Water Reptiles of the Past and Present

by Samuel Wendell Williston

LEATHER-BACK MARINE TURTLES

The most remarkable member of the Chelonia now living is Dermochelys coriacea ([Fig. 128]), the great leathery or leather-back turtle of the warmer parts of the Atlantic, Indian, and Pacific oceans, the sole member of the family Dermochelydidae. It is the largest of all living turtles and the most thoroughly aquatic of all, whether living or extinct. It sometimes reaches a length of six feet, or half that of the largest known extinct forms, and weighs a thousand or more pounds. Agassiz saw a specimen that he said weighed a ton. Unlike other turtles, it has a carapace quite peculiar to itself, composed of a layer of thin, irregularly polygonal bones forming a mosaic, completely hidden in the thick skin, and entirely free from the skeletal bones beneath them. The larger of these skin bones form seven rows above, which appear in the living animal as sharp keels running the whole length of the shell. On the under side there are five rows of smaller-sized bones, under which there are vestiges of bones representing the normal plastron of turtles. The limbs are powerful, flattened paddles, not unlike those of Eretmochelys, but wholly destitute of claws. The front paddles are much larger than the hind ones; the humerus is long and flattened, and the digits are elongated. The leather-back is a powerful and effective swimmer, going long distances. Its habits are not well known; its food is chiefly fish, crustaceans, and mollusks.

Fig. 128.—Dermochelys coriacea.
(From Brehm.)

So very different is the structure of its shell that some excellent naturalists regard Dermochelys as the equivalent in rank of all other turtles combined, the sole representative of the suborder Athecae, as distinguished from the Thecophora. Dr. Hay, whose authority on fossil turtles is of the highest, believes that its line of ancestry has been distinct from that of all other turtles from Triassic times at least. Others believe that the leather-back is merely a highly specialized form derived from the ordinary shelled type, a descendant of some of the marine turtles of Cretaceous times. In support of the primitive ancestry of the leather-back Dr. Hay offers the following:

“The writer holds the view that the earliest turtles possessed practically two kinds of shell, one purely dermal, consisting probably of a mosaic of small bones arranged in at least twelve longitudinal zones. Each zone probably consisted of a row of larger bones bordered on each side by smaller bones. Each of these bones was covered by a horny scute. The nearest approach to such a dermal shell is in our days seen in Dermochelys. Beneath the skin there seems to have existed a carapace more or less complete, which consisted of a nuchal, a median row of neurals, eight pairs of costals, a pygal, probably one or more supraneurals, and about eleven peripherals on each side. To what extent the neurals and the costal plates had become anchylosed to the neural spines and the ribs respectively, it is now impossible to determine. Nor can we say to what extent the various elements of the carapace had become connected with one another. There was a subdermal plastron which was composed of at least eleven bones.

“According to the author’s view, as time went on the external, mosaic-like shell disappeared in most turtles, while a more efficient armor was developed out of the subdermal elements. In the ancestors of Dermochelys, however, the dermal armor was retained, while the more deeply seated one disappeared, with the exception of the nuchal bone.”

Such a hypothesis as the foregoing satisfactorily explains the extraordinary mosaic shell of the leather-back, and is perhaps an acceptable explanation of the rather strange fact that the horny shields of turtles do not correspond with the bones below them, as might be expected. Unfortunately this hypothesis lacks sufficient proof. About the only evidence that is offered in its support is the existence of a row of bones along the middle line in the Cretaceous Toxochelys, and notably in Archelon, both aquatic forms. It is urged that these bones, the epineurals of Wieland, are really the remains of an external layer that persisted in these turtles. However, they might have been new ossifications, such as we know did occur in not a few of the land tortoises later over the tail and limbs. Aside from Proganochelys in the vast interval of time from the Triassic to the Eocene no other evidence of such an external dermal layer has been discovered. The chief argument against such divergent ancestry of the turtles in two chief lines of descent is the fact that in its other structure Dermochelys shows great resemblance to other sea-turtles of the Cretaceous times—so much resemblance that it seems impossible that the ancestors of the leather-back should have paralleled them in almost everything except the shell.

