Water Reptiles of the Past and Present

by Samuel Wendell Williston

The largest and perhaps the best known of all living side-necked turtles is the giant Amazon turtle of South America, which sometimes has a shell nearly three feet in length. Its feet are broadly webbed, and the shell is rather flat in the adult; it is an excellent swimmer in the waters of the Orinoco and Amazon. Six or seven species of the genus to which it belongs are known, all of them South American except one that lives in Madagascar and one fossil found in the Eocene of India. This remarkable distribution is but one more of the many instances known in zoölogy and paleontology that seem to prove an early land connection between India and South America. Had the migration between the two continents occurred by way of Asia and Bering straits, as did that of hosts of mammals, one would certainly expect to find some evidence of it in the North American Tertiary rocks, which, so far, is lacking.

CRYPTODIRA

The chief families of the Cryptodira turtles are the Chelydridae, or snappers; the Emydidae, or marsh tortoises; the Testudinidae, or land tortoises; the Chelonidae, or sea-turtles; the Protostegidae, or ancient sea-turtles; and the Dermochelydidae, or leather-backs. Other doubtful or smaller groups, both living and extinct, may be omitted, or incidentally mentioned.

SNAPPING TURTLES

The family of snapping turtles, the Chelydridae, are of interest because of their peculiar geographical distribution at the present time. Only four species are known, three of them from North America, the fourth from New Guinea. The family is one of the most primitive of living turtles, though no members of it are known with certainty from earlier rocks than the Oligocene. In all probability, also, they have retained, more than have any other group of turtles, unless it be some of the fresh-water tortoises, the primitive habits of the earlier or earliest turtles, though of course there have been modifications, both in structure and in habits. The three species of the United States include two of the snapping turtles proper and the alligator turtles of the southern states, which sometimes reach a length of three feet. All the species are largely aquatic in habit, powerful and active swimmers, with webbed feet and strong claws, and both on the land and in the water they are bold and fierce. They have a relatively large head and very strong jaws. Agassiz saw one bite off a piece of a plank an inch in thickness, and they can usually be raised from the ground by any object which they seize. The carapace and plastron are much reduced, and are rather loosely united. The shell is not large enough for the complete withdrawal of the head and legs within it, and the tail is unusually large and strong. The common snapping turtle, Chelydra serpentina, is found from Canada to Ecuador, and its remains have been found with those of the mammoth and mastodon in Pleistocene deposits; and related species of the same genus have been reported from the Miocene of England.

FRESH-WATER OR MARSH TORTOISES

The family of turtles or tortoises (Emydidae) represented at the present time by the common terrapin, painted tortoise, and box tortoise of the United States, and commonly called fresh-water turtles or tortoises, comprises the largest group of living chelonians—nearly a third of all existing members of the order. They are widely distributed over all parts of the earth except Australia, and are of very varied habits. Some are almost exclusively aquatic; others, like the painted tortoise, are partially so, while others, especially the common box tortoise, are almost as exclusively terrestrial as the true land tortoises, dying even, if forced to live long in water.

The shell in the more aquatic forms is depressed or flattened, but in the terrestrial kinds may be as highly arched as in the true land tortoises. The feet are adapted primarily for walking, but nearly always have the toes webbed, and the digits are longer than are those of the land tortoises. Only the two or three middle toes have claws. Some species have developed hinges in the plastron, whereby they may be completely closed up after the head and legs are withdrawn within the shell. Most of the species are carnivorous in habit, but a few, like the box tortoise, are strictly vegetarian.

Geologically the fresh-water tortoises have a not very ancient history, going back no farther than do the land tortoises, that is, to the beginning of the Cenozoic or Age of Mammals. Fully fifty species are known from the Tertiary rocks of North America, or more than three-fourths as many as are now living upon the earth.

The family at most can be said to be only amphibious in habit, and has no striking aquatic adaptations, since the shell is well developed and is covered with horny shields. The flattened shell of the more aquatic forms is characteristic, as is also the greater degree of webbing between the toes.