Water Reptiles of the Past and Present

by Samuel Wendell Williston

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.

No vertebrate fossils are known in America from the Upper Permian and Lower Triassic. Marine limestones of Middle and Upper Triassic age of Nevada and northern California have yielded numerous remains of primitive ichthyosaurs, the only known remains of the thalattosaurs, and a few others of doubtful affinities, all of which have been described by Dr. Merriam. The Upper Triassic exposures, of considerable extent, occur between the Pitt River and Squaw Creek in Shasta County, California. Reptilian remains from the Middle Triassic are so far known only from the limestones of West Humboldt and New Pass regions of western and central Nevada.

Fig. 29.—Restoration of Varanops, a theromorph reptile from the Permian of Texas;
about four feet long.

Land reptiles of Middle and Upper Triassic age are known from many widely separated localities in the United States, but chiefly from the extensive “red beds” of the Rocky Mountain region. The fossils from these beds occur for the most part at least in the horizon called the Shinarump. Its age is usually considered to be Upper Triassic, but the character of the fossils seems to indicate possibly the Middle Triassic. Aside from the stereospondylian amphibians, the last of the Stegocephalia, the vertebrates from this horizon and these regions are chiefly Phytosauria. A few anomodonts, or what seem to be anomodonts—the only record of their occurrence outside of Africa—are known from Wyoming and Utah. And a single specimen from the Wind River red beds, described by the writer as Dolichobrachium, may represent reptiles allied to the dinosaurs. Phytosaur fossils of this horizon have been discovered in Utah, the Wind River Mountains, and near Laramie City in Wyoming; in southwestern Colorado; in western Texas; and in various places in New Mexico and Arizona. Doubtless when these fossiliferous beds are more thoroughly explored many new and interesting reptiles will be discovered.

Phytosaur remains, probably of about the same age as the Rocky Mountain ones, have long been known from the Triassic of North Carolina. From somewhat more recent Triassic deposits in Connecticut and Massachusetts, several skeletons of small carnivorous dinosaurs, and various parasuchian remains have been described by Marsh, Lull, and Talbot. And these beds have long been famous in Massachusetts for their footprints, for the most part originally referred to birds, but now pretty well known to have been made by dinosaurs and amphibians.

No vertebrate fossils of Lower or even Middle Jurassic age are known from North America. From the Baptanodon beds of Wyoming, limestones of about two hundred feet in thickness, four genera of plesiosaurs, the very peculiar ichthyosaur from which the beds take their name, and a few bones of an ancient crocodile are known.

Immediately overlying the Baptanodon beds, the Morrison beds, of from two hundred to four hundred feet in thickness, probably of Uppermost Jurassic and Lowermost Cretaceous age, have yielded an exceedingly rich vertebrate fauna, consisting chiefly of dinosaurs. Discovered first in the vicinity of Morrison, Colorado, in 1877, hundreds of tons of bones have been collected from these beds for various museums. The dinosaurs include many genera of all three suborders, varying in size from that of a cat to some of the largest known land animals. Of other reptiles a very few jaws of a true rhynchocephalian, a fragment of a wing bone of a pterodactyl, numerous turtles, and crocodiles, only, are known. The beds are predominantly black-clay shales, intercalated with sandstones, and all are of fresh-water origin.

From beds definitely known as Lower Cretaceous (Trinity) in Oklahoma, a few bones of a sauropod dinosaur are known, and from nearly corresponding rocks in southern Kansas, plesiosaurs, crocodiles, turtles, and carnivorous dinosaurs are known from sparse remains. Doubtless the Potomac beds of Virginia, which have yielded bones of various dinosaurs, are also of Lower Cretaceous age.

Fig. 30.—Restoration of Casea, a theromorph reptile from the Permian of Texas,
about four feet long.

With the exception of a single vertebra of doubtful affinities and the cast of a turtle-shell no vertebrate fossils have ever been discovered in the extensive sandstones of Dakota age, the lowermost of the Upper Cretaceous. From the next horizon above the Dakota, the Benton Cretaceous, chiefly marine limestones, at least three genera of plesiosaurs are known from Kansas, Texas, and Arkansas, with two or three more from the limestone shales of Wyoming. A few specimens of armored dinosaurs, two genera of ancient crocodiles, nearly the last of their kind, some marine turtles, and a few vertebrae of ichthyosaurs, the last of the order known anywhere in the world, are also known from the Benton Cretaceous of Wyoming.

Continuous with the Benton limestones above in Kansas are the famous beds of Niobrara chalk; perhaps no fossil deposits in the world are more famous. Exposures covering hundreds of square miles in western Kansas, almost pure chalk, have furnished fossil-hunters during the past forty years literally thousands of specimens of mosasaurs, hundreds of pterodactyls, and scores of plesiosaurs and marine turtles, in addition to the famous birds with teeth and countless fishes of diverse kinds. Two or three specimens of spoon-billed dinosaurs have been found in these deposits, but no other reptiles of any kinds. Beds of like age in Colorado and New Mexico have furnished a few specimens of mosasaurs.

