450 



TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



advances, and the roots of p^ soon lose their firm hold 

 in the alveolus. P^ is always a smaller and simpler 

 tooth than p^, which tends to crowd it out of place. 



Molar evolution. — The Oligocene grinding tooth 

 evolves out of the primitive Eocene grinder and takes 

 its proportions from the skull. Thus we compare the 

 elongate, dolichocephalic grinding tooth of Menodus 



FiGUEE 381. — Upper molars of Menodus giganteus and Alloys marshi 



A, Menodus giganteus. Am. Miis. 496. First left upper molar, posterior view. This tootli was beginning 

 its eruption at the time of the animal's death; in life the tips of the cusps would soon have come into use. 

 The internal cusps (pr) are low; the outer wall of the tooth is produced into a long arc. This outer 

 wall wears down much faster than the inner cusps, and the tooth as a whole slowly rotates downward 

 and inward, the outer side moving faster than the inner side, so that in old animals the outer roots are 

 widely protruded and the surface of the crown is tilted inward. 



B, AUops marshi, Am. Mus, 499. Second right upper molar, crown view. From the base of the metacone 



a small projection, the "crochet, "runs forward into the deep medifossette; from the middle of the crown 

 opposite the mesostyle a second small projection, the "crista," runs inward and forward; from the base 

 of the paracone a third ridge, the "antecrochet," runs back toward the crochet and crista. Crochet, 

 antecrochet, and crista appear in both the deciduous premolars and the permanent molars of Oligocene 

 titanotheres along with the deepening medifossette. They are barely foreshadowed in the upper Eocene 

 Diplacodon and Rhadinorhinus. 

 Two-thirds natural size. 



comparative measurements of the brachycephalic 

 Brontops robustus, the dolichocephalic Menodus gigan- 

 teus, and the brachycephalic Brontotherium gigas 

 elatum. (See p. 451.) 



Upper molars; hypocone and cingulum. — ^The hypo- 

 cone of m^ appears as a low cusp on the posterior 

 cingulum in Brontops dispar and as a prominent cingu- 

 lum cone in Menodus (Diconodon, 

 Anisacodon montanus). In some speci- 

 mens of Menodus the hypocone of m^ is 

 separate and is surrounded by a cingu- 

 lum; in others it is small but distinct or 

 is confluent with the cingulum. Thus 

 the separation of the hypocone would 

 not appear to be a valid specific, much 

 less a valid and constant generic 

 character. 



Cope observed that the strong or 

 feeble development of the cmgulum di- 

 vides the titanotheres into two parallel 

 groups (Cope, 1891.2, p. 9), which are 

 now recognized as follows: 



Menodontine group {Menodus, Brontops, 

 etc.) : Cingulum strongly or distinctly 

 developed. 



Brontotheriine group (Brontotherium, 

 Megacerops, etc.): Cingulum retro- 

 gressive, feebly developed, or wanting. 



with the abbreviated, brachycephalic, transversely 

 spreading grinder of Brontotherium (fig. 382.) 



Special characters. — The most exceptional character 

 is the vertical elongation of the ectoloph (figs. 227, 

 228, 381) which attains twice the height of theproto- 

 cone; thus a disharmonic crown is produced, hypsodont 

 on the outer side and brachyodont on the inner side; 

 this elongation of the ectoloph leaves a deep pit 

 (medifossette) in the central valley of the crown, 

 which is bounded by three secondary foldings of 

 enamel — anterior, median, and posterior — which are 

 comparable to but not homologous with the crochet, 

 antecrochet, and crista of the rhinoceros molar tooth. 

 These secondary folds are rectigradations which are 

 also slightly developed in certain upper Eocene 

 titanotheres. The medifossette is distinctly fore- 

 shadowed in the upper Eocene Diplacodon. A shallow 

 postfossette appears internal to the hypocone. On 

 the antero-internal border of the crown appears a 

 prominent cuspule which is comparable to the proto- 

 style; it never detaches itself from the protocone; 

 in the center of the crown are sometimes observed 

 vestigial or reversional traces of the protoconule and 

 of the metaloph. The terminology of the molars, as 

 compared with that in other perissodactyls, is set 

 forth in Chapter V (p. 263). 



The correlation of dolichocephaly and brachy- 

 cephaly with tooth proportions is illustrated in the 



In Menodus the cingulum is especially 

 strong; in Megacerops it is especially 

 feeble. The cingulum is thus a dis- 

 tinct phyletic or group character. It is not a 

 sex character, as Hatcher suggested (1893.1, p. 216). 



Figure 382. — Extreme dolichocephalic (A) and brachyce- 

 phalic (B) types of upper premolar-molar series in Oligocene 

 titanotheres 



A, Menodus trigonoeeras, Carnegie Mus. 3068, one-fourth natural size; B, Bronto- 

 therium gigas elatum. Am. Mus. 492, tooth row reduced to the same absolute length 

 as in A. 



For example, in the female skull of Menodus giganteus 

 (Am. Mus. 506) the cingulum is quite as strongly 

 marked as in the male skulls (Am. Mus. 505, 1066, 

 1067). The cingtdum is less strong in Brontops and 

 Diploclonus than in Menodus and is almost obsolete 

 in Megacerops acer and Brontotherium, platyceras. 



