CONTENTS
| chapter | page | ||
| I. | Introduction | [ 1] | |
| II. | Classification of Reptiles | [13] | |
| III. | The Skeleton of Reptiles | [19] | |
| IV. | The Age of Reptiles | [44] | |
| V. | Adaptation of Land Reptiles to Life in the Water | [59] | |
| VI. | Order Sauropterygia | [73] | |
| Plesiosauria. | |||
| Nothosauria. | |||
| VII. | Order Anomodontia | [102] | |
| Lystrosaurus. | |||
| VIII. | Order Ichthyosauria | [107] | |
| IX. | Order Proganosauria | [126] | |
| Mesosaurus. | |||
| X. | Order Protorosauria | [132] | |
| Protorosaurus. | |||
| Pleurosaurus. | |||
| XI. | Order Squamata | [138] | |
| Lizards. | |||
| Mosasaurs. | |||
| Snakes. | |||
| XII. | Order Thalattosauria | [171] | |
| XIII. | Order Rhynchocephalia | [176] | |
| Choristodera. | |||
| XIV. | Order Parasuchia | [184] | |
| Phytosauria. | |||
| XV. | Order Crocodilia | [194] | |
| Eusuchia. | |||
| Mesosuchia. | |||
| Thalattosuchia. | |||
| XVI. | Order Chelonia | [216] | |
| Side-necked Turtles. | |||
| Snapping Turtles. | |||
| Fresh-water or Marsh Tortoises. | |||
| Land Tortoises. | |||
| Sea-Turtles. | |||
| Ancient Sea-Turtles. | |||
| Leather-back Marine Turtles. | |||
| River Turtles. | |||
CHAPTER I
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.
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.
About the middle of Cretaceous times, there extended from the Gulf of Mexico on the south to or nearly to the Arctic Ocean on the north a narrow inland ocean or sea, a few hundred miles in width, covering what is now the western part of Kansas and the eastern part of Colorado, and separating the North American continent into two distinct bodies of land. This ocean, because of its location, bordered on both sides by low-lying lands—the Rocky Mountains had not then been pushed up—doubtless was comparatively calm and placid, free from violent storms and high tides. That the climate, in the region of Kansas at least, was warm or even subtropical is fairly certain, since plants allied to those now living in warm, temperate, or subtropical regions were then living much farther to the north; and since the animals which then lived in this sea were only such as would be expected in waters of warm temperature. Its tributary rivers could have been neither large nor swift-flowing, since the sediment at its bottom was free, or nearly free, from in-brought material. This was at least the case not very far from its shores. Its slowly falling sediment was composed, almost exclusively, of microscopic shells of animals and plants, foraminifera and coccoliths. The deposits thus made are almost identical with those now forming in various parts of the world in clear but not deep waters, away from the immediate coasts of the continents, almost pure chalk. Animals dying in this inland sea fell slowly to the bottom during or after decomposition of their softer parts, and the slowly increasing sediments covered up and buried the preservable parts. The many predaceous fishes and other scavengers with which the waters abounded often tore the decomposing bodies apart, separating and displacing the bones of the skeleton; and the currents of the shallow waters washed others apart. Often the teeth of fishes and other carnivorous animals are found imbedded in the bones, and many are the scars and toothmarks observed in the fossil bones.
After the ocean had dried up and the bottom had been raised far above the present level of the oceans, other deposits made in lakes and by the winds covered deeply the consolidating sediments, burying them for millions of years with all that they contained. Long-continued erosion by winds and rains has again laid bare many parts of the old ocean bottom, and has washed them out into ravines and gullies. Many hundreds of square miles of this chalk are now laid bare in western Kansas, upon which the growth of vegetation has been prevented by the arid climate. Here and there may now be discovered protruding from the sloping or precipitous surfaces of this exposed chalk bones or parts of bones of the old animals buried so long ago in the soft sediment of the ancient ocean bottom.
