Water Reptiles of the Past and Present

by Samuel Wendell Williston

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.