Water Reptiles of the Past and Present

by Samuel Wendell Williston

PLESIOSAURIA

It was Dean Buckland who facetiously likened the plesiosaurs to a snake threaded through the shell of a turtle, and the simile was not an inapt one in his day. The vernacular designation of them—long-necked lizards—conveys the same impression of their chief peculiarity, but the name is less applicable than it once was, since recent discoveries have brought to light forms with a relatively short neck.

Though the plesiosaurs are nearly perfectly adapted to an aquatic life, the adaptation was, in many respects, of a very different kind from that of the ichthyosaurs—so very different that we have not yet quite finished conjecturing as to the habits of the living animals. As already suggested in the popular name, the most striking characteristic of the typical plesiosaurs, the one which suggested to Buckland his frequently quoted simile, is the ofttimes enormously long neck, proportionately longer than that of any other known creatures of the past or present. In other truly aquatic animals the neck is actually shortened in the acquirement of a fish-like shape, and the number of bones composing it reduced. In the Sauropterygia the neck is usually longer than any truly land animals ever possessed, the longest-necked forms having as many as seventy-six vertebrae in the cervical region. The elongation of the neck among mammals is always due to an increase in the length of the individual bones, never to an increase in the number from seven, with but a single exception—a South American sloth which has nine cervical vertebrae. The long neck of birds is due both to an increase in the length of the individual vertebrae and to an increase in their number, to as many as twenty-one. But the elongation of the neck among plesiosaurs was very variable indeed; sometimes it was ten or twelve times the length of the head, at other times it was even shorter than the head. And the number of bones composing it was also extremely variable, scarcely any two species having the same, the known extremes being seventy-six and thirteen. In Elasmosaurus platyurus, for instance, the longest-necked plesiosaur known, the head was two feet in length, the neck twenty-three, the body nine, and the tail about seven; on the other hand, in the shortest-necked plesiosaur known, Brachauchenius Lucasi, the head was two and one-half feet in length, the neck less than two feet, and the body about five; the length of the tail is unknown.

Fig. 32.—Skeleton of Trinacromerum osborni, a Cretaceous plesiosaur,
as mounted in the University of Kansas Museum.

Not only was the number of vertebrae so extraordinarily increased in many plesiosaurs, but in the longest necks the vertebrae themselves, as in birds, were more or less elongated, especially the posterior ones, which may be six or seven times the length of the anterior ones. Not only was the neck of such great length in many plesiosaurs, but it also tapered very much toward the head.

The vertebrae are always biconcave, but the cavities are shallow, saucer-like, sometimes almost flat at each end, and very different from the conical fish-like cavities of ichthyosaurian vertebrae.

Fig. 33.—Restoration of Elasmosaurus platyurus, an Upper Cretaceous plesiosaur.

Often the vertebrae are short throughout the vertebral column; sometimes the posterior cervicals and the dorsals are elongated and very robust. The trunk or body proper was never much elongated in the plesiosaurs, having only from twenty-five to thirty vertebrae. The tail was always shorter than the trunk, and it tapered rapidly to the extremity; in some specimens it has been observed to turn up slightly near the extremity, as though for the support of a small terminal fin.

Fig. 34.—Cervical vertebrae, from the side and behind, and dorsal vertebra from in front of Polycotylus, a Cretaceous plesiosaur: az, anterior zygapophysis; pz, posterior zygapophysis, r, r, r, cervical ribs; d, articulation of dorsal rib.

The ribs in the cervical region are short, but so locked together posteriorly as not to permit much lateral motion. They are sometimes double-headed in the neck, sometimes single-headed, but both heads when present articulate or are attached to the body of the vertebrae, distinguishing them at once from those of other animals, except the ichthyosaurs. In the dorsal region the ribs are attached high on the arch to the extremity of the stout transverse processes by a single head, very much as they are in some cetaceans, and quite unlike the condition in any other known reptile. They end freely below, having no attachment to a breast bone or other bony parts. Because of their shape and position as frequently found, the body in life must have been flattened from above downward, and broad; indeed, this shape is quite certain because of the very broad expanse of the coracoids, between the articulations of the front legs.

