Water Reptiles of the Past and Present

by Samuel Wendell Williston

Fig. 106.—Teleosaurus; skull, from above.

These older crocodiles, the Mesosuchia of Huxley, comprise a considerable number of extinct forms which lived as far back as the early part of the Jurassic, and continued nearly to, if not actually into, Cenozoic time, that is, to the Eocene. They differ from all living forms, chiefly in having, not concavo-convex but biconcave backbones, that is, the more primitive vertebrae with which all reptiles began. Nor was the internal opening of the nasal passages so far back in the mouth as in the later forms. In other respects they did not differ very greatly from some of those now living. All the earliest kinds that we know of—the teleosaurs—had a long, slender snout, resembling very much that of the modern gavials. And they were, for the most part, incased in a more complete bony armor, on both the dorsal and the ventral sides; and the front legs were smaller than those of the gavials even. The resemblance of the living teleosaurs to the modern gavials must have been very great, although the heavier bony armor indicates a less exclusively aquatic life. They probably lived more in the shallow waters of the seas near the shores.

Near the close of the Jurassic appeared for the first time, so far as we now know, broad-headed mesosuchian crocodiles, forms having less numerous and stronger teeth, and resembling closely modern alligators. It has been believed that these broad-headed kinds were of later origin than the more slender-nosed teleosaurs, but a moment’s consideration will make it evident how improbable such an evolution must be. The crocodiles must have descended from strictly terrestrial reptiles, and no terrestrial reptiles have a slender nose. That they should have acquired a slender face in adaptation to water habits and then returned to the more primitive land type with a broad face and less strictly aquatic habits is contrary to all our experience in paleontology. From this it is altogether probable that broad-faced crocodiles of later times must have been the descendants of broad-faced kinds that were in existence during all the Jurassic times, but of which we as yet have no knowledge. These broad-faced Jurassic crocodiles were, for the most part, small creatures, much smaller than the teleosaurs even, and smaller than any species of crocodiles now living. Their remains are known only from fresh-water or shore deposits, and are, for the most part, associated with those of land and fresh-water animals. About the time of their first known appearance in geological history, the small mammals and birds had also become more or less abundant, and it was suggested by Owen that these land animals had something to do with the development of the ancient amphibious crocodiles. Perhaps this was the case with respect to their greater abundance and development, and with certain peculiarities of their structure, but that the gavial-like teleosaurs should have come back to the land and reverted to a more primitive form seems quite improbable.

During Cretaceous times, especially in America, numerous forms of these old mesosuchian crocodiles were doubtless in existence, both slender-nosed and broad-nosed, and some of them must have been of considerable size, though none known was as large as some of the late crocodiles. This type, with biconcave vertebrae, continued to live on, in both North and South America, to the latter part of Cretaceous times, and it is even possible that some continued on into the Tertiary. But long before the close of the Cretaceous, the modern kind appeared, those with concavo-convex vertebrae, and more posterior internal nostrils. The earliest are known from New Jersey (Thoracosaurus, Holops), so like the Borneo gavials of today that they are properly classified in the same family, the Tomistomidae or Gavialidae. If all the later, procoelian type, that is, those with concavo-convex vertebrae, originated from a single form when the amphicoelian or mesosuchian type became extinct, Huxley’s classification into the Mesosuchia and Eusuchia would perhaps be proper, but we have much reason to suppose that the change in the kinds of vertebra and in the position of the nostrils was only incidental, and may have occurred in more than one line of descent, that is, it may have occurred in the broad-headed kinds of the Jurassic to the broad-headed crocodiles of today, as also in the gavial-like forms of the Cretaceous to the gavials of the present. And this is the reason why naturalists no longer recognize the classification of Huxley, which, partly perhaps because of the prestige of his name, has so long been accepted in our chief works on natural history.

MARINE CROCODILES,
THALATTOSUCHIA

While the ancient crocodiles of which we have spoken resembled the modern ones so closely in form of body and probably in habits, there were certain others of the old Jurassic seas which departed so widely both in structure and in habits, from their associates that they are by some authors given a place wholly by themselves as a distinct group. This has been called by Professor Fraas the Thalattosuchia, a word meaning “sea-crocodiles.” They were a very early side-branch from the great genealogical tree of the Crocodilia, a branch which departed so widely from their associates in adapting themselves to a peculiar and aberrant mode of existence that they cannot be considered as typical crocodiles, although so closely related to them in other respects that therecannot be the least doubt regarding their ancestry. None of the crocodiles which we have considered, whether ancient or modern, can truthfully be called purely aquatic. They never ceased to use their limbs for locomotion on land, more or less of the time, or for the support of the body; and many of them have subsisted, in part at least, on land animals. How easy it may have been for some of them to become almost wholly emancipated from land habits we may easily conjecture. The gavials, as we have seen, are more at home in the water than upon land; their food is chiefly found in the water; but, so long as their habits restrict them chiefly to fresh-water, or to the vicinity of the shores, their limbs continue to be used as much for crawling as for swimming. Were the gavials to be driven out to sea by the stress of fresh-water conditions or attracted thereto by a greater abundance of more easily obtainable or better food, so far from land that they no longer would have much use for their legs for the support or propulsion of their bodies, in the course of time they would doubtless change to a more purely aquatic type. And in that change there would be material modifications of their structure: their limbs would become better adapted to movements in the water; the skin would become smoother, and the bony and horny scales would be lost, since they would be not only useless in the water, but actually detrimental to the well-being of the animals; and the tail would develop into a more powerful organ of propulsion, as a means of increasing their speed in obtaining food or in escaping their enemies, such as the sharks. In fact, we can only imagine that in the transformation precisely those modifications would occur which we actually find in these old sea-crocodiles, the Thalattosuchia. And they are of especial interest to us here because nowhere do we find a better example of Nature in the act of transforming a terrestrial or subterrestrial animal into a truly aquatic one.

Fig. 107.—Geosaurus, an Upper Jurassic thalattosuchian crocodile, drawn from restoration and figures by Fraas.

The group comprises only a few forms, so far as known. All were of modest size among extinct reptiles, from ten to twenty feet in length, and all are from the Upper Jurassic deposits of Europe. They did not exist very long, probably because they found it impossible to discard old habits and old structures entirely and become absolutely emancipated from the land; their breeding habits possibly were too deeply impressed into their structure readily to change, as did those of other sea-reptiles. Some of their remarkable aquatic adaptations have long been known, but only within a dozen years has our knowledge of them become at all complete. Three or four genera have been described, but only a few forms are well known, of which Geosaurus may be taken as most typical. To this we shall confine our descriptions.