Water Reptiles of the Past and Present

by Samuel Wendell Williston

We recognize in Geosaurus an unusually slenderly built crocodile, in appearance very different from all true crocodiles. The smooth, rounded skull, with its greatly elongated and slender snout, and the deep-lying, small eyes, reminds one most of the ichthyosaurs. The skull merges into the slender, elongated trunk without a visible neck, and the body was provided neither above nor below with horny or bony armor, but was, probably, as in the whales, covered with a smooth, soft skin. The anterior extremities, attached far forward, are developed as paddles, and served rather as organs of equilibration than as a means of propulsion, which was the function of the elongated hind legs and the extraordinarily strong and powerful tail, which supported at its end a large fin. The entire impression given of the animal is that of an excellent swimmer, with all the peculiar aquatic adaptations. In the skeleton, however, all the characters of the original crocodiles are preserved. Most remarkable are the laterally placed eyes, protected by the stout sclerotic bones, and the overhanging bones of the orbits. So, too, the large temporal openings of the skull, doubtless due to the absence of the bony plates in the integument, give to the animal a strangely abnormal appearance for a crocodile.

We have observed that all the truly aquatic air-breathing animals, save the plesiosaurs, have either lost the hind legs or else have them greatly reduced in size, and the disproportionately large size of these members in Geosaurus seems inexplicable. But an explanation is not, I think, hard to find. In the adaptation to water life the first to become modified for the control of the body are the front legs. The hind legs never have any really important use when the tail is a powerful propeller. The hind legs of the geosaurs are still essentially legs and not paddles, and they were doubtless used either occasionally for propulsion on land, or perhaps for pushing the body about on the bottom of shallow waters. And the presence of a well-developed ventral armor of bony ribs possibly also indicates more or less of the terrestrial crawling habit. As soon as the hind legs cease to be used for crawling they take on only a feeble use for the equilibration of the body, and speedily become small, until finally they disappear. That the hind legs of these creatures were of some use in the water is certain, because of the modifications in their structure, and especially because of the loss of the claws; but that they were of important use as propellers is hardly probable. We may therefore infer that the thalattosuchians, while distinctively sea-reptiles, had not entirely lost their land habits. Moreover, it is highly probable that their egg-laying habits, which would hardly change with a greater adaptation to water life, compelled the animals recurrently to visit the shores. To have finally lost their hind legs they must have become viviparous in habit, since it seems to be impossible for any true air-breathers to be hatched in water. Perhaps this insurmountable habit was the final cause of their extinction in competition with the truly viviparous aquatic flesh-eaters. The thalattosuchians had but a brief existence in geological history, during the latter part of the Jurassic period only, so far as certainly known, nor did they become widely dispersed over the earth; they are known from Europe, possibly from Brazil.

CHAPTER XVI

CHELONIA

No order of reptiles of the past or present is more sharply and unequivocally distinguished from all others than the Chelonia or Testudinata. No order has had a more uniformly continuous and uneventful history. None now in existence has had a longer known history, and of none is the origin more obscure. The first known members of the order, in Triassic times, were turtles in all respects, as well or nearly as well adapted for their peculiar mode of life as are those now living, and were they now living they would attract no especial attention from the ordinary observer and but little from the naturalist. From time to time some have gone after better things, and have come to grief, but the main line has remained with fewer improvements, fewer evolutional changes, than any other group of higher vertebrates. The turtles seem very early to have adapted themselves so well to their peculiar mode of life, to have intrenched themselves so thoroughly in their own province, that no other creatures have been able to overcome them, or to drive them from it. The remains of no other air-breathing vertebrates are so omnipresent in the rocks as those of the turtles; they may be expected wherever fossils of air-breathing animals are found, though unfortunately often only in scattered and broken fragments. The loose union of their skeletal bones and their general habits of life in shallow waters left their bodies as food for scavengers, or for dismemberment by the tides and currents.

Relationships with other reptiles they really have none. Some have thought that the plesiosaurs were their first cousins, others the Placodontia, an indeterminate group of extinct reptiles usually placed with the Anomodontia. But their relationship with neither of these is closer than with the crocodiles, dinosaurs, or pterodactyls. They are the only reptiles that we know, besides the cotylosaurs, which have no holes in the temporal roof of the skull, and as the cotylosaurs were the most primitive and the oldest of reptiles, this fact incontestably proves that the turtles had a very ancient origin, though we know them no farther back than the later Triassic. They are the only order of reptiles of which not a single member is known to have teeth, or even vestiges of them. Until recently only a single specimen has been known from the Trias, and of that only the casts of the shell; but the shell was as fully developed and as complete as that of a modern alligator snapper, which it resembled much in form and in size. And doubtless the habits of this ancient Proganochelys were similar to those of the alligator snapper. The early cotylosaurian reptiles were all littoral-or marsh-loving animals, and more or less aquatic, and doubtless the early turtles continued in the same environments and with the same habits after acquiring a shell for protection and losing their teeth, which for some inexplicable reason they seemed no longer to need. Until near the close of the Jurassic period probably all turtles were amphibious animals of the marshes, spending much, perhaps the larger part, of the time in the water, good swimmers, and yet good crawlers. With the beginning of the Cretaceous, however, some of them became ambitious for new and untried modes of life. Various ones went down into the sea and became marine animals, reaching the zenith of their prosperity and the maximum of size before the close of the period, but continuing in diminished size and numbers to the present time, if we may consider the leather-back turtle as really their descendant. Others in the Cretaceous took to the rivers and ponds, and became almost as thoroughly aquatic in their thin shape and soft covering; and their lineal descendants still continue in the rivers of the Northern Hemisphere. Still others, in the Age of Mammals, took to the upland, and competed with the mammals in the open places and prairies, reaching their maximum in Miocene-Pliocene times, when for some unknown reason the giants among them were driven from the mainlands to continue a precarious existence to the present time in some of the larger islands.

Fig. 111

Fig. 112