Water Reptiles of the Past and Present

by Samuel Wendell Williston

MOSASAURS

At St. Pietersberg, a small mountain in the vicinity of Maestricht, Holland, there are immense subterranean stone quarries, which have been worked for more than a thousand years. The stone quarried from them is a sandy limestone of Upper Cretaceous age containing many well-preserved remains of extinct animals that have long been sought by collectors of fossils. In 1776 Major Drouin—an officer of a near-by garrison, one of much military importance in those days—secured from one of these quarries some bones of an extinct reptile, which, though of interest, afforded but little information concerning the structure and affinities of the animal to which they had once belonged. In 1780 a very perfect skull, in excellent preservation, of the same kind of an animal was obtained from the same quarry by Dr. Hofmann, an army surgeon of the same garrison, whose interest in such things had been incited by Major Drouin’s collections. This specimen, so renowned in science, has had a remarkable and eventful human history, in part related by St. Faujas de Fond, a French commissary of the “Army of the North,” and one of the participants:

In one of the great galleries or subterranean quarries in which the Cretaceous stone of St. Pieter’s Mount is worked, about five hundred paces from the entrance, and ninety feet below the surface, the quarrymen exposed part of the skull of a large animal in a block of stone which they were engaged in quarrying. On discovering it they suspended their work and went to inform Dr. Hofmann, surgeon to the forces at Maestricht, who for some years had been collecting the fossils from the quarry, remunerating the workmen liberally for the discovery and preservation of them. Dr. Hofmann, arriving at the spot, saw with extreme pleasure the indication of a magnificent specimen; he directed the operations of the men, so that they worked out the block without injury to the fossil, and he then, by degrees, cleared away the yielding matrix and exposed the extraordinary jaws and teeth, which have since been the subject of so many drawings, descriptions, and discussions. This fine specimen which Dr. Hofmann had transported with so much satisfaction to his collection, soon became, however, a source of much chagrin to him. Dr. Goddin, one of the canons of Maestricht, who owned the surface of the soil beneath which was the quarry whence the fossil was obtained, when the fame of the fossil reached his ears, pleaded certain feudal rights in support of his claim to it. Hofmann resisted and the canon went to law. The whole chapter supported their reverend brother, and the decree ultimately went against the poor surgeon, who lost both the specimen and his money, for he was made to pay the costs of the action. The canon, leaving all remorse to the judges who pronounced the iniquitous sentence, became the happy and contented possessor of this unique example of its kind.

[Translation by Leidy.]

But the canon was ultimately despoiled of his ill-gotten treasure. At the siege of Maestricht in 1795, the famous skull to which Hofmann had devoted so much anxious thought and labor, fell into the hands of the French and was carried off as one of the spoils of war. So widely celebrated had the specimen become during the fifteen years which had elapsed since its discovery, through the writings of several noted scientific men, that the French general commanded his artillerists to spare the house in which it was known to be. The canon, however, shrewdly suspecting that such an unexpected and extraordinary mark of favor was not for his own sake but rather for the sake of the famous fossil, had it removed and carefully hidden in a house in the city. After the capitulation of Maestricht the eagerly sought-for fossil was not to be found, and the offer of a reward of six hundred bottles of wine, so the story goes, was made for its recovery. So tempting was the offer that, ere long, it was brought in triumph to the house of St. Faujas de Fond, by a half-dozen grenadiers, whence it was later transferred to Paris, where it now is.

We may well sympathize with Dr. Hofmann in the loss of his cherished specimen, since, had it not been for his zeal, money, and labor, it would never have escaped the usual fate of such things—complete destruction. But we must remember that St. Faujas de Fond, the recorder of this history, was a Frenchman, and somewhat interested in robbing the reverend canon of it; possibly there is another side of the story which has never been told.

