EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



269 



MECHANISM OF THE TITANOTHERE GRINDING TEETH 



The pattern of the upper and lower grinding teeth 

 of the titanotheres is one that has entirely disappeared 

 among the existing mammals. It has no counterpart 

 among any living ungulates, but it is closely analogous 



PARAGON e: 



FOSSA METACONE 

 FOSSA 



Jvyd 



772-2 



METACONID ENTOCONID 

 FOSSA FOSSA 



Figure 224. — Relations of upper and lower molars in an Eocene 

 titanothere, Telmatherium cultridens 



Princeton Mus. 10027 (type). Two-thirds natural size. A, Second upper molar 

 showing the crushing parts and the cutting parts, also the pits or fossae for the 

 reception of the projections on the lower teeth; B, second lower molar, showing 

 the fossae for the parts of the upper teeth; C, diagram showing how each lower 

 molar articulates with two upper molars, the trigonid of ma wedging between mi 

 and m^, and the talonid of ma receiving the protocone of m^. 



to that of many Eocene and Oligocene mam- 

 mals, both artiodactyls and perissodactyls. 

 There is no perfected grinding function, such 

 as that between the upper and lower molars 

 of the horse, nor could such a grinding function 

 evolve out of the titanothere molar tooth. 



The dental mechanism is a combination 

 of two functions which may be described as 

 crushing and cutting, the crushing being 

 effected by a double pestle and mortar or peg 

 in socket mechanism, and the cutting by a 

 mechanism of double shears or reversed double- 

 bladed crescents. 



In the accompanying diagram (fig. 224) the 

 manner in which this double function is sub- 

 served in the upper and lower grinders is 

 clearly shown. 



(1) The superior molars consist of a double- 

 cutting shear W, composed of the paracone and 

 metacone crescents, which oppose the reversed 

 double shear M, composed of the crescents developed 

 from the protoconid and hypoconid. (2) The chief 

 crushing action is performed by the superior cones, the 

 protocone and hypocone, which fit into the protocone 



and hypocone fossae of the inferior molars. (3) The 

 apex (O) of each of the four primary superior cones (pro- 

 tocone, paracone, metacone, hypocone) has its contact 

 or abrasion point (oblique shading in figure), or fossa, 

 in the inferior molars. (4) Similarly the apex of each 

 of the four inferior .primary cones (protoconid, meta- 

 conid, entoconid, hypoconid) has its contact or ab- 

 rasion point (oblique shading), or fossa, in the superior 

 molars. 



This double cutting and crushing function is me- 

 chanically imperfect in the short-crowned molars of 

 Lambdotherium and Eotitanops (figs. 229, 235, 242, 

 253). It becomes more efficient as the crowns become 

 higher and the cones and crescents are vertically 

 elongated in Telmatherium (fig. 225). The evolution 

 of the titanothere grinders is directed to overcome 

 the deficiency of this cone and crescent mechanism, 

 which proves to be inherently defective in design. 



The crushing function of the grinders is best ob- 

 served in the internal view (fig. 226) of the upper 

 and lower grinders of a telmathere, in which the 

 pestles (p-h) are sinldng into the mortars (m-e) in 

 exactly the same manner as in the primitive insecti- 

 vores. This closely correlated mechanism of the 

 upper and lower grinding teeth, which was first 

 studied by Cope (1889.3) and more fully by Gregory 

 (1916.1), indicates that every new character (recti- 

 gradation, allometron) added to the upper grinders 

 must be correlated with a new and mechanically 

 adaptive character (rectigradation, allometron) in 

 the lower grinders. The cutting function performed 

 by the W of the upper ectoloph and the M of the 

 lower ectoloph is illustrated (figs. 224, 225) and dis- 



FlGTTRB 225 



-Dental mechanism of titanotheres 



Interlocking relations of upper and lower premolar-molar series. One-half natural size. A, Telma- 

 therium cultridens, Princeton Mus. 10027 (type); upper teeth (light line), with pattern of lower 

 teeth (heavy line) projected upon them. Crown view. B, Internal view of the same teeth, 

 showing the crushing action of the cones and conids. 



plays the close mechanical relation of the alternating 

 crests as well as the simultaneous development of 

 the new cusps (rectigradations) of the premolar 

 teeth. 



