive features 

 that accord with the other 

 lower Eocene perissodactyls, 

 such as the lophiodonts (Hep- 

 todon) and horses (EoMppus). 

 The details of the carpus and 

 tarsus remind us also of these 

 lower Eocene perissodactyls, 

 although Eotitanops is decidedly 

 larger and less distinctively 

 cursorial than either Heptodon 

 or Eohippus. The abbrevia- 

 tion of the fifth or outer digit 

 (Mtc V) of the manus forbids 

 our describing the animal as 

 functionally tetradactyl; taken with the extreme dis- 

 placement of the lunar on the unciform (a ti-idactyl 

 specialization) this reduction tends to support the 

 idea of Gregory that these animals, after having 

 entered an evolution toward tridactylism, reversed the 

 process and reentered tetradactylism in middle Eocene 

 time. 



Measurements, in millimeters, of teeth and limb bones referred to 

 Eotitanops, compared with Lambdotherium and Tapirus 



Figure 487. — Atlas and 

 scapula of Lambdothe- 

 rium, popoagicum 



\m. Mus. 4880, Wind Eiver 

 Basin, Wyo., Wind Eiver for- 

 mation. A, Distal portion of 

 left scapula, outer side view; B, 

 rear view of incomplete atlas. 

 Two-thirds natural size. 



Lower grinding teeth (p^-ms). 



Median metatarsal, length 



Median metacarpal, length 



Humerus, length 



Femur, length 



64 



78. 4 



98 

 85 



Lower grinding teeth (pj-ma)- 



Median metatarsal, length 



Median metacarpal, length 



Humerus, length 



Femur, length 



E. princeps E. major 



105 



87 

 205 

 ■250 



118 

 114 

 113 



° Estimated. 



