Water Reptiles of the Past and Present

by Samuel Wendell Williston

It is an interesting fact in the history of the vertebrates, as of all other groups of animals and plants, that the chief divisions arose early in geological history. Every known order of amphibians and reptiles, unless it be that including the blind-worms, was differentiated by the close of the Triassic period. The frogs are now known from the Jurassic. The mammals and birds also quite surely date their birth from the Triassic. And this early differentiation of the chief groups is doubtless due to the fact that the potentialities of diverse evolution are limited by specialization. It is apparently a law that evolution is irreversible, that it never goes from the special to the general, that an organism or an organ once extinct or functionally lost never reappears. And it is also a law in evolution that the parts in an organism tend toward reduction in number, with the fewer parts greatly specialized in function, just as the most perfect human machine is that which has the fewest parts, and each part most highly adapted to the special function it has to subserve. And these laws explain why it is that no highly specialized organism can be ancestral to others differing widely from it. The more radically distinct an organism is from its allies, the earlier it must have branched off from the genealogical tree.

The many new discoveries of extinct forms so often intermediate, not only between the larger groups, but between many of the lesser ones as well, are making the classification of the vertebrates increasingly difficult. At one time it was sufficient to define a reptile as a cold-blooded animal with a single occipital condyle, that is, with a single articular surface between the skull and the first vertebra of the neck; a mammal as a warm-blooded animal with two articular surfaces; but these definitions are no longer strictly correct. Connecting links do not break down classification, as one might think, but they do often spoil our fine systems and compel our classifiers to take a wider view of nature than their own narrow province affords.

We can never hope that most, or even the greater part, of all the animals which have lived in the past will ever become known to us, even imperfectly. Doubtless the species of the past geological ages outnumbered many times, perhaps hundreds of times, all those now living, since many of these latter are merely the remnants of far more varied and extensive faunas. At times the conditions for the preservation of the remains of animal life have been more favorable than at others, and, under such favorable conditions, a fairly good glimpse is sometimes given us of the fauna of some isolated epoch and locality in the earth’s history. Those animals which lived in and about the water have been preserved in greater numbers and more perfectly than the strictly land animals, since fossils are due to the preserving action of water, with few exceptions. Of those animals which lived upon the land or in the air only the rarest of accidents carried the skeletons into the lakes, seas, and oceans. And, even when they had been covered by sediments at the bottoms of lakes and seas and hidden away from adverse agencies, it has often happened that the great erosions of later ages have carried away and destroyed the rocks in which they were inclosed. The records of long intervals of time have thus been lost in all parts of the world. That we are able to obtain even an imperfectly continuous history is due to the fact that the intervals thus lost are not everywhere contemporaneous, that the missing records of one place may be filled out in part elsewhere. But this substitution of records from a distance can never make the history complete. If, in human history, we had only the records for one century in China, for another in England, and for yet another in South America, how imperfect indeed would be our knowledge of human progress. Animals and plants are never quite alike in remote regions, and they never have been. The living reptiles of North and South America are today almost entirely different, and, were their fossil remains to be discovered a million years hence, it would be very difficult to decide that they had once lived contemporaneously; difficult, though perhaps not impossible, since some are so nearly alike that their relationships or possible identity would probably be established after long search. This will serve to make clear how very difficult it is, for the most part, to correlate exactly the geological formations in remote regions of the earth, or even sometimes in adjacent regions where the fossils are scanty, or the conditions under which the animals had lived were very different.

There are long periods of time, millions of years at a stretch perhaps, throughout which our knowledge amounts to little or nothing concerning many land reptiles which we are sure must have existed abundantly. No better example of our oftentimes scanty knowledge can be cited than the following. Until within the past fifteen years it was thought that true land lizards, of which there are about eighteen hundred species now living, dated back in their history no farther than about the close of the great Secondary Period, or the Age of Reptiles. But a single skull of a true land lizard has been discovered in the Triassic deposits of South Africa, a skull of a form so nearly like that of the modern iguana of America that its discoverer, Dr. Broom, has called it Paliguana. The lizards must have been in existence, probably many thousand species of them, during all the great interval of time between the Middle Triassic and the close of the Cretaceous, since it is a law which can have no exception, that a type of life once extinct never reappears. The “ancient iguanas” of the Trias must have been the forbears of many, if not all, of the lizards of later times, though nothing is known of their descendants through a period of time which can be measured only by millions of years.