From the marine beds of Fort Pierre age, next above the Niobrara in the west, have come some excellent specimens of two genera of mosasaurs, three or four forms of plesiosaurs, a few pterodactyls, the largest of all marine turtles, and still fewer specimens of dinosaurs, in Kansas, South Dakota, Wyoming, and Montana. From deposits of approximately like age in Mississippi, Alabama, and New Jersey, many incomplete specimens were found years ago of mosasaurs, plesiosaurs, and turtles, the last of the amphicoelian crocodiles, the first of the procoelian crocodiles, and the famous specimen of Hadrosaurus which served for the Hawkins restoration, the first attempt of its kind.

From the uppermost Cretaceous beds of America, the Lance, Judith River, or Belly River beds as they are variously called, have come the remains of a marvelous reptilian fauna. These beds may be grouped together though not all contemporaneous, and there is dispute about their age, some excellent paleontologists insisting that the uppermost are really of Eocene age. From Colorado east of Denver, from eastern Wyoming, from Montana, and especially from the vicinity of Edmonton in Canada, as also occasionally in western Texas and New Mexico, have come many marvelous specimens of dinosaurs, huge bipedal carnivorous dinosaurs, great spoon-billed aquatic dinosaurs, armored stegosaurian dinosaurs, and many kinds of the great horned dinosaurs, the Ceratopsia, so far known only from these beds. Here at the very close of the Age of Reptiles, at the close of the Age of Dinosaurs, are found the ultimate specializations of all the chief groups of dinosaurs except the long-necked quadrupedal dinosaurs which gave up the ghost in Lower Cretaceous times. Many were provided with horns and spines, some indeed seemed to have bristled with spines throughout, a sure sign that they were approaching the end of their career. The modern type of crocodiles had usurped the ancient forms of the early Cretaceous, and reached the largest size of their race perhaps, though but few specimens are known. Here also in these beds we find the first representatives of lizards and snakes in America, though snakes have been described from earlier strata, perhaps, in Brazil. Those archaic, old-fashioned rhynchocephalians described on a later page as the Choristodera appeared also for the first time in these beds, and persisted for a little while in the Eocene, in Europe and America. And with all these there has very recently been described the last of the plesiosaurs, whose race went out with the dinosaurs at the very close of the Mesozoic. It is needless to say that the turtles also occur, for, as a general rule, wherever vertebrate fossils are found, in rocks of the land or the sea, marine or fresh-water, there will be some bones of turtles among them.

With the beginning of the Cenozoic the record of the reptiles becomes relatively scanty in America. In the warm waters of the old Eocene lakes and rivers of Wyoming lived countless crocodiles, true crocodiles of modern aspect and of large size. But, as the climate of North America grew progressively colder, the crocodiles retreated to the south, till, in the Oligocene, the scanty remains of the last crocodiles are found in the American Tertiary. On the other hand, as the open lands appeared toward the close of the Eocene, and in the Oligocene and Miocene, the land tortoises throve and grew greatly in size. In the Bad Lands of South Dakota one may see their remains in almost incredible numbers. And in equally great numbers are these land tortoises, in shape much like the common box tortoise of today, but vastly larger, found in the rocks of the late Miocene or early Pliocene age in western Kansas. And these are the last records of the big tortoises in North America; their descendants are perhaps yet living in the Galapagos Islands.

The history of the lizards and snakes, the only other reptiles found in the Cenozoic rocks of America, is very brief. A few specimens from the Lower Eocene of Wyoming; a few skinks and amphisbaenas from the Oligocene Bad Lands of South Dakota, and some bones of a python-like snake in the early Eocene of Wyoming are about all that we know of the Squamata in the Tertiary. Doubtless snakes and lizards were just as abundant then as now, though but few were preserved, for they are and always have been distinctly terrestrial animals, that only by accident fell into places where they could be fossilized.

The author has collected reptile bones from nearly all of the horizons here mentioned and believes that the list is complete.

CHAPTER V
ADAPTATION OF LAND REPTILES
TO LIFE IN THE WATER

In the never-ceasing struggle for existence all forms of life upon the earth, whether consciously or unconsciously, are continuously striving for improvement; striving to flee from adverse environments, or to adapt themselves better to those which must be endured; to escape their enemies, or to find means whereby they may withstand them; to find more or better food, or to prevent others from despoiling them of what they have. There is always more or less of unrest, more or less of discontent, if such terms may be used of the lower organisms. It sometimes happens with groups of organisms that by reason of unusual or extraordinary traits they become so perfectly adapted to their environments, to their surroundings, or so easily adaptable to changes in their environments, that they remain for long ages securely protected and little changed. But, as with man himself, improvement is usually the result of adversity—adversity which stimulates but does not destroy. And the word improvement, translated into biological language, means simply specialization, that specialization which adapts the organism better to its mode of life, which fits it the better to excel its less ambitious or less capable competitors. No animals or plants are perfect; if they were, there would be no advancement, no struggle. If all physical conditions stood still, or remained uniform, perhaps life would stand still, but conditions never have and never will stand still, and life must change to meet changed conditions.