After peace was declared, one has regretfully to add that the canon, not Dr. Hofmann, was reimbursed for it, or so it is said. Cuvier rather naïvely says that it was ceded to the Garden of Plants of Paris, perhaps in the way that many other things are ceded to the conqueror in time of war. The specimen is really a good one, even when compared with many found in recent years, and there is little wonder that the cupidity of St. Fond was incited by it. Casts of it are now or have been in nearly every noted museum of the world, and pictures of it illustrated nearly every textbook of geology published during the first three-quarters of the past century. It had been the subject of considerable controversy even before it came into the hands of Cuvier. Peter Camper figured and described the skull as that of a whale or “breathing fish”; while St. Fond himself later called it a crocodile. Crocodile or alligator skeletons were rare in those days, and St. Fond made a special trip to the British Museum to study one. But it was really Adrian Camper, a son of Peter Camper, who deserves the credit, so often wrongly ascribed to Cuvier, for the recognition of the true nature of the fossil. He insisted that the animal was a lizard allied to the living monitors, an opinion which it will be seen has finally been proved to be correct within very recent years.

In 1808 this famous skull, and all other known remains of a similar nature, came under the observation of Cuvier, the renowned French naturalist and paleontologist, who confirmed the views of Adrian Camper. He fully described and figured all the known parts of the skeleton that had later come to light, calling the animal the great lizard of the Meuse, the river near which Hofmann’s specimen was found. Conybeare, a well-known paleontologist of England, some years later formally christened it Mosasaurus, a transliteration of Cuvier’s phrase, from the Latin Mosa, for Meuse, and saurus, a lizard. For more than half a century Cuvier’s figure of the skull of the original specimen appeared in works on geology over the name Mosasaurus hofmanni, or Mosasaurus camperi. One could wish that the former name for the species might prevail, in recognition of the zealous doctor who was so shabbily treated in his possession of the specimen.

For some years the few specimens discovered by Drouin and Hofmann were all that were known of the mosasaurs. A few others of related forms were discovered in England, and some were reported from New Jersey by early explorers, but there was little published about the mosasaurs till 1843, when Dr. August Goldfuss, a noted German paleontologist, described and beautifully figured an excellent specimen from the United States. This specimen also had a rather eventful history. It was discovered early in the fourth decade by Major O’Fallen, an Indian agent, near the Great Bend of the Missouri River, whence it was transported by him to St. Louis and placed in his garden as a curiosity. It happened that Prince Maximilian of Wied, the famous naturalist, in his travels through the United States, saw the specimen and secured it, taking it to Germany on his return. He presented it to the Museum of Haarlem where Goldfuss saw and described it. Rather oddly, this specimen was of a species closely allied to the original one of Maestricht, a species which has since only rarely been found. It was called Mosasaurus maximiliani by Goldfuss, though some time previously, it has since been found, some fragments of the same species were described by Harlan, an American author, under the name Ichthyosaurus missouriensis. Goldfuss’ paper was strangely overlooked by subsequent writers, and it was not till the discovery of numerous remains of mosasaurs by Leidy, Cope, and Marsh in the chalk of western Kansas, nearly thirty years later, that much was added to the world’s knowledge of these strange reptiles.

Fig. 69.—Skeleton of Plalecarpus as mounted in the Paleontological Exhibit. Walker Geological Museum, University of Chicago.

Perhaps nowhere in the world are the fossil remains of marine animals more abundantly and better preserved than in these famous chalk deposits of Kansas. The exposures are of great extent—hundreds of square miles—and the fossil treasures they contain seem inexhaustible. Long-continued explorations by collectors have brought to light thousands of specimens of these swimming lizards, some of them of extraordinary completeness and perfect preservation, so complete and so perfect that there is scarcely anything concerning the mosasaurs which one might hope to learn from their fossil remains that has not been yielded up by these many specimens. The complete structure and relations of all parts of the skeleton, impressions of the bodies made in the soft sediments before decomposition had occurred, the character of their food, the nature of the skin covering, and even some of the color markings of the living animals have all been determined with certainty. Not only from Kansas, but also from many other parts of the world, have remains of these animals been discovered, until now it may truthfully be said that no other group of extinct reptiles is better represented by known fossil remains than the mosasaurs. From England, Belgium, Russia, and France in Europe; from New Jersey, Georgia, Alabama, Mississippi, Texas, New Mexico, Colorado, Kansas, Nebraska, the Dakotas, Wyoming, and other places in the United States; from New Zealand and South America they have been obtained in greater or less abundance and perfection.

