THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



763 



This type of carpus seems to be characteristic of the 

 isotetradactyl perissodactyls. It is in wide contrast 

 to the isotridactyl condition. 



Isotridadyl perissodactyls. — In these animals (fig. 700, 

 K, L, M) the fifth digit (D. V) is nonfunctional, re- 

 duced, or vestigial, while the remaining three digits, 

 Mtc II-IV, equally divide the weight among them. 

 The triplopodines, paleotheres, and helaletids develop 

 this type of foot. It is noteworthy that in all 

 these animals the scaphocentrale entirely covers 

 the magnum; the lunar retains little or no mag- 

 num contact but rests mainly on the unciform; 

 the magnum may be vertically expanded; the 

 scaphoid is closely approximated to the unci- 

 form, so that these bones are almost or quite in 

 contact. These isotridactyl types thus retain 

 the primitive scaphocentrale magnum contact. 



Monodactyl perissodactyls (fig. 700, N-P). — 

 Even in the earliest Sparnacian (Wasatch) horses 

 monodactylism is indicated by the expansion of 

 Mtc III, accompanied by a broadening of the 

 magnum, which, as seen in EoMppus (N), partly 

 supports the lunar; as the magnum expands and 

 flattens (0, P) the magnum-lunar contact in- 

 creases. This is a condition exactly the reverse 

 of that in the isotridactyl types, such as 

 Colodon (M). 



Evolution of the magnum. — This primitively 

 small element (fig. 724, A, C, E) of the carpus is 

 very significant in the variety of its shapes. It 

 expands in all directions in the functional tetra- 

 dactyl types, vertically in the isotridactyl types, 

 and horizontally in the monodactyl type. Its 

 hooklike posterior process (fig. 704) is for attach- 

 ment of a tendon connected with the flexor pro- 

 fundus digitorum muscle (a process lacking in 

 Phenacodus). The magnum is thus one of the 

 most distinctive bones of the carpus, as stated 

 by Gregory (1910.1, pp. 393-395). It under- Figure 693.- 

 goes a special and very characteristic evolution Lower 

 in the titanotheres. As seen from the back the 

 lunar always rests largely on the magnum. 



The manus, summary. — The discovery that in 

 the earliest Eocene perissodactyls the scaphoid 

 rests chiefly on the magnum while the lunar rests 

 chiefly on the unciform proves that the Cope "taxeo- 

 pod" theory of the del'ivation of the perissodactyl 

 carpus from a "serial" type like Phenacodus must be 

 abandoned. The primitive perissodactyl type is a dis- 

 placed type; secondary types may become more or 

 less serial. This reversal of the old and apparently 

 well-established "taxeopod" theory has several impor- 

 tant results. First, it tends to support the view that 

 Euprotogonia and Phenacodus are not to be taken as 

 ancestral types of the Perissodactyla but as inde- 

 pendent offshoots of an insectivore-creodont stock; 

 second, it brings out the new conception that most 



of the early perissodactyls were rather light-limbed, 

 narrow-footed, cursorial animals. To this agility may 

 have been due their wide geographic distribution, 

 in open, semiarid country, and the acquisition of 

 those psychic characteristics associated with speed and 

 alertness of movement which gave them a decided 

 advantage in competition with the small-brained 

 Condylarthra. 



-Fore and hind feet in odd-toed ungulates (perissodactyls) 



pes, upper row manus. A, Brontops roftasius, a titanothere; B, Tapirus terrestris, A 

 modern tapir; C, JRhinoceros (Opsiceros) bicornis, a rhinoceros; D, Equus, the Taodem horse. 

 In primitive titanotheres the main a-Kis of weight, or symmetry, passes through the middle 

 (third) digit both in the fore and the hind feet. In the later titanotheres, through spread- 

 ing of the foot, the main axis in the foot as a whole presents a superficial resemblance to 

 that of the hippopotamus, among artiodactyls. In all recent perissodactyls, however, it 

 remains in the third digit, which finally becomes extremely large, while digits II and IV 

 diminish or dwindle away. 



The mesaxonic or anisotridactyl condition of the 

 manus — the condition in which the median toe was 

 somewhat larger than the two lateral toes — was ac- 

 quired very early. This condition is seen in the early 

 helaletids, tapirs, titanotheres, and chalicotheres. The 

 functional tetradactyl or isotetradactyl condition, in 

 which D. II-V are all functional, is more or less a 

 secondary one, as observed in Menodus and Amyno- 

 don. The isotridactyl condition, in which the three 

 median digits, Mtc II, III, IV, divide the weight, as 

 observed in Triplopus, Palaeotherium, Colodon, is also 

 secondary. 



