“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.)