PECTORAL OR SHOULDER GIRDLE
Those bones which form the framework for the support of the anterior extremity in vertebrate animals are known collectively as the pectoral girdle. In our own skeleton there are but two on each side, or four in all, the scapula or shoulder-blade, and the clavicle or collar-bone. A third bone, however, is represented in all mammals by a mere vestige which early unites with the scapula and is called the coracoid process. In the lowest forms of mammals, the Monotremata, of which the Ornithorhynchus and Echidna are the only examples, not only is this coracoid bone largely developed, articulating with the sternum or breast bone, but there is an additional coracoid bone in front of this; and there is also an interclavicle. Indeed, the pectoral girdle in these mammals is more primitive or generalized in structure than it is in any living reptiles, composed of scapula, coracoid, metacoracoid, and clavicle on each side and an interclavicle in the middle. No living reptiles have the metacoracoid, and, as is the case with many mammals, some reptiles have no clavicles.
Primitively, that is, in all the old reptiles, the girdle is composed of scapula, coracoid, metacoracoid, clavicles, and interclavicle, while in some of the very oldest there is yet another bone, more or less of a vestige, derived from the ancestral amphibians and called the cleithrum or supraclavicle. The scapula is more or less elongated in crawling and climbing reptiles; more slender and bird-like in those which walked erect after the manner of birds and mammals; shorter and more fan-shaped in the swimming reptiles, as we shall see. In some pterodactyls, unlike all other known animals, the scapula articulated at its upper end with the backbone, giving a much firmer support for the anterior extremities. Only in those reptiles allied to the ancestors of the mammals has the scapula ever had a spine or projection on its dorsal side.
Fig. 18.—Cacops, a Permian stegocephalian, ancestrally allied to the primitive reptiles, with rhachitomous vertebrae and large cleithrum above the scapula.
Of the two coracoid bones in the original pectoral girdle the posterior one began to disappear early and is entirely lost in all reptiles that lived later than Triassic times, though it still persists in the lowest mammals, as we have seen. In most later reptiles the remaining coracoid has become less firmly attached to the scapula than it was in the older reptiles. It usually has a small foramen piercing it near the middle of the upper border or end, the supracoracoid foramen. The clavicle, while more constant among reptiles than among mammals, has been lost in some, the Crocodilia, for instance, as also the dinosaurs and pterodactyls. The interclavicle is more constant in reptiles, a more or less T-shaped bone underlying the coracoids where they join, or the breast bone; but there were some reptiles that lost it, the dinosaurs and pterodactyls, for instance. In the turtles both the clavicles and the interclavicle form a part of the under shell or plastron.
Fig. 19.—Scapula (sc), coracoid (cor), and metacoracoid (mcor) of Dimetrodon
The cleithrum is known in only a few of the old reptiles; it is a more or less slender bone which lies along the upper front margin of the scapula, articulating at its lower end with the upper end of the clavicle on each side.
The breast bone or sternum, while not properly a part of the pectoral girdle, may be mentioned here. In reptiles it is rarely well developed or even ossified, the flying reptiles known as the pterodactyls being the most notable exceptions. It was a comparatively late development in this class, the earliest ones not possessing it even in a cartilaginous condition. It was doubtless evolved from the more or less numerous and slender ossifications on the under side of the body called ventral or abdominal ribs, after the coracoids had become reduced and more slender. Whenever it is present the coracoid articulates with it on each side in front. In most lizards it remains as a cartilage throughout life.
Fig. 20.—Clavicles and interclavicle of Ophiacodon, a theromorph reptile from the Permocarboniferous of New Mexico.