There is a well-developed sacrum of three vertebrae for the support of the pelvis or hip bones. The reason for its persistence in animals so thoroughly adapted for life in the water will be understood later. The ilium is slender; it was attached to the sides of the sacrum by ligaments, only, not forming a firm union, but strong nevertheless. The pubes and ischia, the other bones of the pelvis on the under side of the body, like the corresponding bones of the pectoral girdle, were enormously enlarged, forming great flat, bony plates.
Besides these large bony plates of the shoulder and pelvic girdles, the short abdominal region was inclosed by numerous series of strong ventral ribs, that is, overlapping rod-like bones on each side, connected with a central piece. It will be seen that the whole under side of the body, from the base of the neck to the base of the tail, was well protected by bones, rigid and unyielding in front and behind, flexible for a short space below the abdomen; this surface, however, was not flat like the under shell of a turtle, but rounded from side to side.
Fig. 36.—Pelvic girdle from above of Trinacromerum osborni, an Upper Cretaceous plesiosaur: p, pubis; is, ischium; il, ilium.
Many of the characteristics of the limbs of the plesiosaurs are peculiar to themselves; others they had in common with other aquatic reptiles and mammals. The paddles resemble those of the ichthyosaurs more nearly than those of any other reptile, and it was doubtless this superficial resemblance which so long deceived the early anatomists as to the affinities of the two orders. Unlike all other aquatic animals, however, the plesiosaurs have the hind limbs nearly or quite as large as the front ones, and they doubtless were equally effective in function. The humerus and femur are always elongate, though broad and massive. In no other aquatic animals, save the marine turtles, do we find these bones relatively so long and strong; they are very short in the cetaceans, the sirenians, the ichthyosaurs, mosasaurs, thalattosaurs, and the marine crocodiles, in front at least. The strong muscular rugosities of the plesiosaurian bones are very suggestive of powerful swimming muscles.
Fig. 37.—Pelvic girdle of Elasmosaurus:
p, pubis; is, ischium; il, ilium.
Fig. 38.—Paddles of Plesiosaurs: A, right hind paddle of Thaumatosaurus, after Fraas; B, right hind paddle of Trinacromerum; C, right front paddle of same individual; f, femur; fb, fibula; t, tibia; h, humerus; r, radius; u, ulna.
The bones of the forearms and legs, the wrists and ankles are all polygonal platelets of bones, closely articulating with each other. The finger and toe bones have a more elongated, hour-glass shape than those of the ichthyosaurs, resembling more nearly those of the mosasaurs, indicating a greater flexibility than the ichthyosaurs possessed. The ichthyosaur paddles must have been quite like the fins of fishes in function, while doubtless those of the plesiosaurs were capable of a more varied use, as indeed was required of them. Their articulation with the trunk was more of a ball-and-socket joint than in the other reptiles, showing possibility of considerable rotation on the long axis, and an antero-posterior propelling action. The paddles were certainly more powerful than those of any other aquatic air-breathing animals. There were no additional digits, all plesiosaurs having neither more nor less than five in each hand and foot. Hyperphalangy was sometimes carried to an excessive degree, some digits of some species having as many as twenty-four bones, a larger number than has been observed in any other air-breathing vertebrate.