Popular Science Monthly/Volume 60/February 1902/Winged Reptiles
By Professor S. W. WILLISTON,
UNIVERSITY OF KANSAS.
OF the cold-blooded, air-breathing animals known as true reptiles there are now in existence upon the globe more than four thousand species, classified by naturalists in four very distinct groups or orders—the Rhynchocephalia, Crocodilia, Chelonia and Squamata. Of the Rhynchocephalia, there is but a single species now living, the Tuatera or Sphenodon, confined to the islands off the northeast of New Zealand, and very rare. It is a small animal, seldom attaining a length of twenty inches, lizard-like in habits and appearance. Though now so near extinction, the order during Paleozoic times was an important one, both in size and variety. No older reptiles are known.
The Crocodilia, inclusive of the alligators and gavials (better garials) number at the present time scarcely twenty-five species, confined to tropical and subtropical shores of both the old and the new worlds. Of great size, cruel and sluggish in disposition, they seem to be a reminder of those times when size, rapacity and cruelty characterized their class on land, in the air and in the water. Though approaching extinction, these modern representatives of what was at one time a far more numerous and widely distributed order are the specialized descendants of an ancestral line scarcely less ancient than the rhynchocephalian. In form and size, and probably also in habits, the crocodilia have varied comparatively little throughout the greater part of their long period of existence upon the earth.
Were none of the Chelonians—the tortoises and turtles—now living, the extinct forms would appear to us among the most remarkable of vertebrate animals, so little interest do familiar things incite. Like the crocodilia and rhynchocephalia, they are among the oldest of reptiles; yet in number and variety at the present time they are inferior only to the lizards and snakes, and may truly be said, after the lapse of many millions of years, to be only now at or but recently past the zenith of their development. In the past there have been species perhaps twice or thrice as large as any now in existence—from the Bad Lands of Dakota there is known one sea-turtle twelve or more feet in expanse of shell, with a skull nearly as large as that of a horse—but in type of structure the very oldest that we know differed but slightly from some that are now living.
On the other hand, the Squamata (so called because of the scaly covering which characterizes them) are the most modern of all reptiles. They number now more than thirty-four hundred species, about equally divided between the lizards and snakes, the only modern representatives of the order. In the remote past so far as we can trace the broken line of descent, the ancestral forms had but a single conspicuous offset, the rapacious mosasaurs, which had, however, a comparatively brief existence. The lizards, like the snakes, probably go back little or no further than the Age of Mammals. They are, with few exceptions, terrestrial animals of small size and inoffensive habits; the largest living examples scarcely exceed six feet in length, though within the period of man's existence there have been lizards five or six times as long.
Snakes or serpents, the latest, and in many respects the most specialized, of all reptiles, have not yet reached the culmination of their development, a statement which perhaps may not be truthfully made of any other group of reptiles. Their geological history is insignificant and scanty. The venomous serpents especially present the latest modifications of reptilian structure, perhaps the very latest antecedent to the final extinction of the whole class.
No reptiles at the present time walk erect, as did many of the extinct kinds. Their progression is essentially a crawling one, their legs, when present, serving more for propulsion than support. The only exceptions to strictly arboreal, terrestrial or aquatic habits among living reptiles are found in the curious and beautiful little flying lizards, or 'flying dragons,' of the Malayan region. In these reptiles, the flattened body is provided with a broad, wing-like expansion of the skin of its sides, supported by the elongate, movable ribs, and capable of being folded up, fan-like. Similar membranous expansions are also found on the sides of the throat. By these means the creature, which lives for the most part among the tree tops, rarely descending to the ground, is capable of certain aerial movements, though not of true flight. In describing its habits a writer has said: "As the lizard lies in shade along the trunk of a tree, its colors at a distance appear like a mixture of brown and gray and render it scarcely distinguishable from the bark. There it remains with no sign of life, except the restless eyes, watching passing insects, which, suddenly expanding its wings, it seizes with a sometimes considerable, unerring leap." Their flight through the air is very swift, so swift that the expansion of the parachute-like membrane may almost escape notice. As in the flying fishes, flying lemurs and flying squirrels, there is not true flight—a power possessed by no living back-boned animals except birds and bats.
Altogether the direct antecedents of the reptiles now living, that is the crocodiles, turtles, lizards and snakes of the past, took no important part in the great Age of Reptiles. They have existed all these millions of years, some of them at least, because of their comparative insignificance among their many powerful relatives. Of these relatives, the huge dinosaurs, the swimming ichthyosaurs, plesiosaurs and mosasaurs, and the flying ornithosaurs, with others scarcely less renowned in geological history, are known only from the scanty records of the rocks.
