Mammalia

But in the extinct mammal-like reptiles of the Permian and Triassic of South Africa and Russia the skeleton of the pectoral and pelvic girdles and limbs progressively approaches the mam malian type. Their limbs were adapted for running rather than crawling; the body was lifted well above the ground and the trackway became narrower so that the feet gave a more direct support to the weight. In the later mammal-like reptiles the feet were small and short, and from the reduction in the number of phalanges or toe-joints, to the mammalian number (three in each toe, except the first, which has two) we may infer that the fore feet were at least partly "digitigrade," i.e., raised off the ground at the wrist. So too the humerus of the cynodonts, or higher mammal-like reptiles, approached the lower mammalian types, and from the form and position of its joint surfaces we may infer that the angle at the elbow was opening out toward the mam malian condition.

The Fore-Limb.

The typical mammalian shoulder-girdle ex hibits a distinct advance beyond that of the cynodont reptiles in the following features : (I) complete loss of all parts of the outer shoulder-girdle except the clavicles; (2) the scapula has an ante rior shelf or extension supporting the supraspinatus muscle; (3) the dorsal part of the supracoracoid muscle mass has extended upward on to the scapula to give rise to the supraspinatus and the infraspinatus muscles; (4) the coracoid has become greatly reduced and has lost its contact with the sternum; (5) the an terior coracoid plate has disappeared or become vestigial; (6) the interclavicle has disappeared. All these changes have been associ ated with the raising of the body and the drawing-in of the fore limbs so that the feet could be planted beneath the body. The egg-laying monotremes now the lowest of existing mammals, are still largely reptilian in the anatomy of the shoulder-girdle.

The remaining elements of the primitive mammalian fore-limb, the humerus, radius and ulna, carpus, metacarpus and digits, were derived with only minor changes from those of the cynodonts.

The Hind Limb.

The primitive mammalian ilium has appar ently been derived from that of the cynodonts through the pro nounced narrowing of the region above the root of the tail, accompanying the reduction of the tail muscles. The result of this and other changes has been to narrow and lengthen the gluteal area on the back of the ilium and to extend the area on the front inner surface for the origin of the iliacus muscle, so that the primi tive mammalian ilium has become a narrow trihedral rod, with well defined iliacus, gluteal and sacral planes.

Meanwhile the femur has undergone corresponding changes in the passage from crawling to running habits. In the primitive crawling types the short stout femur projected widely from the body, its head, or surface for articulation with the pelvis, was a broad oval set directly on top of the shaft ; there was a large deep pit for the insertion of the obturator muscles on the under-side of the femur below the head, and a high ridge for the adductor muscles, also on the under-side of the shaft. In primitive mam mals, the femur is long, slender, with a cylindrical shaft, the head is globular, set off at a sharp angle and separated from the shaft by a well-defined neck; there is a large flange (greater trochanter) on the outer upper part of the shaft, the proximal pit (or digital fossa) is small, the primary adductor ridge is lost and there is a process, the lesser trochanter, not found in reptiles.

In primitive reptiles the bend between the lower end of the shank and the foot was not sharp ; the two main bones of the tarsus, the astragalus and calcaneum, were flat, more or less circu lar elements located in the same general plane ; the calcaneum did not project backward to form a heel-bone. In the running foot of the typical mammal, on the other hand, there is a sharp bend between the shin-bone and the instep, the astragalus and calca neum are highly differentiated, the former resting upon the latter, the one forming a pulley for the tibia, the other a heel or lever for the powerful muscles of the shank.

Adaptive Radiation of the Limbs.

Among existing mam mals the monotremes are adapted for burrowing and swimming but these are the habits of refugees from direct, above-ground competition with higher types. Among marsupials the primitive forms were arboreal, much like the existing opossums, and these gave rise to the numerous ground-living forms adapted for run ning, leaping and digging. W. D. Matthew has argued that the little-known primitive placental mammals of the early Cretaceous were also arboreal, inasmuch as many of their descendants in the Lower Eocene had five-toed spreading hands and feet, partly divergent thumbs and great toes, and a primitive skeleton not unlike that of an opossum. However, the evidence for arboreal derivation is less decisive than in the marsupials.