270 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



The partial transformation in the titanotheres of 

 a more vertical chopping, crushing, and cutting 

 motion of the jaw into a more oblique sweeping 

 or true grinding action of the molars finally results 

 in the entire loss of the conules and transverse crests 

 (protoloph, metaloph), in the partial molarization of 

 the premolars, and in the development of two gigantic 

 internal crushing cones (protocone, hypocone) and 

 of a very powerful external cutting crest (ectoloph). 



2. The excursion of the mandible was made from the 

 outer side upward and inward, as in rhinoceroses and 

 horses, in contrast with the opposite motion in ruminants. 



3. In Eocene titanotheres the lack of a tetartocone 

 in the upper premolars and of an entoconid in the 

 lower premolars leaves an open space when the jaws 

 are shut. This open space is filled in the Oligocene 

 titanotheres by the opposing tetartocone and entoconid. 



MOLARIZATION OF THE PREMOLARS 



The titanotheres resemble all the other fami- 

 lies of Perissodactyla in the gradual molariza- 

 tion of the premolar teeth — that is, in the 



Figure 226. — Dental mechanism: Grinding teeth of a titanothere (A) 

 and an insectivore (B) 



After W. K. Gregory. Internal view of the opposed upper and lower grinding teeth of 

 Telmatherium cuUridens (A), natural size, and Erinaccus (B), much enlarged. The pro- 

 tocones (p) fit into the talonid basins (fossae) between the metaconids (m) and entoconids 

 (e). The hypocones (ft) fit into the trigonid basins (fossae) between the entoconids (e) and 

 metaconids (m) . Similar relations are found in all primitive mammals. 



Three other interesting features in the evolution of the 

 dental mechanism of the titanotheres are the following: 



1. The marked protrusion of the roots on the outer sides 

 of the upper molars in old Oligocene titanotheres is a result 



HYPOCONID 

 M ESOSTYLE 



PROTOCONE 



Figure 227. — Contrast of molars of a brachyodont Eocene titanothere 

 (A) and a semihypsodont Oligocene titanothere (B, B') 



A, Third left upper molar of Palaeosj/ops leiiyi, seen from the rear. The internal and external 

 cones are subequal in height. B, Third left upper molar (unworn) of Menodus giganteus, 

 seen from the rear. The internal cones are low; the external cones have greatly increased 

 in height and have grown inward at the tip. B', The same seen from the outer side, 

 showing the much deepened ectoloph. 



of the bunoselenodont pattern of the molars and 

 of the vertical-oblique pressure of the lower teeth. 

 As the outer side of the molar crowns becomes more 

 hypsodont, in passing from lower Eocene to Oligocene 

 titanotheres, so the external roots protrude more 

 prominently. 



Figure 228. — Cross sections through second 

 upper and lower molars of Lambdotherium and 

 Menodus 

 A, A brachyodont lower Eocene titanothere, Lambdotherium 

 popoagkum, three-halves natural size; B, a semihypsodont 

 lower Oligocene titanothere, Menodus giganteus, one-half nat- 

 ural size. In A the excursion of the mandible was more trans- 

 verse in direction than it was in B, where, in correlation with 

 the deepening of the ectoloph, the movement of the mandible 

 was more vertical. 



transformation of the premolars into the molar 

 pattern. The mechanical inferiority of the teeth 

 of the titanotheres lies in the fact that this trans- 

 formation is never perfected ; it is very slow or 

 retarded, and the premolars never completely 

 acquire the molar pattern, as they do in the 

 Equidae, for example, in which the premolars 

 become actually superior to the molars both in pattern 

 and in mechanical perfection. The arrested transfor- 

 mation of the premolars in the titanotheres is 

 undoubtedly a defect that is correlated with the 

 abbreviation of the facial region and with the great 

 increase in the relative size of the molars. 



