EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



593 



The association of limb bones with teeth of the 

 same individual is a very rare occurrence. Thus the 

 reference (see table on p. 585) of several of the hind 

 feet is inferred only from the size and degree of slender- 

 ness of the limbs. 



Eotitanops gregoryi Osborn 



There is reason to believe that Eotitanops gregoryi 

 was both small and slenderly proportioned; that E. 



Figure 491. — Astragalus of 

 Lambdotherium popoagicum 



Am. Mus. 2991, Wind River Basin, V^yo., 

 Wind River formation, level B. Left 

 astragalus, front and rear views. Two- 

 thirds natural size. 



horealis and E. princeps were still of light construc- 

 tion, corresponding with the delicately constructed 

 skull of E. horealis ; and that the littl e-known E. major 

 was somewhat more massive in its proportions. The 

 whole range of adaptive radiation of the limbs is 



parts may be conveniently described under a single 

 heading. 



Atlas of Eotitanops horealis 



The atlas of E. horealis is preserved in the neotype 

 skeleton (Am. Mus. 14887, fig. 495). In that speci- 

 men, which is the earliest known, we observe a broad 

 resemblance to the atlas of the middle Eocene Mesa- 

 tirhinus and of the existing Tapirus, both probably 

 representing the primitive perissodactyl type. The 

 characters are (1) centrum relatively elongate, (2) 

 facets for axis in obliquely convergent planes, (3) 

 vertebrarterial canal entering slightly above posterior 

 rim of transverse process and issuing on lower median 

 face of same. Comparison with the atlas of Mesa- 

 tirhinus (fig. 560) indicates that we have here the 

 ancestral titanothere type, which is highly modified 

 in some of the progressive forms. 



Cervical and dorsal vertebrae of Eotitanops princeps (type) 



The two posterior cervicals (C. 6, C. 7), three scat- 

 tered dorsals, and one caudal of the type (Am. Mus. 

 296, fig. 496) show the following characters: (1) 

 Neck relatively short as in Palaeosyops; C. 1 to C. 7 

 estimated at 180 millimeters as compared with 320, 



Figure 492. — Restorations of the lower Eocene titanotheres of the Wind River formation 

 A, Lambdotherium popoagicum: B, Eotitanops princeps; C, Eotitanops gregoryi. One-thirtieth natural size. 



subcursorial or less truly cursorial than either Lamh- 

 dotherium, Heptodon, or Eohippus. Estimates of the 

 heights of these animals, which, it will be recalled, 

 were not successive but partly contemporaneous, are 

 as follows: 



Height of Lambdotherium and Eotitanops 



the estimated basal length of the skull; (2) cervicals 

 with broad depressed centra, facets oblique (length 



ABC D 



Figure 493. — Metatarsals of Eotitanops 

 Median metatarsals of E. gregoryi (A), E. iorealis (B), E. princeps (C), E. major (D). Natural size. 



Eotitanops borealis and E. princeps 



The species and the mutations of E. horealis and 

 E. princeps are so closely related that their skeletal 



of centra, C. 6, 25 mm.; C. 7, 26); (3) three scattered 

 dorsals laterally compressed, centra elongate, meas- 

 uring, ?D. 3, 26 millimeters; ?D. 4, 25; ?D. 10, 29. 



