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.

Fig. 119.—Eretmochelys, loggerhead turtle.
(By permission of the New York Zoölogical Society.)

Extinct members of the family are known from scanty remains in Cenozoic and late Cretaceous rocks. From the earlier Cretaceous deposits of the plains more primitive allied forms occur, often classed in distinct families of which Toxochelys ([Fig. 120]) and Desmatochelys are the more noteworthy. The latter genus, especially, might well have been an ancestor of all the modern forms. About three feet in length, it had all the essential characteristics of the sea-turtles, in its thin form, roofed-over skull, reduced carapace, loose plastron, and flipper-like limbs. The single known specimen, preserved in the museum of the University of Kansas, came from the lower rocks of the Upper Cretaceous of Nebraska. Yet earlier, at the close of the Jurassic, there were shore turtles of considerable size that had begun to develop a fondness for the open seas; to acquire a depressed form and lightened shell, the limbs still retaining, however, more of the terrestrial or crawling form. They are grouped as a separate family, the Thalassemydae, and include the first of the Chelonia to depart from the marsh and fresh-water habits which for long ages, perhaps, had limited the activities and evolution of the turtles.

Fig. 120.—Carapace of Toxochelys bauri,
an Upper Cretaceous sea-turtle: ep, epineural.
(After Wieland.)

ANCIENT SEA-TURTLES.
PROTOSTEGIDAE

Fig. 121.—Toxochelys latiremis; front leg: hum, humerus; rad, radius; ul, ulna; int, intermedium; uln, ulnare; p, pisiform; cen, centrale. (From Wieland.)

Fig. 122.—Desmatochelys lowii;
skull from above and below.

Forty-four years ago the late Professor E. D. Cope, one of the greatest naturalists America has ever produced, in almost the earliest exploration of the great Cretaceous fossil deposits of western Kansas, discovered and collected a remarkable specimen of one of the most extraordinary turtles that is known even yet. By an error somewhat natural for those times, when the theory of evolution was just beginning to attain acceptance by naturalists, he thought that the specimen, notwithstanding its monstrous size, represented a very primitive kind of turtle, and gave to it the name Protostega gigas, meaning gigantic first roof! The late Professor George Baur, to whom paleontology owes so much, showed that, far from being a primitive turtle, Protostega was really one of the most specialized types of the order. Professor Cope’s account of the discovery of the specimen is of so much interest that it may be quoted here:

“In the very young tortoise or turtle the ribs are separate, as in other animals. As they grow older they begin to expand at the upper side of the upper end, and with increased age the expansion extends throughout the length. The ribs first come in contact where the process commences, and in the land tortoise they are united at the end. In the sea-turtles the union ceases a little above the ends. The fragments of the Protostega were seen by one of the men projecting from a ledge of a low bluff. After several square feet of rock had been removed, we cleared up the floor and found ourselves well repaid. Many long, slender pieces of two inches in width lay upon the ledge. They were evidently ribs, with the usual heads, but behind each head was a plate-like the flattened bowl of a huge spoon, placed crosswise. Beneath these stretched two broad plates, two feet in width, and no thicker than binder’s board. The edges were fingered and the surface was hard and smooth. All this was quite new, among fully grown animals. Some bones of a large paddle were recognized, and a leg bone. The shoulder-blade of a huge tortoise came next, and further examination showed that we had stumbled on the burial place of the largest species of sea-turtle yet known. But the ribs were those of an ordinary turtle just hatched, and the great plates represented the bony deposit in the skin, which, commencing independently in modern turtles, unite with each other at an early day. But it was incredible that the largest of known turtles should be but just hatched, and for this and other reasons it has been concluded that this ‘ancient mariner’ is one of those forms, not uncommon in old days, whose incompleteness in some respects points to the truth of the belief that animals have assumed their modern perfection by a process of growth from more simple beginnings.”

Later studies by Doctors G. Baur, E. C. Case, O. P. Hay, and especially G. R. Wieland, of the abundant and excellent material, preserved in the museums of Yale and Kansas universities and the Carnegie Institution, and especially the discovery by Wieland in 1895 of an allied and yet larger form which he called Archelon, have determined practically every detail of the structure of this remarkable group of sea-turtles. A surprisingly complete specimen of Archelon is mounted in the museum of Yale University.

