290 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Standard measurements of teeth of the species of Eotiianops, in 



millimeters 

 [The numbers are those of specimens in the American Museum of Natural History] 



Figure 244. — Skulls of the oldest known titanotheres 

 Reconstructions by L. M. Sterling under the direction of W. K. Gregory. One- 

 fourth natural size. A, Lamhdotherium popoagicum, Am. Mus. 14907, Alkali 

 Creek, Wolton; 14899 and 14903, Alkali Creek, Buck Spring. B, Eotiianops bo- 

 realis, Am. Mus. 14887, Dry Muddy Creek 12 miles above mouth. All specimens 

 from the Wind River Basin, Wind River formation. 



Range of evolution. — As Eotitanops gregoryi, the 

 smallest and simplest form, occurs on a high level, 



having been found 100 feet above the "red stra- 

 tum" on Alkali Creek (see figs. 47, 48), and as 

 specimens referred to E. horealis and E. princeps 

 range from the "red stratum" on Alkali Creek, on 

 the 250-foot level, to the 400-foot level, these species 

 and mutations can not be arranged in monophyletic 

 succession, but they afford evidence that even at this 

 time the titanotheres were polyphyletic. 



Range in size. — The smallest of the Wind River 

 titanotheres, E. gregoryi, measures about 18J^ inches, 

 or 45.6 centimeters, at the shoulders. A larger form, 

 E. princeps, measures about 26 inches, or 66 centi- 

 meters, at the shoulders. The intermediate form, 

 E. borealis, is more slenderly proportioned than the 

 American tapir (T. terrestris); it is between 75 and 

 78 per cent of the height of the tapir, and thus 

 about 75 per cent of the height of Mesatirhinus of 

 the upper levels of the Bridger Eocene. E. major, 

 judged only by the size of the pes, more nearly 

 approaches T. terrestris in size, the median metatarsal 

 of E. major measuring 103 millimeters and that of T. 

 terrestris 108. 



It should be noted that Eotitanops includes the only 

 known large lower Eocene perissodactyl. Even 

 Eotitanops major, the largest Wind River species, 

 appears to be considerably smaller than Palaeosyops 

 jontinalis, the smallest Bridger species. 



Measurements of upper teeth of Eotitanops horealis and Palaeo- 

 syops fontinalis, in millimeters 



M', anteroposterior 



M', transverse 



M', ectoloph, maxiUa 



M', transverse maxilla 



M^, transverse maxilla (pr.-pas.) 



Length of left zygoma (anterior bor- 

 der malar to posterior border post- 

 glenoid process) 



E. borealis 

 from Wind 

 River B, Am. 

 Mus. 148S7 



17.5 

 "18.5 

 19.5 

 23 

 25.5 



» 129 



P. fontinalis 



from Bridger 



A (type). Am. 



Mus. 5107 



137 



The measurements given show that in its dentition 

 P. Jontinalis of Bridger A was much larger than 

 E. horealis of the Wind River formation. The rela- 

 tively small size of the zygoma in the type of P. fonti- 

 nalis is consistent with the fact that the animal was 

 very young, its milk dentition being still functional. 



A comparison of E. horealis (summit of known lower 

 Eocene) with P. fontinalis (lower middle Eocene or 

 lower Bridger) indicates a long period of titanothere 

 evolution between these two species. P. fontinalis, 

 although the oldest known Bridger titanothere, differs 

 in two points — the superior dental series is 25 per 

 cent larger than that of E. horealis; the cranium is 

 elongate and the face abbreviate. 



