754 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



radial ratio decrease; (4) radiohumeral ratio increase; 

 (5) metacarpohumeral ratio increase; (6) tuberosities 

 of humerus become less elevated or prominent, 

 whereas they become more prominent in graviportal 

 digitigrades; (7) entocondj'lar crest of humerus re- 

 duced, ectocondylar crest slightly expanded; (8) spine 

 of scapula abbreviated. 



The speed ratios and speed contours are progres- 

 sively acquired up to and including NeoMpparion, 

 the most light-limbed horse known. The retrogression 

 of these speed ratios through Equus asinvs and E. 

 cahaUus and into the very slow-moving Hippidium 

 is due to the increasing weight of these animals. 

 In this graviportal retrogression (1) the olecranon of 

 the vdna becomes more robust and recumbent; (2) 

 the neck of the scapula broadens; (3) the deltoid and 

 ectocondylar crests become more prominent. 



BRONTOTHERHDAE— THE TITANOTHERES 



The adaptive progression from left to I'ight, as shown 

 in the accompanying diagrams (figs. 685,. 686), is 

 from subcursorial {Eotitanops) through mediportal 

 {Mesatirliinus) to graviportal {Brontothernim) types. 

 In the scapula Palaeosyops, a heavy type, shows 

 incipient trihedral form; Brontops becomes markedly 

 trihedral with prominent tuber spinae. 



Adaptation to graviportal progression in the fore 

 limb is as follows: (1) Elevation of the great tuber- 

 osity of the humerus into a broad plate, (2) lowering 

 on the shaft and great prominence of deltoid crest, 

 (3) production of deltoid tuberosity, (4) elevation of 

 ectocondylar crest, (5) obliquity and asymmetry of 

 humeroradial trochlea, (6) depression and thickening 

 of olecranon of ulna. 



FiGUBE 684. — Limb structure of perissodactyls: Hind limbs of Equidae, showing their cursorial adaptation 



A, Orohippus osbornianus, one-fourth natural size. B, Mesohippus bairdi, one-fifth natural size. C, Hypohippus osbornj; D, Neohipparion whitneyi; 

 E, Hipparion gracile; C-E one-seventh natural size. F, Equus kiang; G, Equus asinus; H, Equus burchelli; J, Equus caballus; F-J one-ninth 

 natural size. Scales approximate. After De Blainville, 1839.1, and Osborn. 



A distinctive and syngenetic character of the 

 Equidae is the doubling of the bicipital groove by the 

 development of a median convexity. The degenera- 

 tion of the ulna and fibula inferiorly is a special equine 

 and also a general cursorial character (ulna reduced 

 and fibula disappears in cursorial ruminants) . 



In the hind limb the chief cursorial progression is 

 as follows: (1) Femorotibial ratio decreases; (2) tibio- 

 femoral ratio increases; (3) third trochanter rises on 

 shaft; (4) second trochanter relatively decreases in 

 prominence. The increasing asymmetry of the femoro- 

 patellar facets may not be regarded as a distinctively 

 cursorial progressive character because it is also 

 observed in the graviportal rhinoceroses. 



The platelike form of the great tuberosity of the 

 humerus is a distinctive titanothere character. An- 

 other distinctive feature is that in the titanotheres the 

 radius increases in length as compared with the 

 humerus, whereas in graviportal rhinoceroses the 

 radius either remains of the same length or abbre- 

 viates {Metamynodon, Teleoceras). 



A relatively straight or vertical hind limb, with 

 little flexure of the knee joint, anterior patellar facet, 

 and relatively short (graviportal) tibia, appears to have 

 characterized the titanotheres from the middle Eocene 

 onward. The graviportal progression, as seen in 

 Figure 686, results in the following changes: (1) Reduc- 

 tion in size of the crest of the great trochanter (rather 



