Water Reptiles of the Past and Present

by Samuel Wendell Williston

CHAPTER IV
THE AGE OF REPTILES

Geologists divide the history of the earth, since life first appeared upon it, into four general eras, the Proterozoic, Paleozoic, Mesozoic, and Cenozoic, that is, into eras of first life, ancient life, middle life, and recent life. These divisions were made long ago by geologists when it was believed that extraordinary changes, great cataclysmic revolutions, marked their limits.

With a fuller knowledge of the life of the past we know that evolution has been continuous and uninterrupted; possibly accelerated or retarded at times, but without break. Were the earth’s history to be written anew, with our present knowledge, and with an unbiased mind, it is very doubtful whether many of the time divisions would have the same limits that they have now—whether the Paleozoic would terminate with the Carboniferous, or the Permian, or the Trias, or whether indeed we should think it necessary to make any primary divisions whatsoever. In other words, our greater knowledge of living and extinct organisms, and of the rocks which contain fossils, has made the problems of classification much more complex than they seemed to be formerly. It is much easier to classify organisms or rocks, or anything else, when we know only a few isolated kinds—much easier to draw divisional lines. Geological history is like a volume in which pages, leaves, and even whole chapters either are missing or are printed in languages which we understand only imperfectly. Where the lost or unknown parts belong, the largest divisions may be made, and possibly such may have been epochs of unusual activity, of diastrophic changes which greatly accelerated organic evolution. No one can say just where the dividing line should be drawn between the rocks of Paleozoic and Mesozoic age, or between the Mesozoic and Cenozoic, for there is none; the most that we can hope for is to make the divisions everywhere in the world conform to those first made for local reasons.

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Fig. 23a.—Range of the Reptilia.Heavy lines indicate occurrence in North America.

The periods of the Paleozoic era are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian, in the order as given; those of the Mesozoic era are the Triassic, Jurassic, and Cretaceous; those of the Cenozoic era, the Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Recent. As a relic of an old classification we still often divide the Cenozoic into two quite arbitrary divisions, the Tertiary and the Quaternary, the latter including the Pleistocene and Recent only. The same may be said regarding the limits of each of these periods as of the eras; the sole problem is to make each period contemporaneous throughout the world, an exceedingly difficult problem, because no faunas or floras have ever been the same over the whole earth. Indeed, with the exception of some of the lowliest and most generalized forms, or man himself, no species are the same throughout the earth today. Inasmuch as we must depend upon the fossils in the rocks for the determination of the ages, where none is quite the same in strata of remote localities the identification becomes very difficult or even impossible. Nor are the periods, as accepted, of equal or even approximately equal duration; the Cretaceous period, for instance, was longer than all the remainder of the Mesozoic, longer perhaps than all the time which has elapsed since its close.

The earliest animals with a backbone, or rather the earliest that we call vertebrates—for some vertebrates have no vertebrae—began their existence, so far as we know, in late Ordovician times, as attested by fish bones in Ordovician rocks of Colorado and Utah. The first evidences of the existence of air-breathing vertebrates in geological history are footprints preserved in the uppermost Devonian rocks of Pennsylvania. We call them amphibian because they resemble footprints associated with amphibian skeletons in later formations, and because the foot itself is still the most important difference we know between fishes and the higher animals.

Fig. 24.—Permocarboniferous landscape (adapted from Neumayr) with restoration of Eryops, a stegocephalian amphibian ancestrally allied to the reptiles; and Limnoscalis, a cotylosaur (in water).