The sharp-eyed searcher after fossils detects these protruding, often broken and weather-worn, petrified bones, which themselves betray the presence often of other parts of their skeletons still concealed in the chalky hillside. Fortunate is he if he has discovered a specimen soon after it appeared at the surface, before the rains have washed away and destroyed most of the remains that had been there preserved. Still more fortunate is he if all or nearly all of the original skeleton has been preserved together in its natural relations. After days, perhaps weeks, of labor, the specimen is secured and shipped to the laboratory. Those parts which have been washed out of the chalky rock before the discovery of the specimen are always more or less injured and for the most part lost, their fragments strewn down the hillside, for erosion is always slow and many years may have elapsed since first the specimen had appeared at the surface. More frequently, perhaps, a few strokes of the pick and shovel disclose but one, two, or three bones remaining in the rocks. The specimen, if large, or composed of many bones, is carefully uncovered sufficiently to show its extent, and then, so far as possible, removed in large blocks of the rock. The bones themselves, notwithstanding their petrifaction, are usually soft and easily broken, and their separate removal from the matrix may require weeks or even months of labor, work which cannot be done prudently in the field.
Of many specimens the rock matrix is so hard that the task of removing it from the bones is slow and difficult, indeed well-nigh impossible, for the bones are usually softer than their surrounding matrix. On the other hand, the matrix may be so soft and friable that it cannot be quarried out in blocks. In such cases the separate divisions, as large as they can be excavated and safely handled, are carefully covered with thick bandages of burlap and plaster-of-paris, often strengthened with rods of iron or boards. The skeleton of a single animal treated in this way may require weeks and even months to collect, prepare, and mount in the museum.
From what has been said the reader will understand how it is possible to make an approximately accurate picture of extinct animals as they appeared in life—approximately accurate, never absolutely so. The flesh and other soft parts of an animal are never petrified, though it is a common belief that they may be. Petrified men and women are still occasionally shown in cheap museums, but they are always frauds. Many times has the writer been called upon to express an opinion as to the nature of some concretion which the discoverer was sure was a petrified snake, turtle, or even some part of the human body, because of fancied resemblances in shape and size. Not too emphatically can it be said that anything dug from the earth having the shape of a living animal and alleged to be petrified is either an accidental resemblance or a deliberate humbug—if we except such extraordinary casts as those of Pompeii. The Cardiff Giant and the Muldoon are still fresh in the memory of some of us. There have been a few instances where flesh has been preserved in the North, frozen for thousands of years, but frozen fossils are very different from petrified fossils. Flesh decays before it possibly can be petrified, and only rarely is the residue of flesh, tendons, and skin, that is, the carbon and mineral matters, preserved.
Fig. 2.—Removing a specimen of fish in a block from
the chalk of western Kansas.
One may sometimes restore extinct animals as in life, knowing fully the shape and structure of the skeleton, and still be far from the real truth. All elephants of the present time have a bare or nearly bare skin. If all that we knew of the extinct mammoth were derived from the skeleton we should never have suspected that the creature was clothed during life with long and abundant hair, such as has been found with the frozen bodies in Siberia. Nor should we suspect that the dromedary and Bactrian camels of today have large masses of fat on their backs, if we knew only their skeletons. It must therefore be remembered that all restorations of extinct animals, representing them as in life, are merely the sum of our knowledge concerning them, as close approximations to the real truth as it is possible to make. Or, rather, they should be such approximations; unfortunately many such restorations have been made by artists wholly unacquainted with the anatomy of the creatures they attempt to represent, often adorned with appendages drawn from a too vivid imagination.
CHAPTER II
CLASSIFICATION OF REPTILES
There is very much doubt, very much uncertainty, among paleontologists about the classification of reptiles. No two writers agree on the number of orders, or the rank of many forms. Some recognize twenty or more orders, others but eight or nine. And this doubt and uncertainty are due chiefly to the many discoveries of early forms that have been made during the past twenty years. The many strange and unclassifiable types which have come to light in North America, South Africa, and Europe have thrown doubt on all previous classificatory schemes, have weakened our faith in all attempts to trace out the genealogies of the reptilian orders; and classification is merely genealogy. It is only the paleontologist who is competent to express opinions concerning the larger principles of classification of organisms, and especially of the classification of reptiles. The neozoölogist, ignorant of extinct forms, can only hazard guesses and conjectures as to the relationships of the larger groups, for he has only the specialized or decadent remnants of past faunas upon which to base his opinions. About some things we can be quite confident; about some groups opinions have crystallized, and we all agree, except perhaps on trifles. The dinosaurs, the pterodactyls, the crocodiles, for instance, offer only minor problems to perplex the systematist, but the origin and the relations, not only of these, but also of nearly all the others, are still involved in obscurity. The question, whence came the ichthyosaurs, the plesiosaurs, the turtles, etc., seems almost as far from solution as it did fifty years ago. With every problem solved a dozen more intrude themselves upon us. Hence, classification simply represents the present condition of our knowledge, our present opinions as to genealogies. It was the fashion a dozen years ago to draw all sorts of genealogical trees on the slightest pretext, to trace in beautifully clear lines the precise descent of all kinds of animals; and very few have been worth the paper on which they were printed. When facts are numerous enough, conclusions are patent even to the novice; when facts are few and obscure, one can guess about as well as another. In general, it may be said that the older a group of animals is the more abstruse are the problems presented; first, because of the lack of abundant material; second, because the forms speak to us in an unfamiliar language that we cannot easily interpret. The classification of the mammals approaches more nearly the ultimate truth than does that of any other group of organisms, because we know more about the extinct forms than we do of any other class, and also because we know more about the living forms than we do about any other living animals.
