Water Reptiles of the Past and Present

CHAPTER I. Introduction

INTRODUCTION

In most persons the word reptile incites only feelings of disgust and abhorrence; to many it means a serpent, a cold, gliding, treacherous, and venomous creature shunning sunlight and always ready to poison. Our repugnance to serpents is so much a part of our instincts, or at least of our early education, that we are prone to impute to all crawling creatures those evil propensities which in reality only a very few possess. Were there no venomous serpents—and there are but two other venomous reptiles known—we should doubtless see much to admire in those animals now so commonly despised; because a few dozen kinds, like the rattlesnakes, copperheads, and cobras, protect themselves in ways not unlike those used by man to protect himself, we unjustly abhor the thousands of other kinds, most of which are not only innocent of all offense toward man, but are often useful to him.

There are now living upon the earth more than four thousand kinds or species of cold-blooded animals which we call reptiles, all of which are easily distinguishable into four principal groups: the serpents and lizards, the crocodiles, the turtles, and the tuatera. Their habits and forms are very diverse, but they all possess in common certain structural characters which sharply distinguish them from all other living creatures. A reptile may be tersely defined as a cold-blooded, backboned animal which breathes air throughout life. And yet, it is not quite certain that this definition is strictly correct when applied to all the reptiles of the past, since it has been believed that certain extinct ones may have been warm-blooded. By this definition, short as it is, we at once exclude a large number of cold-blooded, air-breathing, backboned animals which were formerly included by scientific men among the true reptiles, and even yet are popularly often so included—the amphibians or batrachians. These animals, now almost wholly represented by the despised toads, frogs, and salamanders, were, very long ago, among the rulers of the land, of great size and extraordinary forms. But they have dwindled away, both in size and in numbers, till only a comparatively few of their descendants are left, none of them more than two or three feet in length, and all of them sluggish in disposition and of inoffensive habits. While we may speak of the amphibians as air-breathing, they are, with few exceptions, water-breathers during the earlier part of their existence. Some may pass their whole lives as water-breathers, while a few begin to breathe air as soon as hatched from the egg; but these are the marked exceptions.

In many respects the internal structure of the amphibians of the present time is widely different from that of reptiles, though there can be no doubt that the early amphibian ancestors of the modern toads, frogs, and salamanders were also the ancestors of all living and extinct reptiles, and it is a fact that the living amphibians differ more from some of the ancient ones than those early amphibians did from their contemporary reptiles. Discoveries in recent years have bridged over nearly all the essential differences between the two classes so completely that many forms cannot be classified unless one has their nearly complete skeletons. We know that some of the oldest amphibians, belonging to the great division called Stegocephalia, were really water-breathers during a part of their lives, because distinct impressions of their branchiae, or water-breathing organs, have been discovered in the rocks with their skeletal remains, but we are not at all sure that some of the more highly developed kinds were not air-breathers from the time they left the egg; indeed, we rather suspect that such was the case.

We are also now quite certain that, from some of the early extinct reptiles—the immediate forbears probably of the great dinosaurs—the class of birds arose, since the structural relationships between birds and reptiles are almost as close as those between reptiles and amphibians.

Huxley believed that the great class of mammals arose directly from the amphibians, and there are some zoölogists even yet who think that he was right. But paleontologists are now quite sure that they were evolved from a group of primitive reptiles, known chiefly from Africa, called the Theriodontia; quite sure because nearly all the connecting links between the two classes have already been discovered—to such an extent, indeed, that really nothing distinctive of either class is left save the presence or absence of the peculiar bone called the quadrate, the bone with which the lower jaw articulates in birds and reptiles; and certain elemental parts of the lower jaw itself. And even these bones, in certain mammal-like reptiles, had become mere vestiges. Even the double condyle of the mammal skull, with which the vertebrae articulate, so like those of the amphibian skull that Huxley based his belief of the amphibian origin of the mammals chiefly upon it, has now been found in certain reptiles. Warm-bloodedness, one of the diagnostic characters of birds and mammals, is not really very important, since it must have arisen in these two classes independently, and we may easily conceive that the earliest mammals were cold-blooded or that the immediate ancestors of the mammals were warm-blooded.

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.

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