Thus it is that which makes life easier, which lessens the dangers of destruction, which insures the continued prosperity of the race, is seized upon and utilized by all plants and animals, so far as possible. As said long ago by Tennyson,[2] the first law of life is not the preservation of self, but the prosperity of the race. Whatever the causes may be whereby the offspring are better adapted to conquer in the struggle for existence, whatever may be the laws governing changes and specialization, whether heredity, Mendelism, mutation, natural selection, or Lamarckism, we call the process evolution.

To escape from the severe competition of the overcrowding animals of the sea, some of those creatures we call fishes long ago became air-breathers and took possession of the unoccupied land. From among the myriads which were driven into unbreathable water, by accident or by their enemies, or led there in the search for more easily acquired or better food, some survived and found that the oxygen of the air was quite as breathable as that of the water. Steadily their progeny became better and better adapted to the unusual life until they ceased to be fishes and became amphibians, from which have arisen in like manner all the reptiles and birds and mammals that live or have lived upon the earth.

With more and better powers, developed under better opportunities, not a few of these descendants have repeatedly sought safety from their newly acquired enemies of the overcrowded land, or a better supply of food in the sea; gradually, perhaps incidentally at first, as we shall see is the case with some lizards today, but later with increased adaptation to their new surroundings, they become truly sea or water animals, no longer able to live upon the land. In these changed conditions and with concomitantly changed habits they never reverted to the primitive condition of fishes, never became water-breathing animals again, for that would be actual retrogression, a seeming impossibility in evolution. Nor indeed does it seem possible that a land creature after its reversion to water life ever can return to the land again.

A fish through long ages of evolution has become well adapted to its environments; its shape is the best for speed or varied evolutions in the water; its teeth and mouth-organs are best suited for the food it requires. Now it is evident that if animals of very different habits and form should go back to the water and seek to compete with creatures already well adapted to their surroundings, they must, so far as possible, acquire like forms and like habits. And any improvement on such forms and habits that their higher development permits them to attain will of course be of advantage in their competitive struggles. A fish makes most use of its tail fin for propulsion. It follows that a land animal seeking to compete with it under like conditions must acquire a tail fin or some other organ which subserves its purpose as fully. The body fins are of little use to a fish, save for equilibration, for preserving its position, for stopping quickly, or for changing the direction of its movements quickly—very different functions from those of the corresponding organs, the limbs, of higher vertebrates. There are few better examples of predaceous, fish-eating fishes than the common gar-pike of our rivers, fishes with a slender body covered with very smooth scales, a strong tail, a short neck, and long jaws armed with numerous slender and sharp teeth. Such a fish, darting into a school of smaller fishes, by quick, sudden changes of movement, actively opening and closing its jaws, is sure to seize some of its sought-for prey. In a direct trial of speed with its victims it would most likely be worsted.

There have been many animals of high and low rank which in the past and present have gone back from a terrestrial existence to a life in the water, finding at last a congenial home away from the shores. Or, perhaps, like the monitor lizards of today, they have found temporary safety in the water when hard pressed by their land enemies, and finally found, not only protection, but an abundant supply of easily obtainable food therein. As in every vocation of life there have been many failures in such attempts, many partial successes only. But not a few have found abounding and enduring success and final prosperity—success that has led possibly to undue adaptation to surroundings, and to the acquirement of great size, for that has been the invariable end of water air-breathers of long duration—specializations which finally prevented them from meeting new exigencies. It seems to be a law of evolution that no large creatures can give rise to races of smaller creatures; and as we shall see, the largest sea-animals have been the final evolution of their respective races.

There are no better examples of such success today, nor has there been in all the geological ages, so far as we know, more perfect examples of the adaptation of air-breathing animals to an aquatic life than the great whalebone whales. In Eocene times their ancestors were walking and running land animals; of that there can be not the slightest doubt, since we cannot conceive, as did the older naturalists, of their direct descent from the fishes while having all the essential structure of mammals, i.e., lungs, circulatory system, manner of breeding and rearing the young, etc. Of the living whales, or Cetacea, there are now in existence two very distinct types, so different from each other that some have supposed them to have been evolved from different types of land mammals. One of these is best exemplified by the great baleen whale, having a broad, short head and no teeth. It feeds upon crustaceans chiefly, which are strained from the water by the great fringe or net of “whalebone.” The other type is seen in the porpoise or dolphin. These cetaceans have numerous, pointed and recurved teeth, which they use as did many of the reptiles, hereinafter described, for the seizure and retention of fishes and other swimming animals. So great have been the changes in all these cetaceans, in the adaptation to an aquatic life, that we are almost at a loss to conjecture from what kinds of land animals they have descended. The great zeuglodont whales of early Tertiary times have long been thought to be a sort of connecting link between them and their land ancestors, and it is still probable that they were. The forms of zeuglodont whales that have been discovered in Africa within recent years bear so much resemblance in their skull and teeth to the contemporary carnivores, that many paleontologists think, with good reason, that they were descended from them, that is, from the ancestors of all our dogs, cats, weasels, bears, etc., of modern times. And we have much reason to believe that future discoveries will bring further and more decisive proof of their origin before many years have elapsed. The modern Sirenia, the dugongs and manatees, exclusively aquatic mammals, which feed upon seaweeds at the bottoms of shallow bays and harbors, or in the mouths of rivers, are now known, practically with certainty, to be the descendants in these same African regions of the earliest ancestors of our sheep, oxen, and horses, known so certainly that they are often classed with them, or at least with the elephants, which approach them in their ancestral line even more closely.