Fig. 70.—Tooth
of Tylosaurus.

Their geological history is relatively brief, notwithstanding their wide distribution over the earth in such great numbers and diversity. The earliest are known from near the beginning of the Upper Cretaceous of New Zealand, whence it is believed by some that they migrated to other parts of the world, appearing in North America some time later. They reached their culmination in size, numbers, and variety very soon, and then disappeared forever before the close of Cretaceous time. The largest complete specimen of a mosasaur known measures a little more than thirty feet in length, but incomplete skeletons of others indicate a maximum length of about forty feet. The skulls of the largest species are about five feet long. The smallest known adult skeletons are scarcely eight feet in length. There are now known at the present time seven or eight genera of three distinct types, all belonging to one family, the Mosasauridae, including about twenty-five known species. While a few of the genera are widely distributed over the earth, the species are all of restricted range, indicating, perhaps, non-migratory habits.

The adaptation of the mosasaurs to an aquatic life was very complete, though perhaps not so complete as was that of the ichthyosaurs. The skull is flattened, narrow, and more or less elongate, but large in proportion to the remainder of the skeleton—nearly one-sixth of the entire length; that relative size doubtless is indicative of very predaceous and pugnacious habits. The teeth in the typical forms are numerous, strong, and sharp, conical in shape, and recurved. Not only are there numerous teeth in both the upper and lower jaws, but there are also two rows of strong teeth implanted in the back part of the palate, upon bones called pterygoids, the use of which will be understood later. The teeth were inserted on large, tumid, bony bases, rather loosely attached in shallow pits or alveoli, unlike the teeth of all modern lizards. Such a mode of attachment of the teeth doubtless had some relation to the habits of the animals concerning which we are not quite clear. They were easily dislodged, and, in consequence, of very unequal size, some full grown, some small, and others just appearing above the surface of the gums in the living animals. The frequent loss of teeth and their constant and easy replacement by new ones is a peculiarity of predaceous reptiles, thereby insuring their best functional use.

The external nostrils, of large size, were situated at a considerable distance back of the end of the snout, but not nearly so far back or so near the eyes as were the nostrils of the ichthyosaurs, plesiosaurs, and phytosaurs. Their size and position suggest a use like that of the modern aquatic monitors, as mentioned on a preceding page. The eyes were of moderate size, those of the less purely aquatic forms being directed more laterally than those of species of more distinctly diving habits. They were protected by a stout ring of bony plates, as were the eyes of all truly aquatic reptiles of the past. The ears, also, in most if not all mosasaurs, had a thick cartilaginous ear-drum in place of a simple membrane, evidently, as Dollo has shown, for better protection under undue pressure of the water in deep diving.

Fig. 71.—Clidastes, inner side of right mandible: ang, angular; art, articular; cor, coronoid; pa, prearticular; sur, surangular.

As in all other lizards, the bones with which the lower jaws articulate, the quadrates, were loosely attached at the upper end, permitting great freedom of movement in all directions, more even than the land lizards have. The lower jaws were long and powerful, armed with a single row of teeth on each side, from sixteen to eighteen in number. Just back of the teeth, a little beyond the middle, each mandible has a remarkable joint, quite unknown in land lizards, though a trace of it is found in the monitors, permitting much movement between the front and back parts, both laterally and vertically, though chiefly in the former direction. Furthermore, as in land snakes but not as in land lizards, the front ends of the two sides of the jaws were somewhat loosely attached to each other by ligaments. This looseness of the two sides of the jaws, not only in front but also behind, together with the joint in each, was of the greatest use in swallowing prey, as will be explained farther on.

