Water Reptiles of the Past and Present

by Samuel Wendell Williston

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.

Fig. 81.—Hydrus bicolor; sea-snake.
(From Brehm)

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.

CHAPTER XII
THALATTOSAURIA

Millions of years before the first appearance of the mosasaurs in geological history, another group of reptiles showing many curious resemblances to them attempted a rather precarious existence in the water. Its members survived long enough to acquire many structural adaptations to a water life, long enough to become diversely modified, but not long enough, apparently, to wander far from their birthplace, not long enough to attain that security from their enemies and more ambitious competitors, the early ichthyosaurs and plesiosaurs, to insure them a long existence. They were only a partial success as water reptiles.

It has been only within a few years that we have had any knowledge whatever of them, and that knowledge is still very incomplete, too incomplete to justify any attempt to picture them as living animals, even though we take the liberties that some of our illustrators of extinct animals feel warranted in assuming. The first known specimens of these “sea-reptiles”—for that is the meaning of the word Thalattosauria—were discovered and described by Professor J. C. Merriam less than ten years ago, and all our knowledge of these animals is due to the same author, who has studied attentively the known specimens, all of which are preserved in the museum of the University of California. The first discovered fragmentary specimens were confounded with those of early ichthyosaurs, from the Upper Triassic rocks of northern California with which they were associated. No specimen has yet been found that is even approximately complete; some parts of the skeleton are not yet known, even from fragmentary remains, and not till other and more complete specimens have been found will it be possible to determine the real form of the living animals or to decide what their nearest relationships with other reptiles were. Professor Merriam thinks that they were related most closely with the Rhynchocephalia ([p. 176]) of which the Sphenodon, or tuatera, of New Zealand is the only living representative, but whose direct genealogical history runs back nearly or quite to the time in which the thalattosaurs lived. On the other hand, there are so many resemblances to the mosasaurs shown in the remains that have been discovered, that it is possible the thalattosaurs were only a short-lived branch of the primitive lizards, which we also know were in existence at the time when the thalattosaurs lived. However, even though they resembled the mosasaurs, there could have been no direct genealogical relationships between them, for it is quite certain that the thalattosaurs very soon went out of existence, leaving no descendants. But it matters little which were the land forbears of the thalattosaurs; they present such distinct adaptations to water life—characters all their own—that their ancestral kinship may well be left to the future researches of the curious paleontologist. For the present, at least, they may well be placed in an order of reptiles all their own, as Professor Merriam has proposed—the Thalattosauria.

Fig. 82.—Skull of Thalattosaurus.
(After Merriam)

No thalattosaurs were large animals. If they had the same proportions between the lengths of head, body, and tail as the mosasaurs, none exceeded seven feet in length, and they may have been even shorter, though probably not much. The figure of the skull, as restored by Professor Merriam, shows many striking aquatic adaptations, in the elongated, pointed muzzle, in the large external nostrils, situated far back toward the eyes, and in the well-ossified ring of bones surrounding the eyeball. There is a parietal opening in the roof of the skull, as in the modern lizards and tuatera; but it is not known for certainty whether there were two openings on each side in the roof of the skull, as in the modern tuatera. While this character may seem trivial, it is really one of the most important in the reptilian anatomy in determining the relationship and classification of reptiles. The teeth are conical and pointed in the front end of the upper and lower jaws, but farther back they are rounded, rugose, and obtuse, and could have been used only for crushing hard objects, like mollusks, crustaceans, etc. ([Fig. 82]). And not only was there a row of such teeth on each jaw (only partly seen in the figure), but similar teeth covered a large part of the palate. And the lower jaws, it is seen, are rather massive.

The vertebrae were, of course, of the more primitive kind, that is, with the ends concave, both in front and behind. It would have been strange indeed were they of any other kind, since reptiles with ball-and-socket joints to the vertebrae, that is, concave on one end and convex on the other, as in nearly all living reptiles, did not come into existence till long after the thalattosaurs had disappeared from geological history; and it is also a curious fact that such vertebrae appear to have originated only among animals crawling on land, so that they would not have been a character acquired by the thalattosaurs after descending into the water. It will be seen from the figure of a dorsal vertebra that the rib was attached by a single articular surface, almost exclusively to the body of the vertebra, quite like those of all lizards, snakes, and mosasaurs, and unlike those of other reptiles. This too may seem to be a trivial character to prove relationships with the lizards, but it is a curious fact that no two animals having different kinds of ribs are closely related to each other. Possibly, however, this looser mode of attachment of the ribs in the thalattosaurs was one of their peculiar adaptations to a water life, and may not have been derived from their land ancestors.