A third type of living aquatic air-breathers is seen in the seals, sea-lions, etc. They are much less highly specialized, however, than the whales or sirenians, since they are still capable of considerable freedom upon land, which they recurrently seek for the breeding of their young. They still retain the primitive covering of hair, lost almost entirely by the cetaceans and sirenians and functionally replaced for the conservation of heat by a thick layer of blubber. Instead of losing the hind legs and developing the tail as a propelling organ like the whales, the seals encountered precisely the reverse experience. The hind legs have been developed into most efficient paddles or sculls, and the tail has been for the most part lost. They are fish-eaters, it is true, but they do not have the long jaws possessed by the porpoises and toothed whales.

In the sea-otters, beavers, and even the muskrats, we have examples of less complete adaptation of land mammals to water life, the most of them showing the beginnings at least of structural adaptations similar to those of the seals. From an attentive examination of all these animals, living as well as extinct, which have attained partial or complete success as air-breathing water animals, we find certain laws existing, if we may call them such, which we may discuss a little in detail. As we have seen in the comparison of the whale with the seal, the methods of adaptation have not always been the same, and some recent writers have endeavored to classify aquatic animals under many groups, to which they have given learned technical names, most of which will not concern us here in dealing with the reptiles only.

Beginning with the head, we find that all those reptiles and most of the mammals which have become aquatic fish-eaters have an elongated skull, or rather an elongated face. The jaws are long and slender, and the teeth are not only numerous but also sharp and slender, much like those of the gar-pike, indeed. It is remarkable, too, that in most such animals the external nostrils are situated, not at the extremity of the snout, as in all terrestrial mammals and reptiles, but far back near the eyes. In the whales this position of the nostril enables the animals to breathe without continuous muscular exertion while floating on the surface; that is, the nostrils are at the top of the head. In the sirenians, on the other hand, which live habitually at the bottom of shallow waters, coming to the surface to breathe only, the nostrils are situated so that they are the first to emerge, that is, they are near the front end. The crocodiles, with a more or less elongated face, as also the Choristodera, described farther on, are exceptions, since their nostrils are at the extremity of the snout. Both of these types, however, notwithstanding the elongation of the face, are only partly aquatic in habit, and in the crocodiles the breathing organs have undergone a strange modification in accordance with habits peculiarly their own, as will be explained later on. Whether this recession of the nostril toward the eyes can be explained in all cases by the peculiar breathing habits is, however, doubtful. Possibly in some cases, such as the phytosaurs, described later, the creatures used their long beaks to probe in the mud while breathing. Possibly the posterior position has been in some cases rather the result of the elongation of the face, leaving the nostrils behind in some forms, or carrying them forward in others. Nevertheless posterior nostrils always indicate more or less aquatic habits.

In all the earliest reptiles, as we have seen, the neck was short, like that of their immediate progenitors, the ancient amphibians. The shoulders were close to the skull, with not more than two vertebrae that could be called cervical. It happens that most of the earliest reptiles, as we know them, were more or less amphibious in habit, and all of them were probably good swimmers; nevertheless in all likelihood reptiles began their career as a class with a very short neck. The earliest known distinctly terrestrial reptiles had a moderately long neck composed of six or seven cervical vertebrae. It may therefore be assumed with much probability that all later reptiles with a greater or less number of cervical vertebrae are specialized animals, so far as the neck is concerned. Most living reptiles have eight cervical vertebrae; a few have nine, and still fewer have but five. Birds may have as many as twenty-four, while all mammals, with two or three exceptions, have the primitive number seven. Among extinct reptiles, however, there were not a few with more numerous neck vertebrae, some having the enormous number of seventy-six.

An ordinary fish has apparently no neck whatever, the trunk being seemingly attached to the head, nearly as in the primitive amphibians and primitive reptiles. It is evident that a movable neck of considerable length would not only be of no use to the swiftly swimming fish, but a positive disadvantage to it. The body is quickly and easily turned by the powerful tail fin, and a long neck could be of no use that the tail would not better subserve. It is therefore of interest to learn that, as a rule, aquatic animals of all kinds having a powerful propelling tail have also a short neck, acquired either by the loss of neck vertebrae, or, as in the mammals, by the shortening and coalescence of the normal number of seven. There are very few exceptions to this rule of a short neck and a long tail. Those strange little reptiles of Paleozoic times, the first that we know that returned to the water, the Proganosauria, have not only a long, flattened tail, but also an unduly elongated neck of from nine to twelve vertebrae.