Fig. 35.—Pectoral girdle of Trinacromerum from above: ic, interclavicle; cl, clavicle; sc, scapula; c, coracoid.

The shoulder-girdle or pectoral arch is strangely unlike that of any other reptiles. There is no breast bone, since the breast bone is a comparatively late development in reptiles, not appearing, probably, until after the plesiosaurs had begun their existence. Taking the place of the sternum, the very large and broad coracoidsjoin each other in the middle, forming a sort of subdermal armor on the under side of the body in front. In some of the largest plesiosaurs these two bones measured together about six feet in length by four in width. Though so very large they are thick only in front between the articulations of the forelegs. The shoulder-blades are much reduced in size and are extraordinarily modified. The blade proper, that is, that part extending backward and upward, is narrow and small, affording but little surface for the attachment of muscles. On the inner side, extending toward the middle in front of the coracoids, there is another projection, often broad and large, to which was attached the clavicles when present, and often this projection met its mate of the opposite scapula in the middle in front of the coracoids in a broad union. The clavicles or collar-bones are small and thin, and sometimes absent; they also are united in the middle posteriorly with the coracoids when the scapula did not intervene. And the interclavicle also is sometimes wanting. Altogether the pectoral bones form a very large, broad, and concave trough inclosing the whole of the under side of the anterior part of the body. This extensive surface must have furnished attachment to stout and strong muscles controlling the downward and inward motion of the paddles.

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.

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.

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.

It is not certain that the plesiosaurs were viviparous, though there are good reasons for the belief that they were. Remains of two embryos were found years ago in England associated in such a way that it is reasonable to suppose they were unhatched young, though embryos have never yet been found associated with skeletons of adults, as have those of ichthyosaurs in numerous instances. Bones of young, often quite young, plesiosaurs, are frequently found in shallow water deposits, and if the young were actually born alive they must have swum freely in the open waters while yet of very tender age. Rather singularly, however, the remains of these young plesiosaurs always occur as isolated bones.

In geological range the plesiosaurs were very persistent, extending through nearly all the Mesozoic. They began their career as fully evolved plesiosaurs, so far as we now know, near the close of the Triassic period, and reached their culmination in the Upper Cretaceous, but survived to the close of that period. In the beginning of their career they were associated with the marine crocodiles and the ichthyosaurs, but outlived them to find companions and probably enemies in the huge and voracious mosasaurs of the later Cretaceous times. At no time do they appear to have been especially numerous, nor does it seem probable that they were ever a dominant type of marine vertebrate life, though their remains occur everywhere that marine deposits of the Jura and Cretaceous are known. Indeed, it may be said with almost certainty that rocks of these ages and of that character everywhere in the world contain fossil plesiosaurs. Their bones have been made known from Europe, Asia, Africa, Australia, and North and South America. From North America thirty or more species have been described from New Jersey, Alabama, Mississippi, Texas, Arkansas, Kansas, Nebraska, Colorado, New Mexico, Wyoming, North and South Dakota, California, etc.

The cause of their final extinction no one knows, nor can we conjecture much about it with assurance. That climatic conditions became unfavorable for them is highly improbable, considering their cosmopolitan habits; they were not discriminating in their environments. After successfully withstanding their fiercest foes, the ichthyosaurs, crocodiles, and mosasaurs, and large carnivorous fishes, it does not seem probable that they would succumb to lesser enemies, though it may be that they were finally attacked successfully, not in the fulness of their strength as adults, but while young, by more insidious enemies. More probably after their long life of millions of years they had grown old, as everything grows old, and had become so fixed and unplastic in their structure and habits that even slight causes were at last their undoing. When we shall have bridged over that still imperfectly known transition period between the great Age of Reptiles and the greater Age of Mammals we shall have learned more definitely some of the causes of the extraordinary revolution in vertebrate life that then occurred. The plesiosaurs went out with nearly all of their kind, the mosasaurs, the pterodactyls, the dinosaurs; and, so far as we now know, their places in the sea, land, and air were not immediately taken by any other creatures.

Fig. 44.—Nothosaurus;
restoration after E. Fraas;
landscape by Dorothy Williston.