Fig. 72.—Skulls of mosasaurs. Upper figure, Clidastes, from the side; middle figure, Platecarpus, from below; lower figure, Tylosaurus, from above: an, angular; bs, basisphenoid; c, coronoid; ep, epipterygoid; fr, frontal; j, jugal; l, lacrimal; m, maxilla; na, nasal; oc, occipital condyle; pa, parietal, palatine; pm, premaxilla; pf, prefrontal; pt, pterygoid; po, postorbital; q, quadrate; sp, splenial; sq, squamosal; tr, transverse; v, vomer.

As in most other aquatic reptiles, the neck was short and strong, the vertebrae being less in number than in most other lizards. The trunk was long and slender, more especially so in the surface-swimming kinds, with from twenty-two to thirty-four vertebrae. The tail was long, no longer than the tail of some land lizards, but more powerful, and broader and flatter. It was expanded or dilated more or less toward the free end, that is, with the beginning of a terminal caudal fin, such as the more specialized ichthyosaurs and crocodiles possessed. The vertebrae were procoelous, that is, concave in front and convex behind, like those of most modern lizards and all modern snakes and crocodiles, but quite unlike the biconcave vertebrae of all other aquatic reptiles. This kind of articulation of the backbones gave greater firmness and strength to the spinal column, but decreased the flexibility, and its possession by these animals was doubtless due to their descent from land lizards which had already acquired it. The loss of flexibility, however, was partly compensated by the loss of the additional articulating surfaces of the tail.

Fig. 73.—Platecarpus; occipital view of skull: bo, basioccipital; eo, exoccipital; pf, postfrontal; st, stapes; pt, pterygoid; q, quadrate.

As in all other aquatic reptiles, it is in the limbs that the most striking characteristics of these water lizards or “sea-serpents” are found. The legs were so completely adapted to an aquatic mode of living that the animals must have been practically helpless upon land, able perhaps to move about in a serpentine way when accidentally stranded upon the beaches, but probably never seeking the land voluntarily. The front limbs, like those of all other swimming animals having a powerful propelling tail, were larger than the hind ones, though not very much so. The bones of the first two segments, that is, the arm, forearm, and thigh and leg bones, were all short and broad, resembling those of the ichthyosaurs more than those of any other reptiles, save perhaps the thalattosaurs, discussed below. The articular surfaces of all the limb bones, as in other aquatic animals, were restricted in extent, indicating limited motion between the joints, though doubtless having great flexibility. In the most specialized types, such as Tylosaurus, the wrist and ankle bones were almost wholly cartilaginous, just as they are in the water salamanders, and in whales and porpoises. This tendency of the ends of long bones, the wrists and ankles as well as other bones of the skeleton, to become more cartilaginous, or less well ossified, in animals purely aquatic in habit is a marked one. So much is this the case that paleontologists always suspect water habits in reptiles showing it, even though but few parts of the skeleton are known.

Fig. 74.—Clidastes;
left front paddle: c, coracoid;
h, humerus; r, radius;
sc, scapula; u, ulna.

Fig. 75.—Tylosaurus;
left front paddle: c, coracoid;
sc, scapula; h, humerus;
r, radius; u, ulna.

Increase in the number of bones of the digits is a more or less conspicuous characteristic of all mosasaurs. In those forms in which the wrists and ankle bones had become cartilaginous in great part, as many as eleven phalanges have been observed in the longest toes, though in other forms, those with more completely ossified wrists and ankles, only two or three additional bones have been developed in the longest fingers and toes by aquatic habits. The pliability and flexibility of the fingers and toes were certainly very great, but they could not possibly have been flexed or bent so as to grasp or seize anything; and of course all vestiges of claws had disappeared. Many specimens have been found with all the bones of the limbs, that is, the “paddle bones,” in the positions they occupied when the animals died. Figures of three such specimens, made from photographs or careful drawings by the writer, are shown herewith ([Figs. 74-76]). In several such specimens very clear impressions of the smooth membranes between the fingers have been observed, and in one specimen preserved in the collections of the University of Kansas the outline of the fleshy parts connecting the paddle with the body has been preserved.

