762 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



TETRADACTYI MANUS OF PRIMITIVE PEEISSODACTYIA 



Tetradactylism.— The manus of the primitive peris- 

 sodactyls (see fig. 692, A, B, C) differs widely from 

 that of the primitive Amblypoda (fig. 687, B) and 



-(sicsi) 



dist. 



which have since been discovered reveal the more truly 

 primitive perissodactyl manus, which is alternating, 

 not serial. 



This displacement is partly a cursorial adaptation. 

 The first to comment on the real significance of the 

 primitive displaced lower Eocene perissodactyl foot 

 was Gregory (1910.1, p. 393), who pointed out that 

 the manus of Eeptodon (see fig. 692) has an extremely 

 "displaced" carpus, the scaphocentrale completely 

 covering the magnum, while the lunar rests entirely on 

 the unciform. Subsequent discovery has confirmed 

 Gregory's observation and proved that the primitive 

 perissodactyl carpus, observed in the chalicotheres, 

 tapirs, and titanotheres, as well as in the helaletids, to 

 which Eeptodon belongs, is completely displaced. 



A functional classification and summary of these 

 foot types is as follows : 



Primitive tetradactyls. — The primitive condition of 

 the perissodactyl carpus is well illustrated in Figure 

 700, A-E — namely, (1) magnum very small; (2) scaph- 

 oid with elongate oentrale process resting on magnum; 

 (3) lunar with a very slight magnum contact, thus 

 resting chiefly or entirely on the unciform. This type 

 is seen in the helaletids (A), tapirs (B), titanotheres 

 (C), chalicotheres (E). In the most primitive 

 horses (N) the magnum has a considerable lunar 

 contact. 



Functional isotetradactyls. — In these perissodactyls 

 (fig. 700, D, F-J) the fifth digit is functional— that is, 

 all four digits are used in these forms typified by Eoti- 



FiGtTRE 691. — -Astragali of a condylarth, 

 Phenacodus. -primaevus (A), and of a 

 primitive perissodactyl, Heptodon (B) 



primitive Condylarthra (fig. 687, C, D, E) in being 



tetradactyl and in the arrangement of the carpal bones. 



Both Amblypoda and Condylarthra — in fact, all other 



Ungulata, including the Artiodactyla — are pentadactyl 



in their most primitive forms. It had long been sup- 

 posed, because of the opinions of Marsh on EoTiippus 



and of Deperet (1903.1) on LopModon, that 



there was adequate evidence for pentadac- 



tylism in the most primitive Perisso- 



dactyla. Deperet, in 1903 (op. cit., p. 33), 



observed : " La patte anterieure du LopModon 



etait certainement pentadactyle." This 



conclusion may perhaps rest on the presence 



of a small facet on the internal side of Mtc 



II, a facet that probably served for articu- 

 lation with the trapezium, which descends 



secondarily on the internal side of Mtc II 



in the absence of Mtc I. In the most primi- 

 tive manus loiown of EoTiippus Granger 

 finds a similar facet but fails to find any 

 evidence of Mtc I either as a splint or as 

 a vestige. Such a vestige of Mtc I may 

 yet be discovered, but in the absence of 

 positive evidence the primitive perissodactyl 

 manus must be described as essentially 

 tetradactyl. 



Primitive, alternating, displaced car pals. — 

 Osborn's error in regard to the origin of the 

 perissodactyl foot sprang from regarding as primitive j tanops (D), HyracTiyusiG,!!), LopModon (X),m.A\>j the 

 the partly displaced feet of the middle and upper j upper Eocene tapir Isectolophus (J); the magnum and 

 Eocene perissodactyls; it now appears that these j lunar expand so that there is increasingly broad 

 feet are mostly modified. The feet of the lower magnum-lunar contact; the lunar becomes wedge- 

 Eocene Sparnacian ( = part of Wasatch) perissodactyls | shaped distally, with equal magnum-unciform facets. 



Figure 692. — Manus of Heptodon, Lambdotherium, and Eotitanops 



, Heptodov, Am. Mas. 294, a primitive perissodactyl manus of cursorial type. B, Lambdotherium, 

 Am. Mus. 4880, and C, Eotitanops, Am. Mus. 296 (type), illustrate the ancestral' titanothere type 

 of manus. One-half natural size. 