However, notwithstanding these imperfections of our geological records, we know very much more about extinct reptiles than we do about living ones, so far at least as those parts capable of preservation in the rocks are concerned. Were our knowledge of reptiles confined to the forms now living upon the earth it would be relatively very incomplete since, aside from the lizards and snakes, they are merely the remnants of what was once a mighty class of vertebrates.

Not only do we learn from the remains preserved in the rocks the precise shape and structure of the bones of the skeleton and their precise articulations, but we are often able to determine not a little regarding the forms which the living animals had by the impressions made by the dead bodies in the soft sediment which inclosed them before decomposition of the softer parts had ensued, sediments which afterward solidified into hard rock. But these impressions are, with rare exceptions, only those of profiles or of flattened membranes. The rounded bodies of life do not retain their shape long enough for the sediment to harden; in most cases the flesh has decomposed before being entirely covered by sediment. Sometimes the integument and scales in a carbonized condition are actually preserved, retaining some of the actual structure of the organized material. The carbon pigment of the skin has sometimes been preserved in patterns indicating the color-markings in some of these ancient reptiles; and even the microscopic structure has been detected in carbonized remains of organs. Fossil stomach contents, the bony remains of unhatched young, as well as the delicate impressions of skin and membrane, all add to our knowledge of the structure and habits of the animals which lived so long ago. Many other things also may be learned, or at least inferred, concerning the living animals and their habits from the positions in which the skeletons are found, from the nature of the rocks which inclose them, or from the character and abundance of other fossils found with them. The frequent discovery of bones which had been injured and mended during life, or the living amputation of members, often tell of the characteristics of the creatures. So, too, the climatic conditions under which the animals lived may often be inferred with tolerable certainty; the presence of “stomach-stones” reveals something of the food habits, and even of the structure of the alimentary canal, etc.

All this information is gained slowly, often very slowly, and with much labor and pains. Rarely or never is it the case that all the information obtainable concerning any one kind of an extinct animal is furnished by a single specimen. Skeletons are very seldom, perhaps never, found quite complete, with all their parts in their natural positions; and the nature of the matrix inclosing them usually prevents a study of all parts of any specimen. If a newly discovered fossil is widely different from the corresponding parts of any creature previously known, whether living or extinct, we cannot infer very much from a few bones as to what the remainder of the skeleton is like. Such inferences or guesses in the past have often resulted in grievous error, and self-respecting paleontologists are now very reluctant to speculate much concerning extinct animals from fragments of a skeleton, no matter what those fragments or bones may be; future discoveries are sure to reveal errors. It is, therefore, only by the accumulation of much material, and by the careful study and comparison of all known related animals, that reliable conclusions can be reached. Often it requires scores of specimens to determine the exact structure of a single kind of animal, and, as the collection and preparation of fossil skeletons are tedious and expensive, our knowledge sometimes increases very slowly. In recent years, however, there have been many more students of extinct backboned animals than formerly, and there are now many museums and universities which spend annually large sums of money in the collection and preparation of such fossils. This greater activity of the last twenty years is bringing to light many new and strange forms, as well as completing our knowledge of those previously imperfectly known.

It is commonly, but erroneously, believed that the bones of extinct animals are usually found in excavations made for the purpose. It is true that not a few specimens of fossils have been discovered in excavations made for other purposes, such as railway cuttings, quarries, wells, etc., but if no others were found our knowledge of the animals of the past would be very meager indeed. Fossils are, for the most part, found by deliberate search over the denuded rocks in which they occur. Methods of search and collection will best be understood by the following description of the noted fossil-bearing rocks of western Kansas.

Fig. 1.—A characteristic chalk exposure in western Kansas,
a hundred acres or more in extent.