34 



TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



dently far more potent than ancestry (heredity) in 

 the determination of general form, yet in comparing 

 the limbs of all the members of the different perisso- 

 dactyl families with one another we can generally, by 

 some family characteristic inherited from the ancestral 

 stem form, distinguish the tapir type, the rhinoceros 

 type, the titanothere type, etc. In the limbs, as in 

 the skull and teeth, the titanothere, rhinoceros, or 

 tapu' ancestry respectively seems to keep the evolution 

 of proportion and form within certain limits, so that, 

 for example, the resemblance between the graviportal 

 scapula of the titanothere and that of the rhinoceros, 

 though it may be very close and deceptive, is never 

 quite complete. The stages of muscular and skeletal 



the origin of new characters (rectigradations). In 

 this problem of the origin of new characters in the 

 titanotheres we have two principal subjects of study, 

 namely, the origin of horns on the skull and the 

 origin of cusps on the grinding teeth. 



In the evolution of the grinding teeth the titano- 

 theres are very conservative; in them few new cusp 

 elements originate, though several of the old cusp 

 elements disappear. These animals thus present a 

 striking contrast to the horses in the evolution of the 

 grinding teeth, for in the horses a large number of 

 new cusp elements are successively added. Yet the 

 grinding tooth of the earliest titanotheres {Lambdo- 

 therium and Eotitanops) is in general similar to that 



Figure 26. — Structure of the feet in extinct and living odd-toed ungulates (perissodactj-ls) 



A, Sole of the left fore foot of a tapir (Taphus ierresiris), showing the tripod-like arrangement of digits II, III, and IV, and 

 the reduced condition of V; B, sole of the left fore foot of an Eocene titanothere (Mesatirkinus petersoni), restoration based 

 on Princeton Museum specimen No. 10013; C, sole of the fore foot of a rhinoceros, showing the enlarged hoofs of the 

 three digits (II, III, IV) ; C^, side view of same; C^ longitudinal section of same; D', sole of the fore foot of a horse, show- 

 ing the expanded nail; D^, longitudinal section of same. The central pad (/} in A, B, and C is homologous with the 

 relatively reduced pad or frog (/) in the foot of the horse (DO- All but B after Eber. 



evolution, arranged from latest to earliest, are as 

 follows : 



4. Graviportal; ponderous, relatively slow-moving types, 

 such as Brontolherium, Rhinoceros {C eratotherium) simum. 



.3. Mediportal; of moderate weight and speed, such as 

 Limnohyops, Tapirus. 



2. Subcursorial; of light weight and relatively swift move- 

 ments, such as Eolilanops of the lower Eocene. 



1. Cursorial; swift moving, Ught frame, such as Lanibdo- 

 therium of the lower Eocene. 



ORIGIN OF NEW CHARACTERS AS DISTINGUISHED FROM 

 CHANGES IN PROPORTION 



The continuous gradual changes of proportion in 

 the head, trunk, and limbs (allometrons), as already 

 outlined, present a problem distinct from that of 



of the earliest horses {Eohippus). In these lower 

 Eocene contemporary mammals the grinding teeth 

 are the same, cusp for cusp. In the horse all these 

 cusp elements are preserved and utilized, and the 

 highest degree of mechanical adaptation to the graz- 

 ing habit is gradually evolved; in the titanotheres 

 the browsing habit is generally conserved, and there 

 is little marked increase of mechanical adaptation; 

 in fact, mechanical inadaptation or imperfection of 

 the grinders may have been one of the probable 

 causes of the extinction of the titanotheres at a time 

 when the conditions favorable to grazing gradually 

 replaced those favorable to browsing. 



The adaptive radiation of the grinding teeth in the 

 several families of the Perissodactyla from somewhat 