Fig. 123.—Archelon ischyros; skeleton from above: n, nuchal, r, r, r, ribs; m, m, peripheral bones; h, humerus; r, radius; u, ulna; t, tibia; fi, fibula. (From Wieland.)

About a half-dozen species and two genera of the family have so far been described, all coming from the Upper Cretaceous deposit of Kansas and South Dakota, the genus Archelon from later rocks than those which have yielded Protostega.

The general form and structure of Archelon will best be understood from the accompanying figures after Wieland ([Figs. 123], [124], [125]) and the restoration of the living animal as interpreted by the writer ([Fig. 126]). If the leather-back turtle, described farther on, is really the descendant of these or allied turtles, as many authors believe, it of course represents the very highest aquatic specialization of all Chelonians. If, on the other hand, as some believe, the leather-back is the end of a long and independent line of descent, then Archelon represents the highest aquatic specialization of all other turtles.

In size, at least, Archelon attained the maximum of the order, reaching a length of more than twelve feet, and a weight of more than three tons. Except that the shell was not heart-shaped or elongated as in all modern sea-turtles, but nearly circular in outline, it had all the aquatic adaptations of the sea-turtle in a yet higher degree.

Fig. 124.—Archelon from below, without plastron: h, humerus; r, radius; u, ulna; sc, scapula; c, coracoid; p, pubis; i, ischium. (From Wieland.)

Fig. 125.—Archelon; skeleton from below: hp, hyoplastron; hpp, hypoplastron. (From Wieland.)

The shell was depressed; the dermal plates covering the ribs had almost entirely disappeared, remnants only of their upper ends remaining; the skull ([Fig. 127]) had the temporal region wholly roofed over; the neck was short and not retractile. The front legs were strong flippers, the humerus was long and stout, with the crest for the attachment of muscles far down on the shaft; the digits were greatly elongated and clawless, etc. The plastron only was less reduced than in the case of the modern sea-turtles. No traces of horny shields have been discovered. As to the nature of the covering and the general appearance of the turtle when alive, Dr. Wieland has kindly given the writer his views, as follows:

“After direct study or fairly close examination of all the fossil material of importance thus far collected representing the Protostegidae, it seems certain that in all the members of the group an external leathery layer was well developed. In no instance is there the slightest trace of horny shield sulci, or grooves; though it seems probable that there was some gradation toward a thin and perhaps even slightly horny hide. In Archelon ischyros the reduced condition of the carapace and the presence of the continuous row of large, median, supraneural elements render it quite certain that there was a development of leathery hide comparable to that of Dermochelys. The same may be said of Protostega gigas. But Archelon Marshii had a less reduced carapace, and the leathery skin was probably less well developed; and Protostega Copei, in which no trace of supraneurals remains, must have made some approach to the horn-shield condition. A more distinct suggestion of transition from the leathery to the horny shield covering may be seen in the very different contemporary Cretaceous form, Toxochelys Bauri, where ossified epi-or supraneurals occupy quite exactly the nodal relation of the five vertebral horn shields of later turtles, like Lytoloma, though there are not the slightest traces of sulci.

Fig. 126.—Archelon ischyros, a gigantic sea-turtle
from the Upper Cretaceous of South Dakota.

Fig. 127.—Skull of Archelon ischyros: pa, parietal; f, frontal; pm, premaxilla; pf, prefrontal; ptf, postfrontal; m, maxilla; j, jugal; qj, quadratojugal; sur, surangular; d, dentary; an, angular. (After Wieland.)

“From a purely anatomical standpoint I have suggested that Archelon had seven dorsal keels corresponding to those of Dermochelys. There is much excellent reason for regarding dermogene ossification as essentially double-layered throughout the Reptilia.

“In any restoration of Archelon ischyros only the mid-line should be accentuated as a series of rather sharp supraneural crests. These are shown to have been present by the characteristic groove-like median pits with radiating striae, which are such a prominent feature of epineurals. It is reasonable to believe that the pits mark the attachment of horny crests developed in the leathery hide. Such were doubtless projected, more or less keel-like, to a height of one or two inches, and thus gave to the mid-line of the carapace, when seen laterally, a distinctly sinuous outline not unlike that of Toxochelys.”