On the other hand, certain unrelated reptiles of the past, the dolichosaurs, nothosaurs, and plesiosaurs, with a short non-propelling tail, developed a long neck—sometimes an excessively long one in the plesiosaurs. The turtles, some of which have attained a high adaptation to water life, have invariably a short tail and a freely movable, relatively long neck, a neck which Dr. Hay tells us has increased in length from the beginning of their race by the simple elongation of the vertebrae, as in the giraffe, and never by the addition of vertebrae. We may then account it a rule that swimming animals with a long neck have a short tail, and those with a short tail have a long flexible neck. Even in the plesiosaurs there is some variation of the length of the tail in correlation with the neck. Short-tailed animals must necessarily propel themselves through the water by the aid of their legs, especially the hind legs. If one watches an actively swimming alligator he will observe that the front legs are folded or collapsed by the side of the body, while the hind legs, much bent, are used only slightly in propulsion. The animal swims by a marked sinuous or serpentine movement, like that of a snake upon land, extending throughout the tail and part of the body, at least. An animal propelling itself by its limbs could not move sinuously, and use its legs actively at the same time, and it is probable that the long neck has been evolved compensatorily.

With this shortening of the neck and sinuosity of movement there is developed in every case a long trunk as well as a long tail. The trunk becomes more slender and cylindrical, more like that of a snake, with an actual increase of the bones composing it, reaching the great number of forty-three vertebrae in that most sinuous of all water reptiles with legs, Pleurosaurus of the Protorosauria. And the tail, primitively having perhaps sixty or seventy vertebrae, may have as many as one hundred and fifty in the more typical aquatic forms. This elongation of trunk and tail must be of great advantage to the swimming reptile, just as the racing scull is a more perfect type of speedy craft than a flat-bottomed scow. Dr. Woodward has said that the fate of all fishes, if they continue their evolution long enough, is to become eel-like.

Not only was the tail greatly elongated in swimming reptiles, but it was also more or less flattened. In the beginning of water adaptation the flattening was throughout the tail, as in the living alligators and crocodiles. As the adaptation to water life became more perfect, the flattening became more and more restricted to the extremity; that is, the flattening begins like that of a salamander and in the end becomes like that of a fish, a terminal fin. And some of the actual stages in the evolution of the fish-like fin have been observed by Dr. Merriam in the earlier and more primitive ichthyosaurs of California. In those animals swimming chiefly in a horizontal direction the tail fin has become like that of fishes, that is, vertical; but in those animals which use the tail chiefly for ascending and descending rapidly in the water the fin is developed in a horizontal position, examples of which are seen in the flukes of whales and sirenians.

All animals living upon the land require firm articulations between the different bones of the skeleton, and especially between the vertebrae, for the support and control of the body. Among aquatic animals there is a strong tendency toward looseness of joints, with increasing flexibility. Fishes have the articular processes between the arches of the vertebrae feebly or not at all developed, and the centra or bodies of the vertebrae have thick pads of cartilage between them. Firm union between the vertebrae would restrict freedom of movement, and firmness is not required when the body is surrounded on all sides by water of nearly the same specific gravity as the body itself. And it is doubtless for the same reasons that the articulations of all strictly aquatic reptiles have for the most part become looser and less firm, especially those between the different vertebrae.

The same looseness of articulation is also found in the ribs of aquatic animals. In most animals, and in all those which walk erect, like the mammals, each rib is firmly attached to the backbone by two distinct joints, the head and tubercle, with an interval between them. This double attachment prevents much in-and-out movement of the ribs and gives a firm support for the attachment of the muscles of respiration, as well as for those supporting the viscera. This firmness is unnecessary in animals living always in the water, and the ribs therefore in all aquatic animals tend to become single-headed and loose. The lower or capitular articulation has been lost in part, or almost wholly, in many cetaceans. It has been said that a whale cast up on land will die of suffocation, not for the lack of air, for it is an air-breathing animal like ourselves, but because it can no longer use its respiratory muscles attached to the loosely articulated ribs; it suffocates because the ribs collapse.

As would be expected, the greatest modifications of structure in the adaptation of air-breathers to water life are found in the limbs. No other parts of the body have such different functions in water and on land as the limbs and fins. The limbs of a dog, or a cat, or a man are feeble organs for swimming in comparison with the fins of a fish, and if the land animal must compete with fishes to prey upon them for food it must acquire like swimming powers. As a matter of fact, the limbs of all typically aquatic air-breathing animals have lost nearly all external resemblance to the legs of walking and running animals, and have become more or less fin-like in function—fin-like in shape and function, but never fin-like in actual structure. No creature can go back and begin over again, any more than a man can again become a child with all its possibilities for improvement and development. If an animal cannot modify the organs it already possesses so as to adapt them to new and changed uses by the aid of evolutionary forces it must fail in the struggle. It can never acquire new material, never get new fingers and toes, new organs or parts of organs; all its possibilities lie in the improved and new uses it can make of the material which it received from its ancestors.

