Water Reptiles of the Past and Present

by Samuel Wendell Williston

Fig. 42.—Restoration of Trinacromerum,
a Cretaceous plesiosaur;
length about ten feet.

As is the case with both the ichthyosaurs and mosasaurs, skeletons of plesiosaurs have been discovered with nearly all their bones in their relative positions, and with impressions of skin and outlines of body made before decomposition. Though our knowledge of the external appearance of the plesiosaurs when alive is perhaps not as full as we could wish, it is sufficient to give us a fairly good conception of what the animals really were. The skin was smooth and bare, without scales or plates of any kind, and Dames has described a terminal or nearly terminal fleshy dilatation of the tail, forming a sort of caudal fin, which may have aided as a steering apparatus. Mounted skeletons are preserved in a few museums, notably the British Museum, the American Museum of New York City, and the museum of the University of Kansas. Many nearly complete skeletons, however, preserved as they were found in the matrix, are shown in various museums.

With these, principal facts regarding the structure, size, and external form of these animals we may venture to draw certain conclusions, or at least to offer certain conjectures as to their habits in life.

Because of the rigid structure of the jaws, united in front and incapable of any lateral movement posteriorly, quite as are the jaws of crocodiles, we are sure that prey of any considerable size could not have been swallowed whole. The crocodiles tear away portions of the flesh of their victims by quick, powerful jerks, and it is very probable that the flat-headed plesiosaurs tore their food apart in the same manner. In these kinds the teeth are much larger and more irregular in size than are those of the long-snouted plesiosaurs, and their use was certainly as much for tearing as for seizing. There are the same differences between the size of the head and the size of the teeth among the various plesiosaurs that there are among the modern crocodiles and gavials. While the crocodiles seize and destroy even larger prey, drowning and tearing their victims to pieces, the gavials are more exclusively fish-eating, for which their small, sharp, and more numerous teeth especially fit them. Their food, of small size, is swallowed entire, and they are comparatively harmless, so far as animals of considerable size are concerned.

The long neck, the thickset body, and short, stout tail are not at all what we should expect to find in quick-swimming animals. We may therefore assume that the motions of the plesiosaurs through the water were more turtle-like than fish-like. The tail, even though provided with a terminal, fin-like dilatation, was of little use in the propulsion of the body, since the range of its movements was restricted; it possibly served in a measure as a steering organ, a rudder. The large, freely movable paddles must have been effective organs of locomotion, and this function accounts for the relatively large size of the posterior pair, and the firm union of the pelvis with the vertebral column through the sacrum. With the hind limbs used as oar-like organs, a firmer union with the skeleton was required than the soft yielding flesh would permit. At the same time this union was ligamentous only, not bony and unyielding, since the limbs were never used to support the body upon the ground; and it is of interest to observe that the ilia are directed, not upward and forward, but upward and backward to the sternum, precisely the position that would be expected with the force or thrust coming from behind, and not below the yielding ligaments. Were the tail longer and more powerful, the hind limbs would have been smaller and weaker, of use chiefly in equilibration, involving the loss of any connection with the vertebral column and the disappearance of the sacrum. It is of interest, finally, to observe that many of the slender-jawed plesiosaurs had a relatively short neck; they were doubtless more distinctively fish-eating in habit, and possessed greater speed. That the limbs of plesiosaurs were powerful propelling organs is also conclusively proved by their structure. Quite unlike all those animals whose locomotion in the water is chiefly effected by the tail, the humeri and femora, the upper arm and thigh bones were elongated, and not shortened. They form the rigid and stout handles of oars whose blades are the thinner, flexible forearm, wrist, and fingers, or the corresponding foreleg, ankle, and toes. No other purely aquatic reptiles, save the turtles, which likewise are of the oar-propelled type, have elongated arm and thigh bones.