Fig. 76.—Platecarpus; right front paddle: h, humerus; r, radius; u, ulna.

It will be seen by comparison of the figures of the mosasaur paddles with those of the ichthyosaurs and plesiosaurs that there was a wide difference in their structure, though all have the characteristic shortening of the limb bones and increase in the numbers of the finger and toebones, that is hyperphalangy. It is probable that these differences mean a more powerful and varied use of the limbs in the mosasaurs. It is certain that the mosasaurs were much more predaceous and pugnacious in their habits than were any other truly aquatic backboned air-breathing animals of the past or present. They were the “land sharks” of the ancient seas, and probably the only ones among water reptiles that would be dangerous and offensive to man, were they all living today.

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.

It is a rather remarkable fact that, among the thousands of specimens of mosasaurs which have been collected during the past forty years in both Europe and North America, there never has been found one of a very young animal. Of almost all other animals occurring abundantly as fossils some specimens are sure to be discovered of young and even embryonic individuals. It is quite certain that all such voracious monsters as were the mosasaurs did not die of old age. Some specimens, it is true, have been found that were evidently not full grown animals, but the observed differences in the size of the fossil bones are not great. All are of adult or nearly adult animals. If the mosasaurs were oviparous, as were the ichthyosaurs, and probably the plesiosaurs, and as are some living land lizards, the apparently entire absence of embryonic bones associated with often nearly complete skeletons of the mosasaurs is inexplicable; certainly some mosasaurs must have died a short time before the birth of their young. But embryos have never been discovered, though numerous skeletons inclosing fossilized stomach contents have been found. From this fact it would seem very probable that the mosasaurs were oviparous, as are most other lizards. But this, after all, may be a hasty inference.

No known reptiles lay their eggs in the water. Perhaps there is some reason why the eggs of reptiles and birds, so different from those of fishes and amphibians, cannot hatch in water; and there is no good reason for supposing that the mosasaurs were exceptions to this rule. Unless carefully hidden or protected by the parent, the eggs or very young of the mosasaurs would have been subject to many and grave dangers. Fish eggs are usually small and produced in great numbers, thousands often being extruded from a single female. Among so many there is a greater probability that at least two will hatch and survive to maturity, reproducing their kind. It is unreasonable to suppose that the lizards of the past were more prolific of eggs than are their relatives now living; nor is it possible that their eggs could have been as small as are those of most fishes. Modern lizards seldom lay more than twenty-five or thirty eggs at a time; even the turtles, with their greater vicissitudes, seldom produce more than one hundred. The eggs of the mosasaurs were certainly large and few in number, and the young animals must have begun breathing air immediately after escaping from the shells. If the mosasaurs were oviparous they must have laid their eggs upon the shores and beaches, as do the sea-turtles and the Crocodilia. Nor is it at all probable that the female mosasaurs gave even that protection to their eggs or young that the crocodiles and turtles give. The young mosasaurs, perhaps reaching a foot in length, must have been left entirely to their own devices and their own fate at the very earliest stages of their independent careers.

The waters in which the mosasaurs abounded swarmed with many kinds of predaceous fishes, to say nothing of the hordes of their own kinds, all carnivorous in the highest degree, to all of which the tender saurians must have been choice food. Possibly the shallow waters of the bays and estuaries may have afforded protection to the newly hatched reptiles. It would seem probable that the female mosasaurs went up the rivers for a shorter or longer distance to lay their eggs or give birth to their young, and that the young reptiles remained in such relatively protected places until of a sufficient size to cope with the fierce enemies of the open seas. We know practically nothing of the inhabitants of the lakes and rivers during all the time in which the mosasaurs existed; and this perhaps is the real reason why we have never yet found a specimen of a young mosasaur.

Fig. 79.—Head of Tylosaurus.