The beginning of aquatic adaptation of the limbs lies in the membranous webs between the toes of frogs, salamanders, ducks, seal, otters, etc., where the feet are used largely or entirely for propulsion through the water, in the absence of a propelling tail. And this membrane, in the majority of cases, is the extent of aquatic adaptation in air-breathing animals. In those animals, however, such as most of the reptiles described in the following pages, where the tail has developed as the propelling organ, the limbs lose to a greater or less extent their propelling function and become merely organs of equilibration and control. Of the two pairs of fins of fishes it is evident that the anterior ones have the more important equilibrational function; the hind ones have a much less important use as guiding organs; as a matter of fact, in not a few fishes the hind or pelvic fins have actually migrated forward to supplement the function of the pectoral fins. It is for these reasons that those animals best adapted of all for life in the water—the whales and sirenians—have lost the hind legs completely. In other tail-propelled air-breathers the hind legs have become progressively smaller and less powerful than the front ones. In all short-tailed water animals, however, where the legs, and especially the hind legs, have the important function of propulsion to subserve, they still retain the large size and firm connections with the body, examples of which will be seen in the seals, sea-otters, marine turtles, and plesiosaurs.

Because the legs are no longer needed for the support or propulsion of the body in long-tailed air-breathers, their connection with the body becomes less and less firm, long before their entire disappearance. In animals using the legs for crawling or walking the bones of an arm and thigh are elongated, and the joints are always well formed, permitting varied, extensive, and firm movements. Just the reverse is the tendency in all those animals that propel themselves by the aid of the tail in the water, since here what is needed is broad, short limbs, not long and slender ones.

Most reptiles have five digits on each hand or foot; the bones of the wrist and ankle are well formed, as in mammals, and the digits are elongate, with a very definite arrangement of the bones composing them, as already described, never exceeding five in any one finger or toe.

In the paddles of water reptiles, as the limbs are usually called, the bones of the first segment, that is, the humerus and femur, are always greatly shortened in those having a propelling tail, and even in some with a short tail, such as the seals, and in a lesser degree in the sea-otters. On the other hand, in those animals which use the legs chiefly for direct propulsion these bones are elongated, as exemplified by the plesiosaurs and marine turtles. In all save the seals and their kind, and the otters, whose legs are used rather as sculls than as oars, the bones of the next segment, the radius and ulna of the front pair, the tibia and fibula of the hind pair, are always shortened, and one can tell the stage of aquatic adaptation, as exemplified, for instance, in the plesiosaurs and ichthyosaurs by the degree of shortening of these bones. Indeed, the first suggestion in any crawling animal of water habits is shown in the relative lengths of the epipodial bones, as these bones are called. Furthermore, cursorial or terrestrial habits are suggested by the relative size of the smaller bone of the leg, that on the little-toe side, the fibula. In birds, pterodactyls, and most running animals, it disappears in part or wholly. In swimming animals it tends to grow larger than the tibia, as will be conspicuously seen in the paddle of the mosasaurs.

The bones of the wrist change in two ways: by becoming cartilaginous, as in whales and salamanders, or by becoming more firmly ossified and more closely united, as in the plesiosaurs. The digits always are elongated, often extraordinarily so, either by the elongation of individual bones or phalanges, or by the development of new bones. These new bones, when they occur, are new growths, not the reproduction of the old elements of fishes, and there may be as many as twenty such new elements or phalanges in a single digit. There is one marked exception among reptiles to this hyperphalangy, as the increased number of phalanges is called, and that is the turtles. As we have seen, in the elongation of the neck among turtles there never has been an actual increase in the number of vertebrae; so also in the elongation of the digits the normal number of three in each digit has never been exceeded, except among the river turtles, where there are four in the fourth digit—possibly a relic of original conditions rather than the beginning of hyperphalangy; but the individual bones have become greatly elongated. In living reptiles, birds and mammals of the land, the fifth toe is always shorter than the fourth. In the seals, the sea-otter, and to a less degree in the muskrat, the fifth toe has become elongated. And the elongation of this toe is the first and most decisive indication of a webbed foot of strong propelling power among the aquatic reptiles of the past, as exemplified especially by the proganosaurs. Finally, in one order of extinct reptiles, the ichthyosaurs, there has been an actual increase in the number of digits, in some to as many as nine in each paddle.