Textbook illustrations of the plesiosaurs usually depict the necks, like those of the swans, freely curved, and a popular scientific article in one of our chief magazines a few years ago depicted one of them with the neck coiled like the body of a snake. One noted paleontologist, indeed, not many years ago described the plesiosaurs as resting on the bottom in shallow waters with the neck uplifted above the surface viewing the waterscape! And when we consider the fact that some species of the elasmosaurs had a neck not less than twenty feet in length, such a flexible use of it would not seem improbable. But the plesiosaurs did not and could not use the neck in such ways. They swam with the neck and head, however long, directed in front, and freedom of movement was restricted almost wholly to the anterior part. The posterior part of the neck was thick and heavy, and could not have been moved upward or downward to any considerable extent and not very much laterally. From all of which it seems evident that the plesiosaurs caught their prey by downward and lateral motions of their neck, rather than by quick swimming.

Fig. 43.—Gastroliths and bones of an undetermined plesiosaur from the Lower Cretaceous of Kansas.

About thirty years ago, the late Professor Seeley, a well-known English paleontologist who devoted much attention to the study of these reptiles, found with the remains of a medium-sized plesiosaur nearly a peck of smoothly polished, rounded, and siliceous pebbles. He believed that their occurrence with the skeleton was not accidental, but that they had been intentionally swallowed by the animal when alive, and formed at its death a part of its stomach contents. Even earlier than this the same habit had been noticed. Nearly at the same time that Seeley mentioned the peculiar discovery he had made the present writer found several specimens of plesiosaurs in the chalk of western Kansas with which similar pebbles were associated, an account of which was given soon afterward by the late Professor Mudge. Since then numerous like discoveries have made it certain that the plesiosaurs usually, if not always, swallowed such pebbles in considerable quantities, for what purpose we do not yet feel sure; one can only hazard a guess. The small size of the pebbles, or gastroliths, as they have been called, a half-inch or less in diameter, found with skeletons of large size, indicate much more complete digestion of the hard parts of their food than is the case with many other reptiles; no solid substance of size could have passed out of the plesiosaur stomach, and such is the case with the modern crocodiles, which have a like habit of swallowing pebbles. That the plesiosaurs picked up these siliceous pebbles, sometimes weighing a half-pound, accidentally with their food is highly improbable; they surely had something to do with their food habits. It is not at all unreasonable to suppose that the plesiosaurs, because of their comparative sluggishness, fed upon anything of an animal nature, whether living or dead, which came in their way; that carrion, squids, crustaceans, and fishes were all equally acceptable; they were probably largely scavengers of the old oceans. Barnum Brown found among the stomach contents of a plesiosaur fragments of fish and pterodactyl bones, and cephalopod shells. Gallinaceous birds, most of which have the same pebble-swallowing habit, have a thick-walled muscular stomach or gizzard, in which the pebbles serve as an aid in the trituration of food. Modern crocodiles, with the same pebble-swallowing habit, have a thick-walled muscular stomach, gizzard-like, though of course not as large as in birds; and the same habit has been noted by Des Longchamps in the ancient teleosaur crocodiles.

It is hardly possible yet to decide whether or not the plesiosaurs were denizens of the open oceans for the most part, far from land. That many of them were rovers is quite certain. With the skeleton of a large plesiosaur found some years ago in western Kansas, there were many siliceous pebbles which could have come only from the shores of the old Cretaceous seas about the Black Hills, hundreds of miles distant. Some of the pebbles are red quartzite, quite identical with that of the bowlders brought to Kansas millions of years later by the glacial drift from outcroppings near the northern line of Iowa. The bones of plesiosaurs are often found in deposits believed to be of deep-water origin. But they are also found in Kansas associated with the remains of small turtles, flying reptiles, and birds which could only have lived near the shores. Indeed, their remains have often been found with those of strictly fresh-water animals which had been brought down by the floods to the seas. Their wide but rather sparse distribution in all kinds of marine sediments would rather indicate that they were at home far out in the tempestuous ocean or near the shores in protected bays, though probably they preferred the shallow water littoral regions. One conclusion is quite justified: they were not gregarious, as were the ichthyosaurs.