602 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



rugosity (fig. 500) on the inner side of the shaft for 

 the tendon of the latissimus dorsi muscle. 



Figure 672 gives a comparison of the humeroradial 

 articulations of Tapirus, Palaeosyops, and Rhinoceros. 

 We observe that the asymmetry of the trochlea and 

 capitellum persists in graviportal types, whereas the 

 cursorial Equus acquires a more symmetrical form. 



Although this titanothere elbow joint has points of 

 similarity with that of the tapirs, paleotheres, and 

 other primitive perissodactyls, the form as a whole 

 appears to be somewhat distinctive. Viewed from 

 below, the trochlea is much broader than the capitel- 

 lum; the trochlea is nearly plane, the capitellum 

 sharply convex. In the Oligocene titanotheres (fig. 

 510) the capitellar area widens out so that there is less 

 marked asymmetry with the trochlea. 



Correlated with this is the asymmetry of the upper 

 end of the radius as seen from the front, the trochlear 



Figure 509 



, Metark: 



-Types of scapula in middle Eocene titanotheres 



s, Am. Mus. 1873 (mediportal) ; B, DoUchorhinus sp., Am. Mus. 1833 (mediportal) 

 C, Palaeosyops robusius, Am. Mus. 1680 (subgraviportal). 



portion being depressed, the capitellar portion ele- 

 vated. A series of comparative views of the proximal 

 end of the radius in several genera (fig. 511) brings 

 this point out clearly. 



In the primitive ulna {Lambdotherium) the olecranon 

 is decidedly erect and truncate at the top ; it becomes 

 more depressed and pointed in the progressive weight- 

 bearing forms. In Limnohyops and Mesatirhinus is 

 seen an intermediate mediportal condition (fig. 511). 

 In Mesatirhinus it is rounded and moderately rugose 

 at the extremity; in Limnohyops more heavily rugose. 

 In the graviportal forms {Palaeosyops, fig. 511) it be- 

 comes pointed and heavily rugose at the extremity, 

 prophetic of the condition in the heavy Oligocene 

 titanotheres. In Dolichorhinus it is incurved, abbre- 

 viated, and depressed. The shaft of the ulna is of the 

 flattened, trihedral form. This olecranon process is 

 cleft by a faint groove superiorly, which becomes very 

 distinct in the Oligocene forms. 



Evolution of tJie manus. — The ancestral form of 

 manus and pes is, as shown above, subcursorial. The 

 principles of convergent mediportal and graviportal 

 adaptation, observed in the arches aiid limbs, also 

 dominate the foot structure; the divergence, in fact, 

 is still more conspicuous. On the principles set forth 

 above (pp. 583-584), vertical elongation of every 

 element of the carpus and tarsus as a rule points to 

 speed, while depression and transverse extension 

 point to weight. 



The general trend of the evolution of the manus 

 and pes in the titanotheres is from the relatively high 

 and narrow subcursorial type {Eotitanops) through 

 an intermediate or mediportal type {Mesatirhinus) 

 into a relatively broad and low graviportal type 

 {Palaeosyops). 

 From the conspicuous adaptive divergences either 

 in the mediportal or the graviportal direction, 

 it is first necessary to select the relatively 

 obscure family or universal characters of the 

 manus and pes. The digital formula is that 

 of all primitive perissodactyls. The manus 

 has four digits (mesaxonic to subparaxonic); 

 the pes has three digits (mesaxonic). One 

 peculiar feature of the titanotheres is that (as 

 in the amynodont rhinoceroses alone among 

 all other perissodactyls) the primitive four 

 digits persist in the manus. The pes in the 

 titanotheres, as in all other perissodactyls, 

 because it is connected with the chief loco- 

 motor organ, the hind limb, is more reduced 

 in size and more progressive in form than 

 the manus. 



Primitive manus. — The manus of the ear- 

 liest types known {Lambdotherium, Eotitan- 

 ops) is mesaxonic — that is, the third, median 

 digit (D. Ill) is distinctly the largest, as in Eohippus, 

 Heptodon, and Lophiodon. The retention and expansion 

 of the fifth digit (D. V) to share the weight appears 

 to be partly secondary. 



Adaptive manus. — The titanothere family characters 

 maybe summed up as follows: (1) Persistent tetra- 

 dactylism, with considerable adaptive divergence 

 among the genera in the relative length of the four 

 digits, especially of D. V, which is relatively shorter 

 in some forms {Palaeosyops, Limnohyops) and longer 

 in others {Mesatirhinus, Manteoceras) ; (2) in the front 

 view of the metacarpals, Mtc II, while supporting 

 mainly the trapezoid, articulates broadly against the 

 magnum; Mtc III, while mainly supporting the 

 magnum, also articulates broadly against the unciform; 

 (3) the trapezium is likewise present (in Eocene time) 

 and articulates with the side of the trapezoid, also 

 with Mtc III, and often (especiallj' in Manteoceratinae) 

 with the scaphoid; (4) the proximal phalanges are of 