Of all the reptiles of the past, perhaps the most extraordinary, the ones which departed most from the reptilian type in form and habits, as well as the most highly specialized of all cold-blooded animals of the past or present, were the ornithosaurs or pterodactyls. They made their appearance in geological history, so far as is known, suddenly and in a highly developed state; we know nothing of their antecedents, nor indeed of their descendants. They flourished through millions of years in great numbers and multitudinous forms, and then, evidently, as suddenly disappeared. So different were these flying creatures from any others of the past or present that their proper place in the animal kingdom has been disputed. They were at first thought to be birds, and so described; and this idea of their relationship found expression in the name by which they are more properly known, the ornithosaurs or bird-reptiles. Some authorities would even yet give them a place all their own among animals, coordinate with the birds and reptiles. But they are now so well known to students of extinct life, and in so many forms, that there can scarcely be longer a serious question of their real, though highly modified, reptilian affinities. It is not impossible, however, though somewhat improbable, that they were, unlike others of their class, warm-blooded animals; it is at least highly probable that their circulatory and respiratory systems reached a much higher degree of perfection than is the case in any cold-blooded animals of the present time.
As flying organisms they attained the highest degree of specialization that has ever been reached among animals with a back-bone, at least so far as their skeleton was concerned. Some of them, indeed, seem to have nearly lost all other powers of locomotion—they could move through the air only. Furthermore, the relative proportion between volant surface and body-weight in some of these pterodactyls has been excelled only among insects. They were, par excellence, organisms of flight, to which function all else, so far as was compatible with existence, was subservient.
It was only among the later geological forms, however, that this fight specialization was carried to its extreme, a fact which has its parallel in not a few other classes of animals, where it has been only in the last bitter struggles for existence that the evolution of the type has been carried to its uttermost. In the accompanying figure is given a restoration of Pteranodon, one of these most highly specialized pterodactyls, based upon skeletons from the Cretaceous chalk of western Kansas. As in all restorations of extinct animals which departed widely from any now in existence, entire truthfulness of expression and appearance can not be expected. A more or less close approximation to the living picture is, however, assured from a complete knowledge of the skeleton and the impression in the rocks of form and of membranes, which have been preserved with the bones of allied forms. I use the more widely known name Pteranodon for this animal, though I believe it to be identical with the European genus previously called Ornithostoma.
The body of Pteranodon was short and relatively small; the legs were slender and weak, and loosely jointed; the toes were small and delicate, with little or no grasping power, and without claws; the tail was rudimentary. As if in compensation for the greatly reduced posterior part of the body, the whole anterior part—the head, neck, thorax and anterior limbs—was extraordinarily developed. The head was slender and elongate, with a dagger-like beak, possibly covered with a horny sheath, and the jaws were wholly toothless. In the largest species the head measured over four feet in length, and its equilibrium was maintained by a strange prolongation backward of the skull. The nostrils were placed far back, and the eyes were protected by a ring of bony plates, similar to those of the eyes of hawks and owls. The neck was long and strong, and very flexible, with a remarkable series of additional articulations unlike anything found in the neck bones of other animals, whereby the thrusting and striking power of the beak was greatly intensified.
But it was in the fore limbs and their attachments that the remarkable adaptation for flight of these animals is found. The shoulder-blade and coracoid, functionally replacing the collar-bone of many animals, were fused together into a stout curved bone, articulating in front with the broad breast-bone and behind with the spines of several fused back-bones or vertebrae. These two shoulder bones thus formed a complete, stout ring, firmly attached in front and behind. Such a mode of articulation of these bones is found in no other animal. To add further to the strength and rigidity of the thorax, the strong upper ribs were not movably articulated, but coossified with the vertebrae. Altogether, the large and broad breast-bone, the complete shoulder-ring, the rigidly fixed upper ribs and the immovable vertebrae furnished a support for the attachment of the enormous wing and its necessary muscles that has no parallel for strength among other animals.
Among the higher animals there are three types of structure whereby the fore limbs are modified into organs of flight. In birds, the arm and fore-arm bones are elongated and strengthened; the bones of the wrist are fused together or wanting, while the few bones of the fingers are welded together and made to hinge sideways. In the bat, among mammals, all the bones, save those of the wrist, are elongated and slender. The very slender fingers are spread out fan-like laterally to support the membrane which replaces the feathers of the bird.
In the pterodactyls, the volant surface, as in the bats, was presented by a thin membrane, which extended from the fore limb to the sides of the body. The bones of the arm were strong, with strong projections for the attachment of muscles; those of the wrist were few in number and closely united. The rudimentary first finger, or thumb, consisting of a single slender bone, was turned backward toward the shoulder for the support of the membrane in front of the elbow. The second, third and fourth fingers were entire, but very small and slender, and were provided at their extremities with very sharp and strongly curved claws. While these fingers were flexible and prehensile, their small size and weakness of attachment (they did not articulate with the wrist) suggest that their only use was for clinging. In strange contrast with these small fingers, the fifth digit, that is the one corresponding to the little finger of the human hand, was enormously elongated and strong, including by far the largest bones of the entire skeleton. Between its first and second bones there was a marvelously perfect pulley-like joint, permitting the flexion of the finger through an arc of nearly one hundred and eighty degrees. When not in use in support of the wing membrane, the finger could have been folded back close to the arm, touching each other nearly on the back. In fact these bones have been found fossilized in this position.
Attached to the whole length of the arm and finger to its curved extremity, there was a thin, and certainly strong, membrane, which continued on the sides of the body and legs, probably quite to the rudimentary feet. Impressions of this membrane have been found, showing every fold and crease, as though cast in plaster-of-paris.
