Fig. 39.—Pectoral girdle (in part) and front paddles of Elasmosaurus (after Riggs): sc, scapula; h, humerus; cor, coracoid; r, radius; u, ulna.
In [Fig. 38 on p. 85] are shown two paddles, the front and hind paddles of a single individual of a very specialized plesiosaur from the Upper Cretaceous of Kansas (Trinacromerum). The long arm and thigh bones are followed by remarkably short and broad bones in place of the elongated forearm and leg bones of the land reptiles. Not only are these bones much broader than they are long, but there have been developed additional bones back of them in the same row—new bones which have no counterpart in any terrestrial reptiles. In the first of the three figures is shown a hind paddle of one of the earliest known plesiosaurs, Thaumatosaurus, from the lower part of the Jurassic of Germany. It will be seen here that the tibia and fibula are much more elongated than in Trinacromerum, and much more like the leg bones of land reptiles. A still more primitive stage in the evolution of the swimming paddle of the plesiosaurs will be seen in [Fig. 48 on p. 99], the possibly ancestral, amphibious nothosaur. Here the tibia and fibula, while relatively very much shorter than in any land reptile, still have, together with all the other bones of the leg, a terrestrial or amphibious type. In [Fig. 39] is seen the front paddles of the long-necked Elasmosaurus, which, though one of the latest of all plesiosaurs in geological history, has the structure of its paddles somewhat intermediate between that of the earlier Plesiosaurus and the later Trinacromerum.
Fig. 40.—Skull of Elasmosaurus from the side: pm, premaxilla; m, maxilla; po, postorbital; j, jugal.
The skull of the long-necked plesiosaurs is surprisingly small in comparison with the remainder of the skeleton, often very snake-like in shape, though very un-snake-like in structure. The short-necked plesiosaurs had often a relatively larger skull, in Pliosaurus, for instance, more than five feet long, sometimes rather broad and short, sometimes remarkably long and slender. The external nostrils were situated far back, very near the eyes, and were very small. The eyes, of considerable size, though by no means so large as those of the ichthyosaurs, were directed laterally, and were provided with a ring of bony sclerotic plates—rather small and weak ones, however. The quadrate bones—bones peculiar to the reptiles and birds—to which the lower jaws are articulated, are, as in the ichthyosaurs and crocodiles, rigidly fixed and immovable. The lower jaws, always rather slender, are firmly united in front, sometimes for a long distance, as in the modern gavials. The teeth of the broad-headed plesiosaurs are long, slender, pointed, and recurved, of a murderously cruel shape; they are deeply implanted in sockets, and number from twenty to thirty on each jaw above and below. There are no teeth on the bones of the palate, such as the mosasaurs possessed. The slender-jawed, gavial-like plesiosaurs have more numerous, but smaller teeth. The surface of the skull on each side behind, for the attachment of the muscles closing the mandibles, is of great extent; in some this surface is increased by a high, thin crest in the middle, as in strongly carnivorous animals, all of which give conclusive evidence of the powerful muscles used in biting and seizing. There is but one temporal opening on each side, as in the ichthyosaurs and the mosasaurs, whereas the crocodiles, thalattosaurs, phytosaurs, etc., have two. The brain cavity of all plesiosaurs is small, though the cavities of the internal ears, the semicircular canals at least, are large. The semicircular canals in vertebrates have little or nothing to do with the function of hearing; they serve rather for equilibration, for the co-ordination of muscular movement; possibly we may infer from their large size in the plesiosaurs that they were not at all clumsy in their movements. There is a large opening for the pineal body, the so-called eye in the roof of the brain cavity, though its possession does not necessarily imply the possession of a functional organ.
Fig. 41.—Skull of Trinacromerum from the side: ang, angular; d, dentary; pm, premaxilla; po, postorbital; j, jugal; sur, surangular.
The Plesiosauria included some of the largest aquatic reptiles that have ever existed, equaled, perhaps, though not exceeded, by some of the extinct crocodiles. The largest known are probably those of the Kansas chalk, or the Jurassic of Wyoming, which probably reached a length of nearly or quite fifty feet, of which the neck formed about one-half. Some of them had paddles more than six feet in length. The head of the largest was about five feet in length, or about the size of that of the largest known ichthyosaurs and mosasaurs. The smallest known adult plesiosaurs were nearly ten feet in length. The teeth of the largest and most carnivorous plesiosaurs sometimes measure four inches in length.
