Water Reptiles of the Past and Present

by Samuel Wendell Williston

For a long time it was thought that the mosasaurs had no breast bone, and that, in consequence, the front part of the thorax was expansible. Under this assumption the mosasaurs would have been much more snake-like in habit than they really were. The loose construction of the jaws doubtless permitted the swallowing of prey of considerable size, and the inference was that they habitually preyed upon animals of large size. A snake will often swallow a frog of larger diameter than its own body, the flexible jaws and loosely connected ribs permitting it to pass to the abdominal cavity. But the unyielding ring formed by the anterior ribs connected with the breast bone in the mosasaurs, as in other lizards, conclusively proves that large animals could not have been swallowed whole by the mosasaurs. In several instances the fossilized stomach contents, composed chiefly or wholly of fishes, have been found between the ribs of mosasaurs, and in none were the fishes more than two or three feet in length, though the reptiles were from sixteen to twenty feet long. Possibly the largest mosasaurs, those thirty or thirty-five feet in length, might have captured and swallowed fishes six or seven feet long, but in all probability their usual prey was of smaller relative size.

Fig. 77.—Platecarpus; pelvis, from below: p, pubis; il, ilium; is, ischium.

The very loose construction of the pelvic bones, those to which the hind legs are articulated, is an evidence of more complete adaptation to water life than was or is the case with any other water air-breathers except the ichthyosaurs and cetaceans. The sacrum had entirely lost its function as a support to the pelvis and had disappeared, that is, the vertebrae composing it had become quite like the adjacent ones, by the loss of the ribs connecting them with the ilium. The small pelvis was suspended loosely in the walls of the abdomen, or at the most was feebly connected with a single vertebra by ligaments. It was entirely useless as a support for the legs. The mosasaurs could not possibly have raised their bodies from the ground while on land. It is well known that the land lizards and the crocodiles raise their bodies free from the ground while running or walking; none drags its body over the surface.

Fig. 78.—Photograph of carbonized remains of scales of Tylosaurus.

In several instances complete or nearly complete skeletons of mosasaurs have been discovered with the different bones nearly all in the positions and relations they had after the decomposition of the flesh, together with the carbonized remains of the skin and impressions of the investing scales and membranes. The nature of the body covering is therefore known with certainty from nearly all parts of the body. The body everywhere, save on the membrane between the fingers and toes, and perhaps on the top of the skull, was covered with small overlapping scales, very much like those of the monitors. These scales, however, were small and smooth in comparison with the size of the animals, those of a mosasaur twenty feet in length being almost precisely the size of those of a monitor six feet long. The top of the skull seems to have been covered with horny plates, as in most lizards. In one instance parallel dark bars, obliquely placed, and of narrow width, formed by carbonized pigment, were observed by the writer. As has been stated, in some instances fish bones and fish scales have been observed among the fossilized stomach contents, and it is quite certain that the food of these creatures must have been composed chiefly of fishes, though of course it is not improbable that other small vertebrates, birds, pterodactyls, the young of plesiosaurs, and possibly small mammals, may occasionally have formed a part of their diet. That the mosasaurs were very pugnacious in life is conclusively proved by the many mutilations of their bones that have been observed, mutilations received during life and partly or wholly healed at the time of death. Bones of all vertebrates are repaired after injury by the growth of more or less spongy osseous material about the injured part, forming a sort of natural splint. This material is more or less entirely removed by absorption when it is no longer required for the support of the broken ends. Many such injured bones of the mosasaurs have been found; sometimes the bones of the hands and feet have grown together, and not infrequently the vertebrae have been found united by these osseous splints; occasionally even the skull itself, especially the jaws, attest extensive ante-mortem injuries. In a single instance the writer has observed the loss of a part of the tail, where it probably had been bitten off. It may be mentioned, however, that the bones of the tail had no such “breaking points” in the mosasaurs as have those of many land lizards, whereby a part or all may be lost as a result of even a trivial injury, and then regrown. Such a condition in an organ relied upon entirely for propulsion would have been immediately fatal to the existence of the mosasaurs. The large jaws and teeth are in themselves sufficient evidence of the fiercely carnivorous propensities of the mosasaurs. The constant renewal of the sharply pointed teeth, thereby preventing deterioration by use or accident, preserved, even in the oldest animals, the effectiveness of the youthful structure.

We may now understand how the mosasaurs seized and swallowed their prey. Living constantly in the water, away from all firm objects, with small, short limbs quite incapable of holding struggling prey, and the body not sufficiently serpentine to hold it in its folds after the manner of snakes, the mosasaurs would have found it difficult or impossible to swallow fishes of even moderate size, were their jaws of the same construction as are those of the land lizards. If they preyed upon small animals only, or if they tore their prey to pieces after the manner of the alligators, there would have been no especial difficulty in deglutition. But it is certain that the animals which the mosasaurs devoured were not always small, and they must have been swallowed whole, since their teeth were not adapted, like those of the alligators and true crocodiles, for the rending of bodies. One who has watched a snake swallow a frog or another snake will appreciate the difficulties against which the mosasaurs contended in swallowing fishes a fifth or a sixth of the length of their own bodies. The ordinary snake, no matter where or how it seizes its prey, invariably swallows it head first. Its mandibles are even more loosely united in front than were those of the mosasaurs, and while there is no joint in the snake’s mandibles such as there is in the mosasaurs’, the loose union of the various bones of the mandibles serves the same purpose. The frog or lizard, while firmly held by the slender teeth, is slowly moved sideways by the alternate lateral action of the jaws till the head is reached. Many non-poisonous snakes, if they find it impracticable or impossible to reverse the position of their prey in this way, wrap the folds of their body about it, holding it firmly while they release their mouth-hold and seize it by the head. An amusing instance of these habits came under the observation of the writer not long ago, in Texas. A large “blue racer” (Bascanion), six feet four inches in length, caught an unusually large bullfrog by one hind leg, but in almost less time than it takes to relate, the head of the frog had entered the snake’s gullet and the mouth was closed over it, notwithstanding the vigorous muscular and vocal protests on the part of the frog. Wishing to secure the skull of the snake for his collection, the writer seized an ax standing conveniently by and cut the snake cleanly in two. The peristaltic action of the deglutitional muscles carried the frog slowly on about two feet farther to the ax-made orifice, from which it emerged, and, after a few croakings against such unkind usage, calmly hopped off into the near-by pool of water! Many poisonous snakes release their prey after killing it; other snakes may force their prey down the throat by pressing it against the ground.

Even small fishes could not possibly have been swallowed by the mosasaurs in any other way than head first, since the backwardly projecting, and often long, spines would have rendered any other procedure impossible. Even after the head had entered the gullet, deglutition could have been effectively completed only by the aid of some mechanism whereby the fish could have been pulled or pushed back into the constricting fauces. The strong teeth of the upper jaws and palate held firmly the struggling prey, while the loosely united jaws, bending laterally at the joint back of the middle, either alternately, or more probably in unison, steadily forced it far enough back to be seized by the muscles of the fauces.

The shape of the mosasaurs, though slender, does not suggest extraordinary speed in the water; doubtless most of the fishes that lived in the seas with them could swim faster than they. Their prey was captured, for the most part at least, by sudden and quick lateral movements, for which their powerful and flexible paddles admirably adapted them.