750 



TIT.iNOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASB:A 



EHINOCEROTIDAE, HYRACODONTIDAE, AMYNODONTIDAE 

 The extensive series of riiinoceros limb segments 

 shown in Figure 679 ilkistrates all the principles of 

 contrasting speed and weight progression. These 

 animals have no genetic relationship; they belong to 

 fom" entirely distinct subfamilies. The following 



cursorial (EyracJiyus) , the mediportal (Caenopus), the 

 lobate or vertically elongate graviportal-digitigrade 

 (Rhinoceros (Opsiceros) bicornis), and the triradiate 

 graviportal {Metamynodon, Teleoceras). (4) Contrast- 

 ing the limbs of Triplopus and Metamynodon, we observe 

 the two extremes of cursorial and graviportal allometry. 



Figure 679. — Limb structure of perissodactj'ls: Fore limbs in four American rhinoceros subfamilies 



Limb segments of (A) Triplopus cubitalis, (B) Hijrachyus agrarius, (C) Hyracodon nebrascemis, (D) Caenopus occidentalis, (E) Caenopus tri- 

 dactylus, (F) Amynodon intermedius, (G) Metamynodon planifrons, and (H) Teleoceras fossiger, showing the contrasts between subcursorial 

 types (Hyrachyus, Hyracodon, family Hyracodontidae), a mediportal type (Caermpus), and graviportal types (Metamynodon, faTiily 

 Amynodontidae; Teleoceras, subfamily Teleocerinae). A, about two-sevenths natural size; B, C, about one-seventh natural size; D-H, 

 about one- ninth natural size. 



points appear: (1) It is noteworthy that the Eocene 

 rhinocerotoid types (Triplopus, Hyrachyus, Hyra- 

 codon) and even the Oligocene forms (Caenopus) are 

 either subcursorial or cursorial. (2) A syngenetic 

 resemblance between all the rhinoceroses is seen in 

 the abbreviated spine of the scapula and in the ele- 

 vated position of the tuber spinae. (3) The scapulae 

 represent four adaptive types, namely, the elongate 



We first observe a very pronounced difference 

 between the trihedral graviportal scapulae of Meta- 

 mynodon and Teleoceras and the elongate lobate 

 scapula of all the Old World modern rhinoceroses. 



The graviportal progression, as illustrated in these 

 rhinoceroses, advances through the following changes: 

 (1) The scapulohumeral ratio decreases in the gravi- 

 portal American rhinoceroses (Metamynodon, Teleo- 



