278 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Both animals possess a number of features in com- 

 mon: (1) Tliey show a similar cone and crescent (or 



Figure 231. — Lower jaws of Lamhdotherium., Eotitanops, and 



Tapirus 

 One-fourth natural size. A, Lambdoiherium popoagicum. Am. Mus. 14906; Alkali 

 Creek, Buck Spring, Wind River Basin. B, Eotitanops grcgoryi, Am. Mus. 

 14889 (type); Alkali Creek, Buck .Spring, Wind River Basin, C, E. brownianus, 

 Am. Mus. 4885 (type); Wind River Basin. D, E. borealis, Am. Mus. 14891; 

 west bank of Wind River, 3 miles above canyon (top of banded beds). E, E. 

 princeps. Am. Mus. 296 (type); Wind River Basin. F, Tapirus ierrestris, Am. 

 Mus. 1135; immature rpecimen. 



bunoselenodont) pattern of the superior grinding 

 teeth, with either reduced or vestigial intermediate 



cusps or conules; (2) in both the preorbital (or facial) 

 part of the skull is longer than the cranial (see fig. 232), 

 a primitive characteristic of perissodactyls which at 

 once allies these animals to other primitive perisso- 

 dactyls and distinguishes them from the middle 

 Eocene titanotheres, in which the cranium is longer 

 than the face; (3) they show enlargement of the median 

 digit (D. Ill), or mesaxonic structure of the fore and 

 hind feet. 



It is quite possible (see Chap. VIII) that this 

 mesaxonic specialization is itself common among 

 perissodactyls — that is, that all perissodactyls are 

 descended from quadrupeds with narrow feet, as sug- 

 gested by Gregory. We should interpret this swift- 

 footed structure as an adaptation that enabled the 

 small, defenseless perissodactyls, without horns or 

 tusks, to escape their pursuers. 



CBp7ialic70O% 



FiGTJKE 232. — Restored contours of skulls of La?7ibdothe- 

 rium and Eotitanops 



Showing estimated proportions of face (shaded) to cranium. One-fourth 

 natural size. A, Lambiotheriam, face 65, cranium 35. B, Eotitanops, lace 56, 

 cranium 44. 



The special similarities of foot structure between 

 Lamhdotherium, and Eotitanops embrace the follow- 

 ing characters: (1) Enlargement of the median digit 

 (D. Ill), which is provided with a broad proximal 

 phalanx; (2) reduction of digits II and IV, with re- 

 lative narrowing of the proximal phalanges; (3) reduc- 

 tion of digit V (correlated with this metapodial 

 structure we find that the lunar immediately rests on 

 the unciform, with a narrow {Lambdotherium) or 

 oblique (Eotitanops) facet on the magnum) ; (4) small, 

 relatively high and narrow magnum. These points 

 are well illustrated in the accompanying figures of the 

 manus of Lamhdotherium and Eotitanops. 



The interpretation of this somewhat specialized 

 form of foot as one derived from a cursorial type is 

 considered in the introduction to the study of the 

 skeleton of the Eocene titanotheres (p. 586). 



