Water Reptiles of the Past and Present

by Samuel Wendell Williston

CHORISTODERA

Among the many reptiles of the past which have sought a more congenial or a safer home in the water few have had a more interesting history, or a briefer one, than those to which the late Professor Cope gave the name Choristodera in 1876. Many students of repute consider the group an order, others a suborder of the Rhynchocephalia. The group, whether order or suborder, are interesting because of their long and devious migrations from western North America to Europe, or vice versa, through rivers and ponds; interesting also because of the persistence of certain old-fashioned traits that clung to them long after their disappearance in other animals. Perhaps these traits were among the causes of their merely moderate success as animals of the water, traits that led to their early dissolution. Like the proganosaurs, which they must have resembled in external appearance not a little, they wandered from their birthplace in the western continent, to perish in the eastern; and like them their span of existence was short.

Their history among mankind, too, is brief. The first known specimens, from western North America, were described by Professor Cope in 1876, under the name Champsosaurus. In the following year Professor Gervais of Paris made known another form from Rheims, which he called Simoedosaurus, so closely allied to the American that even yet they have not been sharply distinguished. Some years later these European specimens were more fully described by the well-known Belgian paleontologist, Dr. Dollo, but it has been only within the past few years that our knowledge of the animals has been made at all complete by the discovery and description of several excellent skeletons of Champsosaurus by Mr. Barnum Brown of New York.

Fig. 87.—Champsosaurus; skeleton,
as mounted in American Museum. (Brown.)

These semiaquatic reptiles never grew very large—not more than four or five feet in length; nor did they ever succeed in becoming fully at home in the water, certainly no more so than our modern alligators and crocodiles. They remained to the end of their comparatively brief existence essentially land animals, probably seeking their food in the water but fleeing to the land for protection and for the breeding of their young. Their chief water adaptations are seen in the elongate face and flattened swimming tail. Their legs remained essentially terrestrial, and could have been of but little use in the water for propulsion; the feet even were doubtfully webbed, or if so, not more than are those of the alligator. Singularly, like the proganosaurs, their ribs were heavy and stout, also suggesting bottom-crawling habits, like those of the living Galapagos lizards. The skull was lightly built, and the face was long and slender, like that of the gavials and proganosaurs; but, like those of the former and unlike those of the latter, the nostrils were situated at the extreme tip. The hind legs were firmly attached to the body by the sacrum; and no sclerotic bones of the eyes have been discovered. The neck was neither unusually long nor unusually short. The body was probably covered with horny scales.

Fig. 88.—Restoration of Champsosaurus.

Fig. 89.—Champsosaurus; skull
from above. (After Brown.)

To the student of paleontology these animals are of interest because of the retention of several primitive traits which had long disappeared in other known reptiles. While the vertebrae had ceased to be perforated by the notochord, as in the early reptiles, they were still shallowly biconcave. The first bone of the neck, the atlas, had changed but little from that of their very ancient forbears of Permian times, and the bones of the palate still retained numerous teeth scattered over it, like those of the same Paleozoic ancestors. Most primitive and old fashioned of all was the pelvis, which was so unlike that of all known contemporary or later reptiles that, were a paleontologist to see it without knowing whence it came, he would be almost sure to say that it belonged to a Paleozoic, or at least a Triassic, reptile, and not only to an early reptile but a very primitive one at that. This peculiarity consists in the absence of any opening between the ischium and pubis, which is characteristic of every living vertebrate with legs. And these and other old-fashioned characters could not possibly have been new developments; they must have existed in all the ancestors of the Choristodera from Paleozoic to early Tertiary times, though not a single other reptile is known to have possessed them, for the greater part of this time. Perhaps when Asia and northwestern America have been more thoroughly explored for vertebrate fossils, some of their ancestors which perished on their great migration from the western to the eastern continent in late Cretaceous times will be discovered.

Fig. 90.—Pectoral girdle of
Champsosaurus.
(After Brown.)

Fig. 91.—Champsosaurus;
pelvic bones.
(After Brown.)

The choristoderans began their existence, so far as is now known, in North America in late Cretaceous times and died out in both Europe and North America in early Tertiary times. That is, they were one of the few branches of reptilian life which not only witnessed the extinction of the great dinosaurs and plesiosaurs, but the advent also of the early placental mammals. They lived millions of years after the proganosaurs became extinct, and, similar as they are in form, there is no relation between them. Moreover, in all probability they did not migrate to the eastern continent over the same route.

The structure of the head and teeth of the Choristodera clearly indicates a fish-eating habit, or at least a diet of soft-bodied, free swimming invertebrates. The legs and ribs, as also the armor of ventral ribs, like those of the plesiosaurs, point very insistently toward a bottom-crawling habit while in the water.