Species of reptiles are, for the most part, vague quantities in paleontology; they can be determined with assurance only by the comparison of abundant material. Adult characters in mammals are apparent in the ossification of the skeleton, and size can be used within moderate limits in the determination of species; but size in reptiles means but little; no one could possibly say that the skeleton of an alligator six feet in length is not that of an adult animal if he knew nothing else about the Crocodilia. So also the compression and malformations of bones from the processes of fossilization obliterate specific characters in great part. Nor are specific characters easily distinguishable in the skeletons of living reptiles. The genus, therefore, among fossil reptiles is practically the unit, and we may be sure that for every well-defined genus we discover there existed numerous minor variations, which, had we the living animals to study, we should call species. We classify the living Crocodilia into two families, about four well-defined genera—perhaps even five or six—and about twenty-five species. Of the living lizards there are about eighteen hundred species, twenty families, and four larger groups or suborders. In all probability the lizards have never been more abundant and more varied than they are at the present time. Possibly these proportions of species, genera, families, and suborders may represent approximately the proportions that have existed at some time or other in most of the other groups which we call orders—approximately only, for we can never be quite sure that we evaluate the structural characters of different groups of organisms quite equally. The absence of a molar tooth in a mammal would ordinarily indicate a genus, the absence of a tooth in a reptile might not indicate even a variety or a race. Whence it follows that classification of organisms is not and never will be an exact science. The value of characters used in classification is very unequal, as we have seen. No two persons see these characters from the same viewpoints, and in consequence no two persons whose opinions are worth while ever wholly agree as to classification.
The following scheme differs only in minor details from the more conservative of the generally accepted views, and those differences are, for the most part, the writer’s own opinions, to be taken for what they are worth. It may be said decisively that no classification of the reptiles into major groups, into super-families or subclasses that has so far been proposed is worthy of acceptance; there is no such subclass as the Diapsida or Synapsida, for instance. And we have very much more to learn about the early reptiles before any general classification of the reptiles can be securely founded. It is very probable that the primary radiation of the reptiles into the various lines of descent, into its main branches, occurred much earlier than we have been disposed to believe; that before the close of Paleozoic time, perhaps before the close of the Carboniferous, all the great groups of reptiles had gone off from the main stem, and that since then only smaller and smaller branches have appeared. There have been no new orders of reptiles in all probability since Triassic times, and perhaps none since Permian.
Taxonomists are often disposed to cut the Gordian knots of relationships by raising the ranks of the animals they study to independent positions. More than thirty independent orders of reptiles have been proposed by different students, and quite as many of mammals and of birds; possibly after more forms have been discovered there will be as many proposed for the amphibians. Sometimes, indeed, it is better to make such independent groups than to unite lesser ones on doubtful evidence. But the writer, for one, believes that it is more worthy of the thoughtful scientific student to seek for relationships than for differences. It is far easier to destroy than to construct, to make new genera, families, and orders than to unite those already proposed. To raise every proposed suborder of reptiles to an order, as has been proposed by various writers, and the orders to subclasses, only leaves classification where it was; nothing has been added to taxonomy save a lot of new names to perplex and annoy the student.
