Water Reptiles of the Past and Present

by Samuel Wendell Williston

Fig. 100.

Fig. 99.—Skull of Alligator mississippiensis, from below.

Fig. 100.—The same, from above: bo, basioccipital; bs, basisphenoid; f, frontal; j, jugal; l, lacrimal; m, maxilla; n, nasal; p, parietal; pa, palatine; pm, premaxilla; pf, prefrontal; pr, postfrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; tr, transverse.

The neck is short, as has been stated, but it always includes in living forms nine vertebrae, a number probably slightly in excess of that of their terrestrial forbears. By the peculiar mode of attachment of the short “hatchet-shaped” ribs, much lateral movement of the neck is prohibited, nor is any very great vertical movement possible. The vertebrae of the whole column, save the atlas, the second sacral, and the first caudal—which is a very remarkable anomaly—are concave in front and convex behind, agreeing in this respect with those of all other living reptiles, save the turtles, the tuatera, and some lizards. The ribs of the neck have their two heads attached, one to the body of the vertebra, the other to the arch, but those of the dorsal region, though double-headed, have both become attached to the transverse projection of the arch, a seemingly trivial character, but one which immediately distinguishes all crocodiles from all other water reptiles, and from all terrestrial reptiles, indeed, save the Parasuchia, Pterosauria, and Dinosauria. The pelvis is firmly attached to the spinal column by two sacral vertebrae.

Fig. 101.—Vertebrae of gavial from the side (cervical), and from in front (dorsal): az, anterior zygapophysis; pz, posterior zygapophysis; d, diapophysis; r, cervical rib; c, articulation for head; t, for tubercle of dorsal rib.

The collar-bones, or clavicles, are wanting in crocodiles; there is a slender interclavicle; and the shoulder-blade and coracoid are well developed ([Fig. 102]). The bones of the pelvis are loosely united with each other as they are in most reptiles ([Fig. 104]). The pubes, the anterior bones below, unlike those of all other reptiles, do not help to form the acetabulum or socket for the articulation of the thigh bone, nor do they articulate with each other. This single character sharply distinguishes a crocodile from all other reptiles, living or extinct, and is almost the only character that separates the order from the dinosaurs, aside from the peculiar structure of the nasal passages in the skull. On the under side of the body, connected with the front end of the pubes, there are seven or eight pairs of abdominal ribs, corresponding to the plastron of the turtles and similar to those of the ichthyosaurs and plesiosaurs. The mosasaurs have no such ribs.

Fig. 102.