776 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



hook is seen to support half the lunar, while in the 

 front view of the same carpus the lunar rests mainly 

 on the unciform. In further special studies of the 

 magnum Gregory has compared the magna of Palaeo- 

 syops and of two chalicotheres {Moropus and Macro- 

 therhim) which display family differences and ordinal 

 resemblances, as shown in Figure 703. 



Reduction of digit V in the manus. — To the primi- 

 tive cursorial habit of the perissodactyl ancestors is 

 probably attributable the early reduction of D. I, 

 no vestige of which has thus far been discovered in 

 any perissodactyl, living or fossil, as pointed out 

 above. Digit V suffers incipient reduction as the 

 manus is raised into the unguligrade position, in which 

 the main weight rests upon the horny sheaths of the 

 terminal phalanges, supported by the posterior foot 

 pad, as shown in Chapter I (p. 33). This reduction 

 does not advance very far in the titanotheres because 

 of the early transformation of the foot into the medi- 

 portal, weight-bearing type, in which D. V becomes 

 useful and therefore is retained. Nevertheless, some 



(cf) (H 



Pcicr 



Figure 704. — Left magna of two lower 

 Eocene perissodactyls 



A, Heptodon cakkulus (Cope), Am. Mus. 294; B, 

 Eotitanops princcps (Osborn), Am. Mus. 2% (type) . 

 The small end inQQS anteriorly; the bones are seen 

 from the outer side. Three-halves natural size. 

 lei. c. r.. Tuberosity for ?extensor carpi radialis; ?/?. 

 c. T., tuberosity for ?(]exor carpi radialis; (ce.), facet 

 for hook of the scaphocentrale; (In.), facet for lunar; 

 (unc), facet for unciform; (III), facet for Mtc III. 



of the mediportal titanotheres (such as Mesatirhinus) 

 exhibit a more slender D. V and a tendency toward 

 tridactylism, a secondary mesaxonic condition of the 

 manus, which in course of further evolution would 

 end in a tridactyl manus. All other perissodactyls 

 except the graviportal amynodonts {Amynodon, aqua- 

 tic rhinoceros) rapidly reduce D. V and pass into the 

 isotridactyl condition, with the more or less rapid 

 reduction of D. V conditioned hj the cursorial or the 

 mediportal habit. Thus D. V persists in the medi- 

 portal AceratJierium tetradactylum of the Miocene of 

 Europe, also in the modern mediportal Tapirus. 

 although it disappears in the mediportal rhinoceroses, 

 all of which become typically isotridactyl. Several 

 cursorial branches of the perissodactyl families — the 

 Paloplotheriinae, the Triplopodinae, the lophiodont 

 Helaletidae, the cursorial rhinoceroses (Hyracodon- 

 tinae), the subcursorial horses of the forest-living 

 Hypohippus branch — all retained three digits closely 



compressed into a narrow, solid foot, which reaches an 

 extreme in Oolodon, the terminal member of the 

 lophiodont Helaletidae. 



Reduction of terminal phalanges. — Whereas in all 

 cursorial types the terminal phalanges support rela- 

 tively large horny sheaths adapted to rapid progres- 

 sion over hard ground, the mediportal and gravi- 

 portal types tend to the enlargement of the foot pad 

 and reduction of the terminal phalanges. Conse- 

 quently in the large graviportal titanotheres, as in the 

 Proboscidea, the terminal phalanges are greatly 

 reduced in size, becoming almost vestigial even in 

 such relatively swift-moving forms as Menodus, which 

 has much more elongate digits and limbs than its 

 bulky contemporary BrontotheTiurn. 



SUMMARY OF THE EVOLUTION OF THE PERISSODACTYL 

 FAMILIES 



The closest rivals of the titanotheres were the other 

 perissodactyls, but the titanotheres outstripped them 

 all until the end of lower Oligocene time. It has been 

 noted in Chapter I (p. 24), as shown in the accom- 

 panying diagram (fig. 705), that the nine typical peris- 

 sodactjd families had already diverged from one 

 another in lower Eocene time, and that by the begin- 

 ning of Oligocene time they were widely separated in 

 their dental and osteological structure. In section 1 

 of the present chapter (p. 768) it has been shown that 

 when the titanotheres, horses, tapirs, and lophiodonts 

 first appeared in America they were so similar in struc- 

 ture of skull and feet that they can be separated only 

 by careful analysis of the structure of their grinding 

 teeth. It is not surprising that paleontologists and 

 zoologists of the last century (Flower, Cope, Gill) were 

 disinclined to separate them into distinct families. 



The separation of the Perissodactyla according to 

 their fundamental divergence in the structure of the 

 molar teeth was partly suggested by Schlosser (1886.1) 

 in his "Beitriige zur Kenntnis der Stammesgeschichte 

 der Hufthiere und Versuch einer Systematik der 

 Paar- und Unpaarhufer." In 1892 it was more fully 

 developed by Osborn (1892.67, pp. 90-94) in his thesis 

 "The classification of the Perissodactyla," in which it 

 was shown that the main desiderata of classification 

 are, first, clearness of phyletic relationships; second, 

 convenience; third, structure, since it appears that in 

 this order the teeth are more fundamental than the 

 feet. 



At this time Osborn (op. cit.) divided the Perisso- 

 dactyla into 9 families and 19 subfamilies. In "The 

 rise of the MammaUa in North America" (1893.82) 

 Osborn extended this system, and in "The extinct 

 rhinoceroses" (1898.143, p. 79) he showed that the 

 Perissodactyla may be primarily divided by the fun- 

 damental pattern of the upper grinding teeth into four 

 superfamilies, namely, Titanotheroidea, Hippoidea, 

 Tapiroidea, Rhinocerotoidea, to which, he observed. 