In addition to all these modifications of the skeleton, the bones themselves tend to become softer and more spongy in aquatic animals. The bones of the whale, as is well known, are very spongy in texture, and those of the seals and sea-lions contain an unusually large amount of oily matter. So, too, the bones of the extinct water reptiles—of many of them at least—were more spongy than those of their land relatives; and this is due in part perhaps to their lessened use as muscular supports, in part perhaps to the necessity of a lessened specific gravity. As a rule sea-animals need to be of the same specific gravity as the water in which they live, or a little less. The bones of the living sirenians, the manatees and dugongs, so far from being light and porous, are unusually dense and solid. The sirenians live habitually at the bottom of shallow waters, feeding upon vegetable growths; and doubtless their bottom-feeding habits account for the solidity of the bones. A whale would float to the top, while a dugong would sink to the bottom, on the relaxation of all muscular movement. And we shall see that certain reptiles in the past had in all probability like bottom-feeding habits, because of the solidity of the bones of their skeletons.

Many birds and fishes have a peculiar ossification of the usually tendinous outer covering of the eyeball, called the sclerotic membrane. These ossifications form a flattened or somewhat projecting conical bony ring about the pupil of the eye. The individual bones are flat and more or less imbricated plates, with some motion between them. Accommodation for vision in reptiles, birds, and fishes is not the simple process that it is in mammals, where it is controlled by simple ciliary muscles which compress the lens, causing it to assume a more spherical or a more flattened form, thus changing the focus. In reptiles accommodation is effected by the compression of the eyeball by means of external muscles, elongating it and causing its front part to expand or project. The imbricated sclerotic plates permit this expansion and contraction of the eyeball. Under great internal or external air pressure the cornea, the only unprotected part, must necessarily change its contour unless some compensatory force is brought to bear to counterbalance it; and this doubtless was the function of the sclerotic plates so commonly present in aquatic reptiles.

Among terrestrial reptiles there are not a few examples of the ossification of such sclerotic plates, notably among the skink lizards. Every known form of extinct reptiles of aquatic habit had them, and even some of the subaquatic dinosaurs, like Diplodocus and Trachodon. One may say with assurance that it is impossible for any reptile to become thoroughly adapted to aquatic life without acquiring large and strong sclerotic plates.

Most land reptiles are or were covered by horny scales or bony plates; the pterodactyls are the only order of terrestrial reptiles with no such covering of which we have any evidence. Such coverings are wholly unneeded for animals living in the water. Not only are they unnecessary, but the increased resistance to the water would be more or less detrimental to rapid swimming. It is for these reasons doubtless that bony plates or horny scales disappeared for the most part from the skin of all truly aquatic reptiles and mammals.

The foregoing are the chief acquired characteristics of aquatic air-breathing animals and especially aquatic reptiles in adaptation to their new mode of life. The resemblances, sometimes striking, thus brought about in animals of very different origin and remote relationships have often been mistaken for evidences of kinship, that is, direct inheritance from common ancestors. Such acquired resemblances in unrelated animals are known as parallel or convergent evolution. It has often been difficult to distinguish between convergent evolution and direct evolution, and difficulties still perplex and trouble the student of natural history in every branch of life. Not till all such problems are solved can we hope to attain the true classification of animals and plants. The whales a century ago were considered merely breathing fishes; the ichthyosaurs until a quarter of a century ago were supposed to be the direct descendants of fishes; lizards and crocodiles were grouped together in a single order; and salamanders were called reptiles not very long ago.

Perhaps the reader will be able from the foregoing to understand and appreciate better some of the difficulties that confront the paleontologist in his attempts to solve the problems of past life; to understand why he sometimes makes mistakes, for he has by no means yet learned all the permutations of the skeleton in any class of vertebrates, and is not sure that the laws he accepts are not subject to modifications and exceptions. If he is truly scientific he hesitates long in prophesying or conjecturing.

CHAPTER VI
SAUROPTERYGIA

Very scanty are the early human records of those strange reptiles known as the plesiosaurs. Were one to search through the many works published during the latter half of the seventeenth century and all of the eighteenth, devoted to “lapides petrifacti,” “figured stones,” “reliquia diluvii,” or by whatever other fanciful names fossils were known, here and there he would probably find descriptions and figures of bones of these reptiles. It would hardly seem that plesiosaurian bones could have been overlooked by the curious, so abundant are they in many places. But there is no such history of the early discovery of the plesiosaurs as there is of the ichthyosaurs and mosasaurs. Their birth into human history was very formal and proper, under the ministrations of a learned doctor of science, the renowned Conybeare, of whom we shall speak again. It was he, who with De la Bêche, late Director of the British Geological Survey, described for the first time, in 1823, one of these reptiles, to which he gave the name Plesiosaurus, meaning “like a lizard.” He distinguished the plesiosaurs from ichthyosaurs, with which it is possible that they had previously been confounded, and gave a good description of considerable material. Cuvier, a little later, gave a more complete description of the same remains which had served Conybeare and De la Bêche for their original description, and for the first time made it evident that fossil plesiosaurs were widely and abundantly distributed over the earth. The closing sentence of Cuvier’s chapter devoted to the discussion of these creatures in his Ossemens Fossiles was really prophetic, not only of the many discoveries of the plesiosaurs yet to be made, but of all other extinct animals as well: “I doubt not that, in a few years it may be, I shall be compelled to say that the work which I have today finished, and to which I have given so much labor is but the first glimpse of the immense creations of ancient times.”

