Water Reptiles of the Past and Present

by Samuel Wendell Williston

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.