We have seen that many skeletons of ichthyosaurs are found entire, and but little disturbed in position, suggesting, if not proving, that the animals as a rule lived and died far out in the deep seas, away from the disturbing effects of currents of water on their decaying bodies. Among the thousands collected, the great majority of the specimens of mosasaurs consist of a few bones or a part of the skeleton only. Moreover, nearly all specimens show the disturbing effects of currents of water; and the bones are usually associated with those of turtles, birds, and flying reptiles, which probably did not often venture far from the shores; all of which goes to prove that the mosasaurs in general lived in the comparatively shallow waters of the seas, and not far from the shores. That some were excellent divers, descending probably many fathoms deep in the water, is certain, because of the extraordinary protective structures of the eyes and ears.

But the various kinds of mosasaurs differed not a little in their habits. Some, like Mosasaurus and Clidastes, were doubtless chiefly surface swimmers, as is evidenced by their better ossified bones, firmer articulation, and the presence of the additional zygosphenal articulations of the vertebrae, wanting in other forms, as also by the structure of their paddles. They had a relatively long body and short tail, the tail having a more pronounced distal expansion than in the case of other forms, and the eyes looking laterally, not at all upward. The feet, as shown in [Fig. 74], were broad and short, with most of the wrist and ankle bones well ossified, and with but few extra bones in the digits. Tylosaurus ([Fig. 79]), on the other hand, had a more slender skull, the nostrils were situated farther back from the tip of the snout, the tail was longer and more powerful, and the feet were very highly specialized ([Fig. 75]). The wrist and the ankle were almost wholly cartilaginous, the fifth finger and fifth toe were much longer, and the number of phalanges was greatly increased. Moreover, the bones of the skeleton are more spongy, the joints are more cartilaginous, and the ears were better protected by a heavy coat of cartilage. In most of these respects the genus Platecarpus was intermediate between Clidastes and Tylosaurus ([Fig. 76]).

Like nearly all other lizards, the mosasaurs had a pineal opening in the skull, but it is not at all probable that they possessed a functional pineal eye.

Many and varied have been the opinions of scientific men regarding the relationships of these animals, as has been intimated. They were thought to be a kind of whale or breathing fish by Peter Camper; crocodiles, by St. Fond; and aquatic lizards, by Adrian Camper and Cuvier. The late Professor Cope thought they were more nearly related to the snakes than to the lizards, and that they might even have been the ancestral stock from which the snakes have descended. Because of this belief he gave to them the name Pythonomorpha, meaning python-like, and this name, really the first ever applied to them, is yet often used instead of Mosasauria. A more complete knowledge of the mosasaurs, however, and especially the recent discoveries of the semiaquatic connecting links, called the aigialosaurs and described on a preceding page, have set at rest all doubt as to their real affinities. They are real lizards, differing less from the living monitor land lizards than do the monitors from some other land lizards, especially the amphisbaenas and chameleons. And to Adrian Camper is due the credit for the recognition of their real relationship, though it required more than a century to prove that he was right.

Fig. 80.—Globidens alabamensis.
Part of mandible, with teeth.
(From Gilmore.)

Very recently, and since the foregoing was written, a remarkable new type of mosasaurs has been discovered in Alabama and Europe. Only fragmentary jaws, a few vertebrae, and some skull bones are known, so that it is impossible yet to decide how closely the new form is related to the true mosasaurs, but so far as the evidence goes the only distinguishable character is the teeth. These, instead of being elongated and pointed, are nearly spherical, as shown in [Fig. 80]. Such teeth could have been used only for crushing shell-fish, and not at all for the seizure and retention of slippery fishes. The genus, which was called Globidens by its discoverer, Mr. Gilmore, includes two known species, from Alabama and Europe, the latter recently described by Dollo. It has been suggested that this peculiar kind of dentition was a more primitive or intermediate one, a kind that the first mosasaurs had before they became fully adapted to the water; but this is doubtful, since Globidens comes from late Cretaceous, and must be one of the later types. If Globidens is a true mosasaur, and it seems to be one, its life-habits must have been remarkably different from those that have long been known. Possibly when the limbs and more of the skull are found, Globidens will prove to be of a distinctive type.