On the other hand, those who disagree with this view believe that the modern leather-back is the descendant of such Cretaceous marine turtles as Protostega or Archelon, some of which had lost nearly all of the costal plates and had the neurals and marginals reduced. It is urged that some of these early marine forms, after they had practically lost the ordinary bones of the carapace, for some reason or other found a bony shell again necessary for their welfare. Possibly they had become littoral in habit; possibly they again became subject to new and dangerous enemies in their unprotected condition, notwithstanding their great size; perhaps the zeuglodons were among their enemies. Now, as we have seen, an animal never takes a back track and recovers a thing it has once lost. It was impossible for the ancestors of the leather-back again to acquire an orthodox shell, and they forthwith proceeded to acquire quite another kind that would serve the same purpose.

Possibly the truth lies between the variant views, in the theory recently expressed by Versluys: “The shell of tortoises and turtles is formed by a combination of two layers of dermal ossifications, a thecal layer and a more superficial epithecal layer, the latter generally represented by the marginals only. The leather-back is a member of the Cryptodira, and is allied to the other marine turtles. The problem of the origin of the aberrant shell of the leather-back seems to find its solution in the hypothesis that it is a secondary proliferation of the marginals and such other epithecal elements as were present in its thecophorous ancestors.”

In other words, Versluys believes that Hay’s and Wieland’s views of the primitive double layer of exoskeletal bones is essentially correct, but that Dermochelys was derived from later forms in which some of them, only, as in Archelon, had remained. Baur’s contention that “Dermochelys is not the least, but the most specialized marine turtle” seems to have been fully justified.

RIVER TURTLES.
TRIONYCHOIDEA

No reptile is more familiar or more exasperating to the river fisherman than the turtle, variously known as the river, soft-shelled, or mud turtle. It lives, often in great numbers, in most of the rivers, ponds, and bayous of the interior east of the Rocky Mountains, and especially in those of sluggish current and muddy bottoms. It is voraciously carnivorous in habit, feeding upon the smaller fish, mussels, and such other living food as it can capture. With its long, sinuous neck and snake-like head, and soft, mottled skin, it is repulsive enough to most persons, but is especially annoying to the fisherman, since it devours with impunity his bait so long as he feeds it, and can seldom be caught on the hook because of its hard and bony mouth, in which only by good luck will the hook catch. And the luckless string of fish that the fisherman leaves in the water may be almost completely devoured in a few hours by these fiercely predaceous feeders. However, if so annoying while seeking for better game, it in part makes up for the annoyance it causes by furnishing in its own body a not unpalatable food for those who like to eat reptiles.

Fig. 129.—Trionyx, river turtle.
(By permission of the New York Zoölogical Society.)

The river turtles will be readily recognized from the accompanying illustration ([Fig. 129]). They are very flat, covered with a soft, smooth skin, with a long, sinuous neck and a small, snake-like and vicious-looking head which has a protuberant snout with the external nasal orifice at its end. Their feet are webbed and somewhat paddle-like, but always with three stout claws—whence comes the name of the group—on the anterior digits, which are used for burrowing in the mud and excavating holes for their eggs. These turtles burrow more or less in the mud, with the long neck free, lying in wait for their prey, and coming to the surface from time to time to breathe. As the shape of the body and the paddle-like feet would suggest, they are active swimmers and purely aquatic in habit, never leaving the water unless compelled to. They bury their hard-shelled eggs on the shores only a few feet from the water, and leave them to their fate. If the pools in which they live dry up, they burrow deeply in the mud and await the rains and floods. In captivity they feed upon all kinds of food, vegetable as well as animal, and are active and aggressive.

Fig. 130.—Aspideretes, a trionychoid turtle
from the Basal Eocene of New Mexico;
skull from above. (From Hay.)

