GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



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and mastoid; superior semicircular canal near the rim of the subarcuate fossa; mastoid not inflated, 

 forming a prominent tuberosity. Vertical plate of palatine separated by a notch from alveolar region 

 of maxilla. Basioccipital condyles extended vertically on occiput. Occiput triangular in general out- 

 line. All cranial foramina of primitive placental type. Foramen pterygospinosum present (piercing 

 pterygoid plate of alisphenoid) . Brain macrosmatic with large olfactory lobes, small frontal lobes and 

 small cerebellum. 



From present evidence it seems probable that these characters, or the great bulk of them, will also 

 be found to describe the common Paleocene stem of all the divisions of the primates. 



SUMMARY OF THE KVOLFTIOX OF THE LIMBS OF PRIMATES 



At least as far back in geologic time as the Lower Eocene, certain primitive Primates (Pelycodus) 

 which were the direct ancestors of Notharctus exhibited the same general type of limbs as that described 

 above. Let us consider briefly the probable origin and evolution of this type. Many millions of years 

 earlier, in Triassic times, the remote ancestors of the mammals probably arose from cynodont reptiles 

 with very short hands and feet and stout limb bones. The existing monotremes may possibly be the 

 remote descendants of these cynodonts, or, more probably, they may have been derived from some of 

 the Therapsid group, such as the deinocephalians or the anomodonts which they resemble in many char- 

 acters of the skull and skeleton. If the monotremes ever passed through an arboreal stage they have 

 retained no clear trace of it at the present day, and more probably they branched off before the arboreal 

 life was assumed. During the long ages of the Mesozoic era the ancestors of the existing marsupials 

 gradually became adapted to arboreal life. They lost the su1)reptilian character of the pectoral and 

 pelvic girdles that have been retained by the monotremes, and especially by sacrificing the bony connec- 

 tion between the coracoid and the sternum and by making a movable joint between the clavicle and 

 the sternum, they gained great freedom of the forearms, which was a distinct gain in climbing. 



Meanwhile the pads and friction ridges on the palms and soles and beneath the digits became segre- 

 gated, differentiated and located at advantageous points, so as to increase the grasping power of the manus 

 and pes. The pollex in the forefoot and still more the hallux on the hind foot became divaricated from 

 the rest of the digits; the hallux especially increased in size and power of flexion and adduction. At the 

 same time the muscles, joints, and ligaments of the hands and feet were improved so that the palms and 

 soles could at one moment be spread out very flat and at the next moment drawn together, so as to press 

 the epidermal pads and friction ridges against the branches. 



The movements of the earliest arboreal mammals in climbing were probably cautious and rather 

 slow. A striking and probably very primitive characteristic of the existing opossums (which are sur- 

 vivors of the Cretaceous ancestors of the marsupials) is their deliberate and cautious method of climbing, 

 so that a close observer, Mr. S. H. Chubb, describes them as "flowing" over a surface. They do not 

 fling themselves carelessly and lightly about among the brandies, but grasp each branch very firmly, 

 bringing the strong hand-like hind foot well forward and obtaining a firm grip before letting go with the 

 hands. In climbing downward the tail trails along close to the branches in the rear, ready to seize hold 

 if the feet slip. But the main grasping organs are the hind feet. 



Certain arboreal marsupials on the contrary, especially the flying phalangers of several families, 

 have entirely overcome this cautious timidity in climbing and take long leaps and gliding flights. So 

 also we may infer that after the primiti\'e marsupio-placental stock had learned to climb cautiously, 



