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: