Water Reptiles of the Past and Present

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.

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.

SNAKES

The chief differences between snakes and lizards have already been given and need not be repeated, save very briefly. Snakes are always functionally legless, though some have vestiges of the hind pair; the brain-case is wholly bony; the upper temporal bar is wanting; the lower jaws are united in front by ligaments only, like those of the mosasaurs; the vertebrae are greatly increased in number, and always have the additional zygosphenal articulations like those of Clidastes and Mosasaurus and some lizards; there is but one lung, and the eyes are always without free eyelids. But these characters are really not very important, since every one of them is found in the lizards or mosasaurs, except the complete ossification of the brain-case, and even this is partly ossified in the mosasaurs. It is rather the presence of all these characters which distinguishes a snake from a lizard.

The number of living snakes is nearly as great as that of the living lizards, and their distribution over the earth is very similar. Snakes are for the most part strictly terrestrial in habit. Some live more or less among trees, and some live in the water, though with but few exceptions all are fully capable of rapid progression upon land. They are almost invariably carnivorous in habit, swallowing their prey whole, and usually alive, as has been described. Some poison their prey or crush it to death before swallowing it. Some feed upon eggs which are swallowed whole and then crushed in their stomachs by projecting bones from the under side of the vertebrae developed for that purpose. In size snakes vary from a few inches in length to twenty-five or more feet, no known extinct forms being larger than the living anacondas and boas. In geological history the earliest remains known date from the latter part of the Cretaceous, and it is quite probable that they have a briefer history than that of the lizards of which they are the descendants. Venomous serpents are known only from comparatively recent geological times, and it is probable that venomosity is the latest and final specialization of importance in the reptilian class.

Of strictly aquatic snakes there is no known geological history, and it is improbable that there is any such history. There are a few snakes now living—very venomous ones, allied to the deadly cobras—which have become so completely adapted to life in the water that they are unable to exist or even move about on land. These are the well-known sea-snakes of the Indian Ocean and adjacent waters. Perhaps the most highly specialized and typical of these is the black-banded sea-snake, Distina cyanocincta, which reaches a length of four or five feet, and is a rapid and excellent swimmer. From the figure (Fig. 81) it is seen that the body is very much flattened from side to side, and lacks or has but a few vestiges of the transverse scales on the under side so characteristic of all other snakes, and which enable them to move about on land. So helpless are these snakes on land that it is said sailors will handle them carelessly, because of their inability to bite while out of water, though the bite is very venomous. They never come on land for any purpose whatever, and their young, unlike those of most other snakes, are born alive. There are a number of species of these sea-snakes, though comparatively little is known of their habits. They are of especial interest as another example of the ways in which air-breathing land vertebrates have become adapted to water life. The adaptation, however, was simple, for nearly all snakes swim freely in water by undulatory movements; it would require not much change to convert an ordinary water snake into one like these sea-snakes.