Because of certain peculiarities, they are usually classed in a separate suborder all their own, the Trionychoidea, especially distinguished from the Cryptodira, which in general they resemble in most respects, aside from the absence of the usual horny dermal plates, in the lack of a marginal row of bony plates around the carapace—not a very important distinction. Less than thirty living species are known, all of them exclusively, or chiefly of fresh-water habit. Six species are known from North America; the remainder inhabit Africa, south of the Sahara Desert, southern India, and most of the East Indian islands; none is known from Australia. No species lives in South America and none is known to have lived there in past times. During Eocene, Oligocene, and Miocene times these fresh-water turtles lived in the region of Europe in great numbers, but for some inexplicable reason they became extinct there and never returned. Nearly seventy species of the Trionychoidea, belonging in two families, are described by Dr. Hay from the Tertiary rocks of North America, more than twice the number now living throughout the world. Some of these were of relatively large size, measuring fully two feet in the length of the shell. And in some places they must have been very abundant. The writer has seen, in the Bad Lands of the Continental Divide, their weathered-out remains so numerous that they might be raked into windrows miles in length along the sloping bluffs, all in small fragments, for their bones, like those of most turtles, are only loosely united by sutures and readily drop apart before fossilization. Their shells may be readily distinguished from those of all other turtles by the granulated, pitted, or sculptured exterior surface, that was covered by the skin in life; other turtles have the surface smooth below the horny shields, the margins of which are marked on the bones by grooves or sulci; the few marine turtles of the past that were probably covered with a soft skin instead o£ horny shields had the shell smooth and much less completely ossified.

Fig. 131.—Aspideretes, a trionychoid turtle
from the Eocene of New Mexico;
front leg. (From Hay.)

As to the origin of the soft-shelled turtles there has been not a little difference of opinion. The earliest ones known in geological history date back only to about the middle of the Cretaceous; perhaps they branched off from the horny-shelled turtles somewhat earlier, but probably not much. There are some, however, who think that this group of turtles was very primitive, perhaps the most primitive, but the writer agrees with Dr. Hay in rejecting this view. Unlike those of all other turtles, the fourth digit in front and hind feet has one or two more phalanges than have other turtles. We have seen that the oldest known reptiles had the digital formula 2, 3, 4, 5, 3 or 4. Most other turtles have the same numbers of bones in the digits that mammals have, that is, two phalanges in the thumb and big toe and three in each of the other digits. The river turtles have a larger number in the fourth digit, either four or five. It seems to be a law that evolution is irreversible, and if so could the river turtles have been descended from forms with a less number of phalanges? But, the skeleton of the Trionychoidea resembles the more specialized turtles in so many ways that one can hardly believe they were all accidental or parallel.

We may then assume that at about the time that the ordinary marsh turtles took to the sea to become marine, others took advantage of the fresh-water ponds and rivers, and in doing so, like the marine turtles, lost their horny epidermal shields, and became thinner in shape, thereby reducing the resistance to the water. Instead, however, of reducing the costal plates over the ribs, they retained them intact and complete for some reason or other, but lost instead the marginal row of bones, unlike the marine turtles which retained them even after they had lost nearly all of the costal plates. Possibly also they regained additional bones in the fourth digit, a sort of hyperphalangy like that of the more strictly aquatic reptiles. Or, possibly, they may have descended from some branch of the turtles which had not yet lost these bones, retaining them because they were still serviceable for swimming. We know nothing yet about the structure of the feet of the early turtles, and it is possible that not all had acquired the reduced phalangeal formula.

In the development of aquatic habits the river turtles do not show the same degree of specialization in the limbs that the strictly marine forms do. The humerus ([Fig. 131]) is a slender bone, with the tuberosities for the attachment of the muscles situated near the proximal end. The radius and ulna are relatively short, and the foot is long. The hind legs, as would be supposed, are less highly specialized as swimming paddles, and are relatively smaller. Nevertheless the Trionychoidea present an interesting type of adaptation to water habits, both in body and in limbs.