38 



TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



different perissodactyl families, are indicated by dif- 

 ferent kinds of shading. 



Forest-living habits among perissodactyls are some- 

 what more rare, especially the extreme adaptation to 

 forest living, consisting of relatively slow locomotion 

 and marked special adaptation to browsing on the 

 leaves of trees. Types that are more or less fully 



Aquatic branches of the perissodactyls are also more 

 or less readily distinguishable. Among the titano- 

 theres we have a group of swamp or river living forms, 

 with short limbs and spreading feet, whose remains 

 are preserved in many river-channel sandstones, 

 namely, the genera DolichorMnus and Metarhinus, 

 which are clearly distinguished from all other titano- 



FiGURE 30. — Adaptive radiation in tlie evolution of the upper molar teeth in the odd-toed hoofed mammals 



(perissodactyls) 



After W. D. Matthew. The earliest members of each family had low-crowned (brachyodont) teeth, of relatively simple pattern. In the 

 titanotheres and paleotheres the internal cusps remain low and the two outer main cusps form a W. In the horses (hypsodont) the whole sur- 

 face of the crown is thrown into complex crests and ridges and the crown becomes very long. In the tapirs (brachyodont) the molar crown 

 takes the form of two sharp cross crests. A somewhat similar pattern is seen in the lophiodonts, e.Kcept that in this family (brachyodont) 

 the outer cusps form an irregular outer wall. In the rhinoceroses (brachyodont to hypsodont) the outer wall (ectoloph) becomes very much 

 flattened, elongate, and oblique, and the cross crests also become oblique. 



adapted to forest living are represented, we believe, 

 among the chalicotheres, among certain forest-living 

 horses {HypoJiippus), and among certain forest-living 

 tapirs (Tapirus terrestris), all relatively slow in move- 

 ment and all without conspicuous weapons of offense 

 or defense, except that the chalicotheres, such as 

 Moropus, are provided with heavy claws. 



theres by their apparent adaptations to river-border 

 or aquatic life. Certain tapirs frequent river borders 

 and swim freely for long distances, but they do not 

 acquire distinctive aquatic adaptations. Among the 

 rhinoceroses the pronounced aquatic division is the 

 amynodonts, which have marked aquatic features 

 about the head, simulating those of the hippopotami. 