Fig. 31.—Restoration of Plesiosaurus guilelmi imperatoris (left figure) and Thaumatosaurus victor (right figure), Liassic plesiosaurs. (From E. Fraas.)

In quick succession there followed many other discoveries of plesiosaurs, not only in England but elsewhere in Europe. The famous English anatomist and paleontologist, Sir Richard Owen, to whom we owe, perhaps, more than to anyone else our present knowledge of these animals, the eccentric Hawkins of England, the learned von Meyer of Germany, and, in later times, more especially Seeley and Andrews of England, Fraas of Germany, Bogalobou and Riabanin of Russia, as well as many others, have brought to light during the past century many and varied forms of those sea-reptiles. Blaineville in 1835 gave to the plesiosaurs an ordinal rank under the class Ichthyosauria, and even the astute Owen in 1839 united them with the ichthyosaurs as a suborder of his Enaliosauria, or “sea-saurians.” He called them Sauropterygia, or “reptile-finned,” and these terms, Enaliosauria, Ichthyopterygia, and Sauropterygia, have long persisted in works on natural history because of the prestige of Owen’s name. As we shall see later, the plesiosaurs are really of remote kinship to the ichthyosaurs, and there is no such natural group as the Enaliosauria. It often takes years to distinguish between apparent and real relationships among living organisms, and both of these groups of sea-saurians have had a sorry experience in the treatment they have received from nomenclators.

Perhaps because of the writings of Dean Buckland in his famous Bridgewater Treatise, in large part a theological disquisition, though of real scientific merit, the ichthyosaurs and plesiosaurs early became widely and popularly known, and, even to this day, these reptiles, together with the dinosaurs, first made known by Rev. Dr. Mantell, are often supposed to be the most typical and horrid of monsters. Many and fabulous are the tales that have been told of them in literature both grave and gay. The preacher adduced them as evidences of the great world-catastrophe told in biblical history, and the German student sings of them to the tune of the “Lorelei”:

Es rauscht in Schachtelhalmen, verdächtig leuchtet das Meer;

Da schwimmt mit Thränen in Auge ein Ichthyosaurus einher.

Ihn jammert der Zeiten Verderbniss, denn ein sehr bedenklicher Ton

War neuerlich eingerissen in der Liasformation.

Der Plesiosaurus, der alte, der jubelt in Saus und Braus;

Der Pterodactylus selber flog jungst betrunken nach Haus.

Der Iguanodon, der Lümmel, wird frecher zu jeglicher Frist;

Schon hat er am hellen Tage die Ichthosaura geküsst.

We now know that they were not the monsters of horrid mien that they were once supposed to be: the largest plesiosaurs, were they living today, would find unopposable foes in the vicious and cruel crocodiles. They were relatively stupid and slow, cruel enough to the smaller creatures, but of limited prowess. But in structure and habits they are among the most remarkable of all the animals of the past or present.

Although their remains are among the most abundant and widely distributed of all fossil reptiles, the plesiosaurs as a whole are less perfectly known than either the ichthyosaurs or the mosasaurs, and it has been within a comparatively few years only that an approximately complete knowledge of any form has been obtained. This is partly due to the fact that the order comprises vastly more kinds, more species, genera, and families than does any other order of marine reptiles; partly because their remains, though widely distributed over the earth, and in rocks of many geological epochs, are seldom found completely preserved; usually specimens comprise only a few bones or single bones, and complete skeletons are rare. Were there but few kinds, the many specimens discovered would mutually supplement each other, finally completing our knowledge; but the fragments of many kinds only add to our confusion. Nevertheless, because the plesiosaurs lived so long in geological history, their remains are found in rocks of many different kinds, and since it is improbable that any of them had great specific longevity, it is very probable that all these described species, or most of them, often made known from single bones, will eventually be found to be distinct, and that many more will be added to them. It does not seem improbable that within the next forty or fifty years not less than a hundred species of plesiosaurs will have been discovered in North America alone. At the present time perhaps that many have been described from the whole world.

When Blaineville gave the name Plesiosauria to the aquatic reptiles described by Conybeare, Cuvier, and others, he had no knowledge of others of an intermediate kind between them and land reptiles. His group-term then can be properly applied only to the truly aquatic forms, and Owen’s name Sauropterygia becomes available in a wider sense to include all the known types belonging to the order of which the plesiosaurs form a part. Of this order then there are two clearly marked divisions or suborders, the Plesiosauria and the Nothosauria, the former having a complete aquatic adaptation, the latter only a partial one. While the two suborders are evidently allied, some authors have suggested that their differences are only familial; others have thought that they are really orders. We shall see how close the relationships are.