In the following scheme of classification three groups provisionally called orders are prefixed by an asterisk.
| Order | COTYLOSAURIA | ||
Primitive reptiles with notochordal vertebrae, imperforate temporal region, persistent intercentra; two coracoids; plate-like pelvis, with all or most of the amphibian skull elements; short legs and short neck; phalangeal formula primarily 2, 3, 4, 5, 3(4). | |||
| Suborder | Diadectosauria | Permocarboniferous, North America. | |
| Pantylosauria | Permocarboniferous, North America. | ||
| Labidosauria | Lower Permian, North America. | ||
| Pareiasauria | Upper Permian, Europe, Africa. | ||
| Procolophonia | Triassic, Europe, Africa. | ||
| Order | CHELONIA | ||
Temporal region imperforate. Head and limbs more or less retractile within a box formed chiefly by the exoskeleton. | |||
| Suborder | Pleurodira | Triassic to recent. | |
| Cryptodira | Jurassic to recent. | ||
| Trionychoidea | Cretaceous to recent. | ||
| Order | THEROMORPHA | ||
Primitive reptiles with notochordal vertebrae, perforate temporal region, persistent intercentra; two coracoids; plate-like pelvis with median vacuity; no free dermosupraoccipitals in skull; longer legs and neck; phalangeal formula 2, 3, 4, 5, 3(4). | |||
| Suborder | Pelycosauria (sens. lat.) | Permocarboniferous, North America, | |
| Europe. | |||
| Dromasauria | Upper Permian, Africa. | ||
| Dinocephalia | Middle and Upper Permian, Africa. | ||
| Order | THERAPSIDA | ||
Reptiles with a single temporal perforation on each side; vertebrae not notochordal; intercentra not persistent; pelvis with vacuity; skull bones reduced; teeth heterodont; phalangeal formula, 2, 3, 3, 3, 3. | |||
| Suborder | Anomodontia | Permo-Trias, Africa, North America. | |
| Therocephalia | Upper Permian, Africa. | ||
| Theriodontia | Trias, Africa. | ||
| Order | SAUROPTERYGIA | ||
Aquatic reptiles with a single temporal vacuity; no supratemporal bone, or quadratojugal; ribs single-headed, diapophysial; coracoids large, meeting in middle line, single; neck long, tail short. | |||
| Suborder | Nothosauria | Triassic, Europe. | |
| Plesiosauria | Triassic to close of Cretaceous, | ||
| cosmopolitan. | |||
| ⃰Order | PROGANOSAURIA | ||
Primitive aquatic reptiles; single (? upper) temporal perforation; neck elongate; nares posterior; vertebrae notochordal; intercentra persistent; pelvis plate-like; phalangeal formula 2, 3, 4, 5, 4(6). Permocarboniferous, Africa, South America. | |||
| Order | ICHTHYOSAURIA | ||
Reptiles with all aquatic adaptations; a single, upper temporal perforation; both supratemporal and squamosal present; a single coracoid. Middle Triassic to Benton Cretaceous, cosmopolitan. | |||
| ⃰Order | PROTOROSAURIA | ||
A single, upper temporal vacuity, quadrate fixed (neck vertebrae elongate); bones hollow; cervical ribs single-headed, articulating with centrum; pelvis plate-like. Permian, North America, Europe. | |||
| Order | SQUAMATA | ||
A single, upper temporal vacuity, or, secondarily none; quadrate loosely articulated with cranium; teeth on palate; intercentra more or less persistent; a single coracoid; ribs single-headed, central. | |||
| Suborder | Lacertilia | Trias to recent. | |
| Mosasauria | Upper Cretaceous, cosmopolitan. | ||
| Ophidia | Upper Cretaceous to recent. | ||
| ⃰Order | THALATTOSAURIA | ||
Aquatic reptiles; two (?) temporal vacuities; ribs single-headed, attached to centrum; single coracoid; no intercentra. Trias, California. | |||
| Order | RHYNCHOCEPHALIA | ||
Two temporal vacuities on each side; palate with teeth; intercentra persistent; a single coracoid; teeth acrodont; ribs articulating with centrum and arch. | |||
| Suborder | Rhynchosauria | Triassic, Europe. | |
| Sphenodontia | Triassic to recent. | ||
| Choristodera | Uppermost Cretaceous, lowermost | ||
| Eocene, North America, Europe. | |||
| Order | PARASUCHIA | ||
Subaquatic reptiles, with two temporal vacuities; an antorbital vacuity; no false palate; pubis entering acetabulum; ribs double-headed, diapophysial. | |||
| Suborder | Phytosauria | Upper Trias, cosmopolitan. | |
| Pelycosimia | Trias, Africa. | ||
| Pseudosuchia | Trias, Europe, North America. | ||
| Order | CROCODILIA | ||
Two temporal vacuities; teeth thecodont; a false palate; pubis excluded from acetabulum; single coracoid; ribs double-headed, diapophysial; subaquatic or aquatic. | |||
| Suborder | Eusuchia | Jurassic to recent. | |
| Thalattosuchia | Upper Jurassic, Europe. | ||
