GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



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gave rise to more active arboreal types, which began to leap from branch to branch. In the course of 

 this line of evolution the tarsus was slightly lengthened and the head of the astragalus became separated 

 from the trochlea by a distinct neck which is wanting in the primitive marsupials. At the same time a 

 closer fitting joint was developed between the astragalus and the tibia, through the upraising of the fibular 

 facet and the strengthening of the internal malleolus of the tibia; the digits became quite long and 

 slender, but claws were still retained, the expanded finger tips not being developed below the Lemuroidea. 



This stage is illustrated in the Lower Eocene Plesiadapidse which, as suggested by Matthew, seem 

 to be near the borderland between Primates and Menotyphla. These animals also had long delicate 

 fingers and grasping hands and feet. Even at the present day the pen-tailed tree shrew Ptilocercus 

 retains extremely primitive hands and feet of arboreal type. 



In short, present indications suggest that the Menotyphla-primate stock may have been derived 

 directly from primitive arboreal or semiarboreal placental mammals of the Mesozoic era and that all the 

 other orders of placentals sooner or later became terrestrial, so that in many Lower Eocene mammals 

 the grasping power of the hallux especially was reduced. 



This provisional conclusion as to the origin of the primates, based on a comparative study of the 

 limbs, is in harmony with the evidence drawn from a comparative study of the skull and dentition, which 

 suggests that even in the Eocene the Primates and the Menotyphla, although at that time closely related 

 with each other, were already widely separated from any other order of the placentals. 



The primitive Lemuroidea are distinguished from arboreal unguiculate mammals of other orders by 

 their very long, slender phalanges and more or less flattened nails. The flexor muscles of their hands 

 and feet are relatively slender and the heads or lower ends of the metacarpals are ball-like. In arboreal 

 unguiculate mammals, on the other hand, such as Cercoleptes, the phalanges are very shoi't and wide, 

 the claws are very large, compressed and pointed, the flexor muscles, which are short and powerful, ride 

 above the large paired sesamoids and between the sesamoids there are strongly marked keels on the 

 metacarpals. 



The modern Indrisidse have certain highly specialized characteristics and functions of the limbs 

 and of the hands and feet, which were already evolved in less degree among the Eocene lemuroids. Ob- 

 servers tell us, and their anatomical construction indicates, that the indrisine lemurs make long leaps 

 from branch to branch and at the end of every leap clutch the branches and limbs of the trees with their 

 great hands and feet. They also spend much time in perching quietly among the branches. Hence it 

 is that their limbs, including the wrist and ankle, are self moving compound levers for leaping, while 

 their extremities, especially the hind feet, are so modified for grasping that they look like long narrow 

 mittens, the great toe being set off over against all the others. In Nothardus the long phalanges and 

 partly expanded finger tips, the very large hallux, which is sharply divaricated from the other digits, 

 and many other characteristics throughout the skeleton, constitute certain proof of a protracted course 

 of adaptation to arboreal life. Such specializations for climbing and leaping among the branches could 

 hardly be derived from any other of the specialized modes of terrestrial life which are exliibited by Eocene 

 mammals of other orders, and we are again forced to conclude that the immediate ancestors of the Meno- 

 typhla-primate stock were not terrestrial mammals but arboreal mammals of a more primitive evolu- 

 tionary stage, represented rather by the existing opossum than by the terrestrial insectivores. . 



The relatively high specialization of Eocene lefnuroids has in its turn become the point of departure 

 for a new adaptive radiation or embranchment during the millions of years of post-Eocene time. Just 

 as in countless other groups, many of the csenotelic characters of earlier generations have persisted as 



