The vertebrae are always biconcave, but the cavities are shallow, saucer-like, sometimes almost flat at each end, and very different from the conical fish-like cavities of ichthyosaurian vertebrae.
Fig. 33.—Restoration of Elasmosaurus platyurus, an Upper Cretaceous plesiosaur.
Often the vertebrae are short throughout the vertebral column; sometimes the posterior cervicals and the dorsals are elongated and very robust. The trunk or body proper was never much elongated in the plesiosaurs, having only from twenty-five to thirty vertebrae. The tail was always shorter than the trunk, and it tapered rapidly to the extremity; in some specimens it has been observed to turn up slightly near the extremity, as though for the support of a small terminal fin.
Fig. 34.—Cervical vertebrae, from the side and behind, and dorsal vertebra from in front of Polycotylus, a Cretaceous plesiosaur: az, anterior zygapophysis; pz, posterior zygapophysis, r, r, r, cervical ribs; d, articulation of dorsal rib.
The ribs in the cervical region are short, but so locked together posteriorly as not to permit much lateral motion. They are sometimes double-headed in the neck, sometimes single-headed, but both heads when present articulate or are attached to the body of the vertebrae, distinguishing them at once from those of other animals, except the ichthyosaurs. In the dorsal region the ribs are attached high on the arch to the extremity of the stout transverse processes by a single head, very much as they are in some cetaceans, and quite unlike the condition in any other known reptile. They end freely below, having no attachment to a breast bone or other bony parts. Because of their shape and position as frequently found, the body in life must have been flattened from above downward, and broad; indeed, this shape is quite certain because of the very broad expanse of the coracoids, between the articulations of the front legs.
Fig. 35.—Pectoral girdle of Trinacromerum from above: ic, interclavicle; cl, clavicle; sc, scapula; c, coracoid.
The shoulder-girdle or pectoral arch is strangely unlike that of any other reptiles. There is no breast bone, since the breast bone is a comparatively late development in reptiles, not appearing, probably, until after the plesiosaurs had begun their existence. Taking the place of the sternum, the very large and broad coracoidsjoin each other in the middle, forming a sort of subdermal armor on the under side of the body in front. In some of the largest plesiosaurs these two bones measured together about six feet in length by four in width. Though so very large they are thick only in front between the articulations of the forelegs. The shoulder-blades are much reduced in size and are extraordinarily modified. The blade proper, that is, that part extending backward and upward, is narrow and small, affording but little surface for the attachment of muscles. On the inner side, extending toward the middle in front of the coracoids, there is another projection, often broad and large, to which was attached the clavicles when present, and often this projection met its mate of the opposite scapula in the middle in front of the coracoids in a broad union. The clavicles or collar-bones are small and thin, and sometimes absent; they also are united in the middle posteriorly with the coracoids when the scapula did not intervene. And the interclavicle also is sometimes wanting. Altogether the pectoral bones form a very large, broad, and concave trough inclosing the whole of the under side of the anterior part of the body. This extensive surface must have furnished attachment to stout and strong muscles controlling the downward and inward motion of the paddles.