| Order | DINOSAURIA | ||
Ambulatory reptiles, with two temporal vacuities; no false palate; pubis entering acetabulum; ribs double-headed, diapophysial. | |||
| Suborder | Theropoda | Upper Trias to close of Cretaceous, cosmopolitan. | |
| Orthopoda | Close of Trias to close of Cretaceous, cosmopolitan. | ||
| Sauropoda | Upper Jurassic, Lower Cretaceous, cosmopolitan. | ||
| Order | PTEROSAURIA | ||
Volant reptiles; fourth finger greatly elongated to support patagium; neck vertebrae elongated; bones hollow; ribs double-headed, diapophysial; a single coracoid; no clavicles or interclavicle; two temporal vacuities. | |||
| Suborder | Pterodermata | Jurassic, Europe. | |
| Pterodactyloidea | Upper Jurassic to Upper Cretaceous, | ||
| Europe, North America. | |||
CHAPTER III
THE SKELETON OF REPTILES
The bony framework, or skeleton, that which gives form and stature to the body, and which serves for the support of the soft parts and the attachment of muscles, is, with rare exceptions, all that is ever preserved of fossil animals. Because, therefore, students of extinct animals must rely so much, if not exclusively, upon the skeleton much attention has been given to the study of comparative osteology, the science of bones. Not only are most of the bones of the skeleton characteristic of the genus to which they belong, but the more general plan of the skeleton, or parts of the skeleton, is likewise characteristic of the larger groups. The paleontologist may become so expert in deciphering the characters of single bones, or even parts of bones—often all that are known of animals new to science—that he is able to hazard guesses as to the general structure of the skeleton to which they belong. But such guesses usually will approximate the real truth only in the degree that the bones upon which they are based approximate like bones of other animals that are better known. Not all parts of the skeleton are equally characteristic of the type of animal which possessed them. A tooth of a mammal may positively determine the species to which it belongs, while the toe bone of the same animal might not enable one to guess at its family, even. As a rule one can seldom be quite sure of the species of a reptile unless the larger part of the skeleton, or at least the skull, is available, although almost any bone of the skeleton, if one is expert, will permit a decision as to the family, if not the genus.
One must often depend upon the positions and relations of the bones, as found in the rocky matrix, for the final determination of many characters. One can, for instance, never be sure of the number of bones in the neck, trunk, tail, or feet of a reptile, until specimens have been found with all such bones in position. It is for this reason that much care is exercised in the collection of specimens of fossil animals, and especially of fossil reptiles, to preserve all parts of the skeleton, so far as possible, in the relations they occupied in the rocks until they can be studied in the laboratory. Many grievous errors have been made in the past by hasty inferences from fragmentary and poorly collected specimens.
Fig. 3.—Limnoscelis, a subaquatic cotylosaur, from
the Permocarboniferous of New Mexico.
Because of the reliance which must be placed upon the skeleton it will be necessary to speak somewhat in detail of its structure in the reptiles, and to use not a few terms in its description that are unfamiliar to the general reader. So far as possible technical terms will be avoided, though some must be used, as there are no equivalents in the English language for them. The reader may use this chapter as a sort of explanatory index or glossary for the better elucidation of the necessary details of the following chapters.
It is needless to say that the skeleton of a reptile is arranged on essentially the same plan as that of our own; the bones have the same names that they have in our own skeleton, but there are more of them, and the individual bones, as a general rule, are less highly specialized, that is, are not so well adapted for special functions. In a word, the skeleton of a reptile for the most part is generalized, though particular parts may be highly specialized for particular uses. As a rule, if not as a law, the course of evolution has been to reduce the number of parts and to adapt those which remain more closely to their special uses, either by increase in size, or by modifications of their shape and structure.