As regards the habits of these ancient sea-turtles, we may offer tolerably certain conjectures. In the opinion of the writer, the less reduced plastron indicates a bottom-feeding habit, a view that is strengthened by the more rounded form of the shell, like that of the river turtle. All in all it would seem that Protostega and Archelon lived habitually on the soft bottoms of the shallower seas, feeding upon the hordes of large shell-fish, for which their powerful parrot-like beak was admirably adapted. That the species of Protostega did not commonly frequent the deeper oceans is indicated by the general absence of their remains in the deeper water deposits. The writer, in a long collecting experience, always found their remains associated with those of the smaller Toxochelys, toothed birds, pterodactyls, and the smaller mosasaurs.

Perhaps no one can speak more authoritatively as to the habits of these gigantic sea-turtles of the Cretaceous than Dr. Wieland:

“With regard to the general habits and appearance of Archelon much might doubtless be said if the present-day sea-turtles were more familiar objects. Dr. Hay thought that Archelon ischyros was a clumsy or even a sluggish, mainly littoral animal, moving slowly about the bottom of quiet inlets in quest of shell-fish; I, on the contrary, much struck by the powerful flippers, and especially by the flattening of the humerus, with its low radial crest and obviously strong musculature, have held that unusual swimming power and adaptation to a strictly marine life were indicated. Perhaps, as usual where experts differ, it is probable that both views are in part correct, and that Archelon was only a moderately good swimmer. It may be noted that, notwithstanding the almost circular body, the femoral notch, that for the hind leg, lies far back, so that it is not necessary, on the score of bulk, to assume slowness of motion, or the inability to pursue a sea-going life. Furthermore, it is now known that the development of the digits fell little short of that seen in Colpochelys ([Fig. 117]) or Eretmochelys, truly marine turtles.

“Therefore, while there can be no doubt that Archelon was strictly carnivorous in habit, and well able to navigate the open seas, it is not likely that it fed on other than relatively slow-moving prey. Lydekker looked upon the broad mandibles and broad palate of Lytoloma as specializations for a mussel diet; and very similarly in Archelon, while the decurved beak would easily transform him into a most formidable enemy, the heavy premaxillaries and vomer, and the flat but deep lower jaw, suggest an adept crusher of crustaceans. The presence of vast quantities of Nautilus dekayi, which I found accompanying one of the specimens, was doubtless accidental, but it plainly suggests that this cephalopod was one of the teeming sources of food in the Archelon environment.

“The huge bulk of the mature Archelon might account for the shearing off and loss of the flippers of younger forms caught between the shells of the ‘elder boatmen of the Cretaceous seas,’ as Cope has called them, during any sudden rush while herding on the shores. But probably the young turtles did not much frequent the shores at either egg-laying or other times. Whence it is much more likely that it was a mosasaur or some of the gigantic fishes like Portheus which bit off the right hind flipper in the type-specimen of Archelon ischyros, well above the heel, as I have described it. That this happened rather early in life is shown by the arrested growth of the right femur and remaining portions of the tibia and fibula, which are all uniformly 10 per cent smaller than the corresponding bones of the left flipper.”

While there were many small fishes in the Niobrara seas which the Protostegas inhabited, the most striking thing in the fauna is the great abundance of molluscal shells, especially Ostrea congesta. And with them were great hordes of larger pelycypod mollusks, some of them of enormous size. Some of the largest reach a diameter of nearly four feet, with shells so thin that one can hardly understand how they could have supported such large, oyster-like creatures. One can imagine that such shell-fish might have afforded an almost inexhaustible source of food for the large turtles; and several times the writer has found remains of Protostega associated with such shells. From all of which evidence it seems very probable indeed that Dr. Wieland is right in imputing to these gigantic turtles a shell-feeding habit, a habit which required neither speed nor great prowess; and perhaps the formidable beak was used more in social quarrels than for food-getting. That these marine turtles departed from the usual reptilian habit of laying their eggs upon land is improbable. The tortoise shell turtles of the Bahamas lay three or four hundred eggs in a hollow scooped out in the sand and then leave the young to their own devices; certainly many a one is gobbled up by birds of prey or other enemies on their way to the water. Perhaps the young Archelon lost its hind leg in some such mishap.