Water Reptiles of the Past and Present

by Samuel Wendell Williston

The records of the lower part of the Triassic period are scanty everywhere in the world, save perhaps in Africa. Before the close of the period, however, probably every important group of cold-blooded air-breathing animals had made its appearance in geological history, if we except the snakes; even the mammals had appeared, and possibly the birds. The Cotylosauria, Theromorpha, and Therapsida disappeared, the latter giving birth to the mammals; the nothosaurs and plesiosaurs, the ichthyosaurs, dinosaurs, crocodiles, phytosaurs, rhynchocephalians, lizards, and turtles have all left records of their existence in Upper Triassic rocks; and the pterodactyls had also, in all probability, begun their career, though none is surely known till the Jurassic.

During Jurassic times all these orders of reptiles waxed prosperous and powerful, and branched out in many ways and in countless numbers; many new kinds of each appeared—the marine crocodiles, the quadrupedal dinosaurs, etc.—but no order or suborder, so far as we know, disappeared before its close. And this prosperity continued on into the Lower Cretaceous and for many even into the Upper Cretaceous. The largest dinosaurs disappeared in the Lower Cretaceous, so far as our knowledge goes, but the old-fashioned crocodiles continued on into the Upper, to give place to the new-fashioned kinds. The ichthyosaurs lingered on for a while on the western continent, but the mosasaurs appeared, and the plesiosaurs reached their highest evolution and continued to the end. The flying reptiles attained the zenith of their evolution, but disappeared before the close. The marine turtles attained the maximum of specialization and size. The upright-walking dinosaurs continued on unabated to the close of the period; and a new kind of dinosaurs appeared near the end.

Fig. 28.—Restoration of Dimetrodon, a pelycosaur reptile from the Permian of Texas;
about eight feet long.

With the opening of the next great era—the Cenozoic or Tertiary—the reptiles dwindled away to their present insignificant position, while the birds and mammals appeared in great numbers and varied forms. The Age of Reptiles was closed and the Age of Mammals had begun.

The history of the reptiles during the Cenozoic is an uneventful one; they ceased their dominion upon land, in the water, and in the air. Their remains are scanty, for the most part, in the rocks of the Tertiary, and such as are known differ only in details from those now living. The land tortoises only, like the mammals of Oligocene and Miocene times, seized the opportunities of open prairies and prospered. A few of the late Mesozoic forms continued a short while into the Eocene. No new groups, perhaps few new families, came into existence during the greater part of this time; it was the age only of land tortoises and the poisonous snakes among reptiles.

EXTINCT REPTILES OF NORTH AMERICA

The oldest known fossil reptile of North America, or indeed of the world, is represented by a single specimen, lacking the skull, from black shales of Middle Pennsylvanian age overlying a coal seam at Linton, Ohio. The specimen was originally described as an amphibian, but was later recognized by Professor Cope as a true reptile. It was more fully described by the writer under the name Eosauravus Copei, who agreed with Cope as to its reptilian nature. Until the skull is discovered, however, the precise relationships of the animal must remain doubtful.

The next later rocks that have yielded reptilian remains are those of Illinois and Texas formerly supposed to be of Permian age. Later evidence, furnished by invertebrates, however, seems to prove that the lowermost of the strata are of uppermost Carboniferous age. The Illinois deposits, so far as known, are of very limited extent, consisting practically of a single bone-bed in black shale in the immediate valley of the Kaskaskia River near Danville. The known fossils from this bone-bed—all isolated bones—are preserved in the museum of the University of Chicago, and include the types of several genera later recognized in the Texas deposits.

The deposits of Texas, extending northward through Oklahoma to the south line of Kansas, are of considerable extent, for the most part lying along the Wichita River and its tributaries, north of Seymour, Texas. They are composed chiefly of red clays and sandstones of fresh-water or delta origin, perhaps eight hundred feet in total thickness. Beds of like character and yielding similar fossils are also known from northern New Mexico on the tributaries of the Chama River. Their chief characters, as well as restorations of some of the more noteworthy forms, have already been given.