In size these highly specialized pterodactyls reached, as outspread in flight, the enormous expanse of more than nineteen feet from tip to tip of fingers. The length of the body from tip of bill to toes was about eight feet of which the head was nearly a half. Contemporaneous with these gigantic species were many others of smaller size, while related forms of an earlier period scarcely exceeded the stature of a common sparrow.
The earlier pterodactyls were smaller and in many other respects much less specialized than the latter ones, that is they departed less widely from the true reptilian type. Of these perhaps the best known is the Rhamphorhynchus, a restoration of which is given herewith, based upon a nearly complete skeleton in the Yale Museum, a specimen of great interest, because it shows so clearly the impressions of the wing membrane, wholly without hair, feathers or scales. In this pterodactyl, the jaws were provided with long and sharp teeth, the neck was less stout, the shoulders were loosely articulated, the bones of the wings less elongated, the legs stouter. Furthermore, it had a long, slender and flexible tail, provided at its extremity with a diamond shaped membranous expansion, which doubtless served as a steering organ or rudder in flight. Between the Rhamphorhynchus and Pteranodon there were many intermediate forms, both large and small, with and without teeth. Species of Ornithocheirus, nearly as large as those of Pteranodon, which they closely resembled, except in the presence of teeth, are known from the chalk of England. Rhamphorhynchus and the still earlier Dimorphodon, as well as the later Pterodactylus are from the Jurassic deposits of England.
That the Pteranodon had a marvelous capacity for flight, there can be hardly a question. Their remains are often found in the Kansas marine chalk deposits associated with others of deep-sea animals, and many miles away from the ancient shore lines; found so completely preserved that they never could have drifted far. With their remains have been found the fossilized stomach contents, including comminuted fish bones and scales. The bones of their skeletons were very hollow and light, perhaps more so than in any other animals that have ever lived; so light indeed that a finger bone twenty-six inches in length might be best likened to a hollow cylinder of blotting paper two inches in diameter; and this hollowness extended into nearly every bone of the skeleton, from the toes to the ribs and the skull. Moreover, the bones were all of fine and firm texture, and the sutures or immovable joints throughout the body were fused or anchylosed, as is the case with birds, thus giving the maximum of strength with the minimum of weight. The larger bones had each one or two pneumatic openings in them, through which doubtless entered ramifications of the bronchial tubes, as in birds.
Altogether the weight of a Pteranodon of the largest size in life could scarcely have reached twenty-five pounds—the bones of the skeleton having an expanse of twenty feet, even as petrified, do not weigh more than five or six pounds. This extreme lightness, together with the vast expansion of the volant membranes, suggests as a comparison the hull and sails of a Columbia or Shamrock. With the perfect and delicate construction and their strong articulations, the fusion of all loose bones, the presence of sclerotic plates in the eyes giving greater control over vision in high altitudes and in the dark, the possession of a relatively large and bird-like brain, all conclusively demonstrate the flight powers of these singular reptiles.
Upon land even the best of the pterodactyls must have been awkward and ungainly, creeping about upon hands and feet, impeded by the cumbersome head and flapping membranes. It is doubtful, indeed, whether the more highly specialized forms like Pteranodon often voluntarily sought the surface of the ground; they doubtless spent the most of the time while not flying, suspended from cliffs and trees by means of their slender, clawed fingers. In the air they reigned supreme and alone, save for the small toothed birds which were then making their appearance in geological history, and which may often have served as delicious tid-bits for their insatiable maws. The sharp, dagger-like beak, driven by the powerful neck, was certainly a murderous weapon against their enemies, whether upon land or in the air. That they laid eggs, like most reptiles, is almost certain. Did they build nests in inaccessible heights?
That the ornithosaurs for a long time enjoyed a wide distribution throughout the world is certain—their remains have been discovered in the most remote parts of the earth, and wherever the conditions were favorable for their existence and the rocks for the represervation of their remains, traces of them may be confidently expected to be found. In North America, with a single exception, their fossilized bones have been discovered only in the Cretaceous deposits of western Kansas. Throughout this region there are wide exposures of pure chalk, the sediment of the old Cretaceous ocean, which at one time extended from the Gulf of Mexico to the Arctic Ocean, and from central Kansas on the east to far into Colorado on the west. Because of the protected situation of this inland body of salt water, and its placid surface, it was a favorite region for countless and divers animals of strange aspect. The waters swarmed with swimming reptiles of unfamiliar shapes and great size, while over its surface hovered and soared innumerable pterodactyls searching for their prey in the waters beneath. The delicate bones of the pterodactyls, beneath the pressure of the superincumbent rocks, have been, almost invariably, crushed flat, requiring skill and care in their excavation. Sometimes, though rarely, has a skeleton been found nearly complete; more frequently are the scattered bones found here and there. Considering the great buoyancy of their bodies floating upon the surface of the water, or sinking slowly through the ocean's depths, swarming as it was with many predaceous scavengers, one can only wonder that so many have been preserved all these millions of years for the delight and amazement of the modern student of geology.