EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



279 



EXPLORATIONS AND DISCOVERIES 



The explorations in the Wind River Basin for re- 

 mains of titanotheres and the types collected were as 

 follows : 



1880. J. L. Wortman, for E. D. Cope; EoHtanops borealis 



(Cope), E. brownianus (Cope), and Lambdotherium 



popoagicum (Cope). 

 1891. J. L. Wortman, for American Museum of Natural 



History; EoHtanops princeps Osborn. 

 1894. F. B. Loomis, for Amherst College Museum; Lamb- 



dolheriavi primaevum Loomis. 

 1909, 1910, 1911. Walter Granger, for American Museum of 



Natural History; L. priscum Osborn, L. progressum 



Osborn, and E. gregoryi Osborn. 



The American Museum expeditions of 1909, 1910, 

 and 1911, under Granger and Sinclair, made a series of 

 important discoveries — first, in locating all these 

 titanotheres in the Lamhdotherium zone; second, in 

 proving that EoHtanops and Lambdotherium were 

 contemporaneous; third, in collecting the remains of 

 more than eight specimens of EoHtanops (Am. Mus. 

 14887-14894); fourth, in collecting a complete skull 

 and jaws of Eotitanops iorealis, affording proof that 

 EoHtanops is more primitive than the middle Eocene 

 or Bridger forms and belongs to a distinct generic 

 stage. 



The Wind River Lamhdotherium zone is 400 feet 

 thick and may be divided into four levels, each in- 

 cluding 100 feet. It is a remarkable fact that nearly 

 all the remains of the 124 specimens of titanotheres 

 found by the American Museum parties were collected 

 between the 250 and 400 foot levels, as shown in the 

 section taken from Granger's field records (fig. 48). 



Thus Eotitanops and Lamhdotherium occur contem- 

 poraneously. It appears that the mutations or spe- 

 cies of Eotitanops do not exhibit a continuously graded 

 evolution or succession in ascending levels, for it hap- 

 pens that the smallest and most primitive form known, 

 E. gregoryi, occurs on a high geologic level, showing 

 that Eotitanops was already polyphyletic in early 

 Eocene time. This is an example of the extreme 

 importance of an exact record of levels. 



SYSTEMATIC DESCRIPTIONS OF THE LOWER EOCENE 

 TITANOTHERES 



Superfamily Titanotheroidea Osborn 



Perissodactyls with bunoselenodont superior molars 

 and selenodont inferior molars. Distinguished from 

 the Chalicotherioidea by normal limbs and hoofs. 

 Distinguished from the Hippoidea by a persistently 

 tetradactyl manus and tridactyl pes. 



Family Brontotheriidae Marsh 



Related to the type of Brontotherium. Evolving 

 between early Eocene and early Oligocene time. 

 Primitively cursorial in gait but early evolving into 

 mediportal and graviportal forms. Terminal pha- 

 langes and hoofs progressively reduced. Forward 

 101959— 29— VOL 1 21 



portion of skull originally elongate but early becoming 

 abbreviate; cranial portion progressively elongate. 

 Earlier genera hornless; paired nasofrontal horns 

 developing in middle Eocene time and becoming the 

 dominant character of the skull. 



Subfamily Lambdotlieriinae Osborn 



Lower Eocene titanotheres, small, light limbed, of 

 slender, cursorial proportions. Skull mesaticephalic; 

 facial region elongate; cranial region abbreviate. 

 Superior molars brachyodont, incipiently bunoseleno- 

 dont, with lophoid paraconules and metaconules; 

 lower molars with pronounced metastylids; premolars \ 

 absent; lower premolars p2_4, progressively molariform. 



Lambdotherium Cope 



Plate LIV; text figures 27, 33, 103, 114, 143, 146, 154, 221, 

 228, 230-242, 244, 483, 484, 486-492, 503, 504, 510, 512, 521, 

 522, 661, 685, 688, 692, 694, 700, 701, 723 



[For original description and type reference see p. 168. For skeletal characters see 

 p. 590] 



Generic characters. — Skull of decided proopic doli- 

 chocephaly. Anterior premolars wanting. Superior 

 molars broadened transversely, with prominent para- 

 styles and mesostyles, oblique ectolophs, large, free 

 protoconules, and low metalophs; m' with prominent 

 hypocone; inferior molars with metastylids; first up 

 per and lower premolars wanting; posterior lower pre- 

 molars progressive; ra^ with large crescentic hypoco- 

 nulid. Manus numerically tetradactyl, functionally 

 aniso tridactyl; lunar resting chiefly on unciform an- 

 teriorly; magnum small, high, and narrow. 



We know neither the ancestors nor the descendants 

 of this animal. It is already more highly specialized 

 in many respects than certain of the middle Eocene 

 titanotheres. Lamhdotherium is readily distinguished 

 by its fine, delicate construction for swift movement, 

 in which it has analogies to the lower Eocene horses. 

 It has a long, slender snout and delicately prehensile 

 jaw. We note especially that (1) the face is much 

 elongated, the faciocephalic index being 65 as com- 

 pared with 56 in Eotitanops and 60 in Eohippus; (2) 

 this elongation is correlated with a very slender snout-, 

 but the first upper and lower premolars are wanting; 

 (3) the third and fourth lower premolars are more 

 complicated than in Eotitanops, p4 in some forms 

 closely resembling a molar tooth; (4) its sharply 

 piercing canine teeth and chisel-shaped incisors are 

 other features of specialization. 



History of discovery of Lamhdotherium. — Wortman' s 

 discovery in the Wind River valley (1880) and Cope's 

 original description have already been cited. Cope 

 at once recognized the ancestral relationship of this 

 form to the titanotheres. In 1889 he made Lamhdo- 

 therium the type of a distinct family — "Lambdo- 

 theriidae"- — to embrace all the Eocene titanotheres, a 

 family name that was adopted by Flower and Lydek- 

 ker. In his "Tertiary Vertebrata" (1885.1, pp. 709, 