Fig. 83.—Dorsal vertebra
of Thalattosaurus.
(After Merriam.)

Fig. 84.—Thalattosaurus:
bones of front extremity: s, scapula;
c, coracoid; h, humerus;
r, radius; u, ulna.
(After Merriam.)

Of the limbs, only a few bones are known, but these are very instructive. The arm bones, as shown in [Fig. 84], are strikingly like those of the mosasaurs, as will be seen by comparing the figure on [p. 157]. The humerus is a little more elongated than that of the mosasaurs, more nearly like the mosasaurian femur. The shoulder-blade and the coracoid are imperfectly ossified, as is seen from the figure—another characteristic of aquatic life. What the fingers and toes were like cannot be said; probably they were bound together by membrane, forming swimming paddles similar to those of the mosasaurs. Some of the bones referred to the pelvis are known, but it is not known whether they are united to the spinal column by a sacrum, as in land animals. Nor is anything certainly known of the hind leg or much of the tail. Since the front legs show marked aquatic adaptations, it is altogether certain that the hind legs will be found to be modified more or less, though not so much modified as the front legs, because, as we have seen, the front legs are always more specialized in aquatic animals than the hind ones, even as the hind legs are more specialized than the front ones in land animals. Possibly the hind legs will be found to be more like those of the Thalattosuchia, as shown on [p. 212], that is, partly terrestrial in character. Doubtless the tail was long and flattened, possibly with a terminal fin-like dilation, though this is less probable.

As regards the habits and food of the thalattosaurs, no better summary can be given than that of Professor Merriam, in his own words:

The remains of thalattosaurs are known only in purely marine deposits containing little or no material of terrestrial origin. They are associated with a fauna consisting of numerous forms, both vertebrate and invertebrate, which are not known to have existed away from marine areas. In the structure of the skeleton we find the abbreviated and broadened proximal segments of the limbs, the slender snout with prehensile terminal teeth, and the median superior nostrils, indicating a purely aquatic type. There can scarcely be room for doubt that the thalattosaurs as a group were typical marine forms. The larger and more specialized species comprised in the genus Thalattosaurus were strictly natatory. They may have visited the shore, but, like the plesiosaurs, were better fitted for swimming than for crawling. Of the smaller Nectosaurus we unfortunately do not know the limbs. They may have been considerably less specialized, and the animal to a correspondingly greater degree a shore-dweller. Nectosaurus is, however, found in the same deposits with other forms and appears to be as common as the others; so that it is safe to consider it as having passed the greater part of its life away from the shore.

From what we know of the vertebral column of Thalattosaurus it appears that the animal had a relatively short neck and a long dorsal region, the proportions being nearly those in the vertebral column of some mosasaurs. Only the anterior portion of the caudal region is known. The slender, rounded neural spines with well-developed articulating processes seen here are not such as commonly appear in forms with a highly specialized sculling tail, and it is hardly probable that a caudal fin of large size was developed.

The anterior limbs evidently formed paddles of moderate size. The posterior pair may have been larger, in compensation for lack of a strong sculling tail. It is, however, possible, that as in Geosaurus (of the thalattosuchian crocodiles) the hind limbs were not typically natatory, and the distal end of the tail was vertically expanded.

No specimens have yet been found which are well enough preserved to show any remains of the stomach contents, and we have no definite evidence concerning the food of the thalattosaurs, more than is furnished by the general structure of the animal. The character of the paddles, the form of the skull, and the presence of slender prehensile teeth in the terminal portions of the jaws would indicate that they fed in part upon some swiftly moving prey which was caught by a quick snap of the jaws, deglutition being assisted by the curved teeth of the pterygoids. The heavy vomerine and posterior mandibular teeth may have been used for crushing the light shells of ammonites, which existed in vast numbers in the same seas.