EVOLUTION HE SKULL AND TEETH OF EOCENE TITANOTHERES 



281 



Lambdotherium popoagicum Cope 



Plate LIV; text figures 27, 33, 103, 143, 228, 230, 231, 233-237, 

 244, 483, 484, 486-492, 503, 504, 512, 521, 522, 661, 694, 700 



[For original description and type references see p. 168. For slieletal characters 

 see p. 590] 



Type locality and geologic horizon. — Wind River 

 Basin, Wyo.; Lambdotherium zone (Wind River B). 



Specific characters. — P2-m3 69-56 millimeters; ps with 

 paraconid, metaconid, and hypoconid intermediate 

 in development. In superior molars the protoconules 

 more or less free and distinct, metaloph low but dis- 

 tinct, cingula not surrounding the crown internally. 



Materials. — The type species (figs. 234, H; 236, C) 

 of a series of mutations of specific character is repre- 

 sented by over 70 specimens in the collections of the 

 American Museum, chiefly from the typical Wind 

 River formation, but also from contemporaneous 

 deposits in the Big Horn Basin, Beaver Divide, and 

 Huerfano Basin. These specimens consist mostly of 

 scattered upper and lower teeth and fragments of 

 jaws but include several nearly complete jaws. One 

 specimen (Am. Mus. 4880) affords a limited but signi- 

 ficant knowledge of the skeleton. 



SJciill.— The imperfectly known skull is analo- 

 gous to that of the primitive horses rather than 

 that of Eotitanops or any of the typical middle 

 Eocene titanotheres. There are only two speci- 

 mens (Am. Mus. 14903, 14907) in which fragments 

 of the skull are associated with the teeth, from 

 which the conjectural restoration (fig. 233) is as- 

 sembled, the outlines of the anterior part, or pre- 

 maxillaries and nasals, being inferred from the 

 attenuate structure of the lower jaw. The prin- 



General features of the teeth. — The dental formula, so 

 far as known, is If, C\, Pf , M|. The inferior incisors, 

 as observed in Am. Mus. 14899, 14906, 14920, repre- 

 sented in Figures 233, 236, 237, are semicircular in 

 arrangement, semiprocumbent, with spatulate or 

 chisel-shaped crowns; the median incisors especially, 

 which are distinctly chisel-shaped, are quite different 

 from those of Palaeosyops, which are bluntly pointed. 

 The inferior and superior canines are rounded, slightly 

 compressed laterally, and sharply pointed. 



Ty2)e preinolars. — No trace of p^ or pi is to be found 

 in any of the specimens; this tooth is ordinarily very 

 persistent in the Perissodactyla. In the L. popoagicum 

 type premolar series p2 is an elevated, laterally com- 

 pressed cone, with a rudimentary paraconid and low, 

 narrow heel bearing a hypoconid; ps presents an 

 anterior lobe composed of a low paraconid, an elevated 

 protoconid, a postero-internal metaconid elevated but 

 slightly developed, a somewhat broader posterior 



Figure 233. — Skull of Lambdotherium "popoagicum, reconstructed 



cipal characters are the following: (1) Dolicho- MadebyL. M. sterling under the direction otW. K.Gregory. About two-fifths natural size. 



,. . iUi"! IJ '^'''^ reconstruction is made from three specimens in the American Ivluseum, collected in 



Cephaly OI prOOpiC type that is, long, slender ^^^ ^^^^ jji^^j. Basin-No. 14899,Alkali creek, Buck Spring, lower jaw; No. 14907, Alkali 



skuU, in which the facial greatly exceeds the cranial creek, Wolton, maxilla, malar, and skull top; No. 14903, Alkali Creek, Buck Spring, 



, ,1,1 <•• 11--1 1- nr- squamosal and condyle. Missing parts conjecturally restored by comparison with Systemo- 



length, the faciocephalic mdex being 65, as com- ionB.niEoMppm. 



pared with 56 in Eotitanops; (2) sagittal crest rather 

 low and slender; (3) external auditory meatus open 

 inferiorly; (4) infraorbital foramen placed above the 

 second premolar — that is, decidedly anterior in posi- 

 tion as compared with that in the typical titano- 

 theres; (5) an attenuated rostrum associated with 

 the elongated symphysis of the jaw, suggesting the 

 conformation of the skull of a ruminant rather than 

 of a perissodactyl. 



Comparison. — This skull is very close in its propor- 

 tions to that of Eohippus, and if it were not for the 

 differences in the teeth might be mistaken for it. 

 The faciocranial indices are similar, namely: 



heel or hypoconid, with a rudiment ry internal crest 

 representing the entoconid; p4 is a more progressive 

 or submolariform tooth with an anterior transverse 

 crest composed of protoconid and metaconid behind 

 which is a low, incomplete posterior crest supporting 

 an elevated hypoconid and a depressed internal ridge. 

 Lower premolars, primitive and progressive muta- 

 tions or specific forms. — The structure of the cusps in 

 P2, Ps, P4 is very important. The accompanying 

 diagram (fig. 234) shows the wide range of progressive 

 evolution in the lower premolar teeth which are 

 exhibited in the large number of specimens in the 

 American Museum collection. They embrace stages 

 ranging from far less primitive to stages far more 

 primitive than the lower premolar teeth of the type 

 of L. popoagicum. These stages are especially impor- 

 tant and interesting because they are recorded as 

 coming from similar geologic levels. These records of 

 geologic levels may be confused, but accepting them 



