Water Reptiles of the Past and Present

by Samuel Wendell Williston

LAND TORTOISES

Perhaps the last of the more noteworthy specializations of the Chelonia, and indeed among the last of the more important specializations of the Reptilia, are the upland tortoises, of which the common “gopher” of the southern states is almost the only remnant in North America. They formed a part of the great hegira of forest and marsh animals to the open prairies, away from the lowlands and water which the turtles had inhabited almost exclusively for millions of years.

Fig. 118.—Testudo sumeirei, a giant upland tortoise.
(From Hay, after Rothschild.)

They began their career, Dr. Hay thinks, at about the beginning of the Cenozoic, that is, with the great development of the mammals, and reached the maximum of their development in the Miocene; and they have been on the decline ever since. In the Northern Hemisphere, at least, the slowly cooling climate throughout the Eocene, and a decided decrease in moisture, brought about the prairies and prairie plants before its close. Just as the horses, rhinoceroses, camels, and other herbivorous mammals took to these open places for the better and more abundant food found therein, so also the lowland tortoises found better food and fewer enemies there, for they are all strictly herbivorous in habit. The mammals became more conspicuous to their enemies when they went into the open, and it was only by the development of speed, more sober coloration, and perhaps greater cunning that they found safety from them. The tortoises were handicapped by low intelligence, and they could not develop speed, for they were not constructed to that end. But they did find protection in their bony shell, which became thicker, higher, and more convex, and with smaller openings. To quote Dr. Hay: “We may suppose that it would be much more difficult for a carnivorous animal to effect an entrance into such a shell than into one depressed, and whose borders may be spanned by the jaws of their enemies.” Perhaps also the highly arched form of the shell gave greater capacity for the lungs, and the tortoises in general, it is said, do have better lung capacity than the more aquatic or lowland types of turtles. Possibly, also, the heavier shell lessened the evaporation of the body fluids, and made the tortoises less dependent upon the vicinity of water. Certain it is that the common box tortoise, of like form and habits, occurs not rarely on the arid plains, far from water.

The neck and legs became fully retractile within the shell; the digits were shortened up, without a vestige of webbing membrane between them; the phalanges were reduced in number to two in each toe, and nearly all the toes have well formed claws. The feet are placed squarely upon the ground, and the body is elevated in walking. They can swim, when by accident they are thrown into the water, only as any terrestrial mammal can.

About forty species of land tortoises are known throughout the world at the present time, though North America, the probable original home of the tribe, has but three, all small. The larger species are all now denizens of islands, especially the Galapagos Islands, where the giant tortoises have long been famous. And many of our living forms have changed but little since Eocene times. In the Oligocene and Miocene they inhabited western North America in enormous numbers. In the Bad Lands of South Dakota one can often see the remains of a dozen or more of these giant tortoises at one time, specimens varying from one to three feet in length of shell. In river deposits, those of the late Miocene or early Pliocene, the writer has seen areas of an acre or more literally strewn with their remains, as though droves of them had been overwhelmed and perished together. About fifty species of these land tortoises are known from the American Tertiary, thirty-two of them belonging to the modern genus Testudo, which comprises the giant tortoises of the Galapagos. The largest known species of the group is one of Testudo from the Pliocene of India, which had a shell six feet in length. Why the larger species became extinct in Pliocene times on the mainland to survive only in the islands is not known; possibly their carnivorous enemies became too cunning and too numerous.

SEA-TURTLES.
CHELONIDAE

The sea-turtles, or Chelonidae comprise five or six living species, inhabitants for the most part of tropical and subtropical oceans, of which the green or edible turtle (Chelone), the hawksbill turtle (Caretta), and the loggerhead (Eretmochelys [[Fig. 119]]) are the best known. They are all thoroughly aquatic in habit, and of large size, from three to five feet in length. The carapace is heart-shaped, and reduced, that is, with large openings between the ribs; the plastron also is reduced and loosely united to the carapace. The neck is short and the head is not retractile within the shell. The temporal region of the skull is roofed over. The four legs form large and powerful flippers, and the hind legs are relatively small. The body is flattened and the tail is small. The aquatic characters of the limbs are seen especially in the broad and strong humerus, with the radial crest for the attachment of powerful muscles situated far down on the shaft; in the relative shortness of the radius and ulna, and the large size of the latter bone; in the flattened carpal bones; and in the great elongation of the digits and the absence of all but one or two of the claws. Unlike the leather-back turtle and the Cretaceous sea-turtles, the carapace and plastron are completely covered with horny shields, from which indeed the tortoise shell of commerce is derived. Except the green turtle, all members of the family are carnivorous.