800 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



families — the Eqiiidae, Helaletinae (Lophiodontidae), 

 and Hyracodontidae — are cursorial or subcursorial. 

 This prevalence of primitive cursorial types points 

 to the hypothesis that the most primitive titanotheres, 

 like the other Perissodactyla, the Artiodactyla, and 

 the Condylarthra, were largely (if not wholly) cursorial 

 or subcursorial animals, from which the larger medi- 

 portal and graviportal titanotheres evolved. This hy- 

 pothesis, as Gregory (1910.1) points out, is sustained 

 by the carpal and tarsal structure in most of the early 

 perissodactyls and by what is known of Lamhdothermm. 

 In accordance with the principle of survival through 

 natural selection, the early perissodactyls, being of 

 diminutive size and without defensive weapons of 

 any kind, would survive by adopting a relatively 

 swift gait to escape their enemies. 



In the remote ancestral perissodactyl types, includ- 

 ing the titanotheres, so far as known, the arches and 

 limbs were slender, the manus and pes were relatively 

 elongate, and the carpals and tarsals were high and 

 narrow. 



The details of structure of the Jimbs of the lower Eo- 

 cene LamhdotJierium and of Eotifanops are described on 



pages 590-598. 



THE TITANOTHERE PES 



The early titanotheres show the following char- 

 acters in common with other early perissodactyls: 

 (1) Tridactyl, no trace of D. I or D. V; (2) digits 

 relatively elongate, laterally compressed; (3) astrag- 

 alus with elongate neck and three entirely separate 

 astragalocalcaneal facets (Osborn, 1890.51), the ectal 

 (e), the vertically elongated "sustentaculars, " and the 

 "inferior" (i), the last usually small and separate; 

 (4) astragalocuboidal facet narrowing to a point 

 anteriorly, whereas in the middle Eocene titanotheres 

 it is progressively broad; (5) calcaneofibular facet, 

 or pit, functional in titanotheres and equines. 



The proportions and facets of the pes of titano- 

 theres must be interpreted by considering the common 

 ancestry of the titanotheres with the other perisso- 

 dactyls and the special submediportal divergence of the 

 pes. Thus, in the evolution of the feet of the several 

 forms of the perissodactyls the contrasts are as follows : 



Forms of the elements of the pes in three types oj perissodactyls 



Judging by these tests the gradations as to weight 

 and speed among these lower Eocene titanotheres, as 

 compared with contemporary perissodactyls, were as 

 follows : 



Ancestral titanothere Eotitanops, larger, least cursorial. 



Cursorial titanothere Lambdotherium, smaller, transi- 

 tional. 



Cursorial lophiodont Heptodon, more slender and 

 cursorial. 



Cursorial horse Eohippus, most slender and cursorial. 



Thus all the elements of the pes of Eotitanops may 

 be distinguished in all their facets and other parts as 

 broader and more expanded laterally than those of 

 Heptodon and Eohippus, which are compressed laterally 

 and expanded anteroposteriorly. 



PROGRESSIVE STAGES OF THE MANUS 



A comparison of the evolution of the manus in the 

 ascending series of titanotheres, as illustrated in 

 Figures 722-723, shows a complete transition from 

 the cursorial through the mediportal to the graviportal 

 type, modified throughout by certain peculiar titano- 

 there family tendencies, namely, (1) increase in 

 size; (2) secondary enlargement of D. V, which 

 finally nearly equals D. II in size, a titanothere 

 peculiarity; (3) secondary substitution of paraxonic 



formesaxonic condition, a titanothere peculiarity; (4) 

 flattening of all the small bones of the carpus; (5) ab- 

 breviation of all the phalanges, especially of the ter- 

 minal phalanges supporting the horn sheaths; (6) pro- 

 gressive expansion of the lunar upon the magnum in 

 adaptation to tetradactyly, to the paraxonic condi- 

 tion, and to the progressive graviportal condition; 

 (7) gradual atrophy of the trapezium, which becomes 

 vestigial. 



THE TITANOTHERE MAGNUM 



The progressive stages in the evolution of a single 

 bone, the magnum, demonstrate that each bone of the 

 carpus passes through a complete succession of stages 

 from the cursorial to the graviportal type; that each 

 bone reflects what it is correlated and coadapted 

 with, namely, the bodily history of the entire or- 

 ganism; that each bone if examined with sufficient 

 thoroughness and minuteness would reveal generic and 

 probably specific characters capable of definition. 



The magnum, as the central bone of the carpus, in 

 the course of adaptation from the cursorial condition 

 in Eotitanops to the extreme graviportal condition in 

 Brontotherium, changes its diameters completely, 

 evolving from a high, narrow form (Eotitanops) into 

 a broad, low, flattened form (Brontotherium). It has 



