Water Reptiles of the Past and Present

by Samuel Wendell Williston

The number of caudal vertebrae in reptiles is exceedingly variable, from a dozen or fifteen up to a hundred and fifty or more. In snakes but two regions are distinguishable, the caudal and precaudal, and the number altogether may reach nearly five hundred. With the exception of the first few basal caudal vertebrae (pygals) and the minute ones at the extreme tip, all caudal vertebrae of reptiles bear a slender, usually Y-shaped bone below in the interval between the centra, for the protection of the vessels and nerves. Because of their shape they have been called chevrons, and are really outgrowths from the intercentra.

The ribs of reptiles are of more importance in classification than one would suppose. The primitive rib was a slender, curved bone, with the vertebral end dilated to articulate continuously with the intercentral space—that between the centra and the anterior part of the arch. And this is the condition still remaining in the tuatera. Very soon, however, the lower end of the articular surface (capitulum) became separated from the upper (tubercle) by a notch, and the ribs became distinctly double-headed. And this mode of articulation is the rule among mammals. Among later reptiles, however, there were many modifications. In nearly all the head migrated a little backward on the centrum. By the loss of the tubercle in lizards, the head became truly single-headed, and attached solely to the body; and this condition is characteristic of the order Squamata. In another large group the head of the rib gradually migrated up on the arch and on the transverse process (diapophysis), so that both head and tubercle are attached to the diapophysis; and this condition is equally characteristic of the orders of reptiles known collectively as the Archosauria—the crocodiles, pterodactyls, dinosaurs, and phytosaurs. In the Sauropterygia, the ribs are single-headed and attached to the end of the diapophysis. Finally in most ichthyosaurs the capitulum and tubercle both articulate with the body of the vertebra.

Fig. 17.—Ostodolepis, a primitive theromorph reptile. Vertebrae from in front and side, with primitive double-headed rib and intercentrum.

Ribs primitively were probably attached to all the vertebrae to the end of the tail. In the earliest reptiles that we know they are present on all vertebrae as far back as the tenth or twelfth caudal only, those of the caudal for the most part co-ossified with the centra. The ribs of the neck vertebrae more quickly disappeared, or became fused with the vertebrae, and only in the crocodiles among living reptiles are there ribs on the atlas. The sacral ribs, on the other hand, became much larger and stouter and developed an articulation at their outer ends for the support of the ilium ([Fig. 16]).

The so-called ventral ribs are slender ossifications in the connective tissue under the skin, on the under side of the body, and are characteristic of most reptiles. The anterior ones doubtless fused together more or less to form the sternum or breast bone, which was otherwise absent in the early reptiles.

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.