 CHAPTER 18 OF DRAGONS OF THE AIR by Harry Seeley Ornithosauria have many characters inseparably blended together, which are otherwise distinctive of reptiles, birds, and mammals, and associated with peculiar structures which are absent from all other animals. They are not quite alone in this incongruous combination of different types of animals in the same skeleton. Dinosaurs, which were contemporary with Ornithosaurs, approximate to them in blending characters of birds with the structure of a reptile and something of a mammal in one animal. If an Ornithosaur is reptilian in its backbone, in the articular ends of each vertebra having the cup in front and ball behind in the manner of crocodiles, serpents, and many lizards, a dinosaur like iguanodon, which had the reversed condition of ball in front and cup behind in its early vertebrae, may be more mammalian than avian in a corresponding resemblance of the bones to the neck in hoofed mammals. But while pterodactyls are sometimes mammalian in having the head of the thigh bone molded as in carnivorous mammals and man, the corresponding bone in a dinosaur is more like that of a bird. And while the pterodactyl shoulder girdle is often absolutely bird-like, that region in dinosaurs can only be paralleled among reptiles. Such combinations of diverse characters are not limited to animals which are extinct. There were not wanting scientific men who regarded the platypus of Australia when first sent to Europe as an ingenious example of Eastern skill in which an animal had been compounded artificially by blending the beak of a bird with the body of a mammal. Fuller knowledge of that remarkable animal has continuously intensified wonder at its combination of mammal, bird, and reptile in a single animal. It has broken down the theoretical divisions between the higher vertebrae, demonstrating that a mammal may lay eggs like a reptile or bird, that the skull may include the reptilian characters of the mallar arch and prefrontal and postfrontal bones otherwise unknown in mammals and birds. The groups of mammals, birds, and reptiles now surviving on the earth prove to be less sharply defined from each other when the living and extinct types are considered together. But in pterodactyls, mammal, bird, and reptile lose their identity as three colors would do when unequally mixed together. This mingling of characteristics of different animals is not to be attributed to interbreeding but is the converse of the combination of characters found in hybrid animals. It is no exaggeration to say that there is a sense in which mammal, bird, reptile, and the distinctive structures of the ornithosar have simultaneously developed from one egg in the body of one animal. The differences between those vertebrate types of animals consist chiefly in the way in which their organization is modified by one strain of characters being eliminated so that another becomes predominant while a distinctive set of structures is elaborated in each class of animals. The earlier geological history of the higher vertebrate is very imperfectly known, but the evidence tends to the inference that the older representatives of the several classes approximate to each other more closely than do their surviving representatives so that in still earlier ages of time the distinction between them had not become recognizable. The relation of the great groups of animals to each other among vertebrata is essentially a parallel relation like the colors of the solar spectrum or the parallel digits of the hand. It was natural when only the surviving life on the earth was known to imagine that animals were connected in a continuous chain by successive descent but mammals have given no evidence of approximation to birds. And birds discover no evidence that their ancestors were reptiles in the sense in which that word is used to define animals which now exist on the earth. When the variation which animals attain in their maturity and exhibit in development from the egg was first realized it was imagined that nature by slow summing up an accumulation of differences which were observed would so modify one animal type that it would pass into another. There is little evidence to support belief that the changes between the types of life have been wrought in that way. The history of fossil animals has not shown transitions of this kind from the lower to higher vertebrata but only intermediate parallel groups of animals analogous to those which survive and distinct from them in the same way as surviving groups are distinct. The circumstance that mammals, birds, and reptiles are all known low down in the secondary epic of geological time is favorable to the idea of their history being parallel rather than successive. Such a conception is supported by the theory of elimination of characters from groups of animals as the basis of their differentiation. This loss appears always to be accompanied by a corresponding gain of characters which is more remarkable in the soft vital organs than in the skeleton. The gain in higher vertebrates in the bones is chiefly in the perfection of joints at their extremities. But the gain in brain, lungs, heart, and other soft parts is an elaboration of those structures and an increase in amount of tissue. The resemblances of ornithosaurs to mammals are the least conspicuous of their characters. Those seen in the upper arm bone and thigh bone are manifestly not derived from mammals. They cannot be explained as adaptations of the bones to conditions of existence because there is no community of habit to be inferred between pterodactyls and mammals in which the bones are in any way comparable. Other fossil animals show that a fundamentally reptilian structure is capable of developing in the mammalian direction in the skull, backbone, shoulder girdle, hip girdle, and limbs so as to be uniformly mammalian in its tendencies. This is proved by tracing the North American Texas fossils named labyrinthodonts through the South African theriadonts towards the monotrimata and other mammalia. Just as those animals have obliterated all traces of the bird from their skeletons, birds have obliterated the distinctive characters of mammals. The ornithosaur has partially obliterated both. With the skull and backbone marked by typical characters of the reptile, it combines the shoulder girdle and hip girdle of a bird with characters in the limbs which suggest both those types in combination with mammals. The bones have been compared in the skeleton of each order of existing reptiles and found to show side by side with their peculiar characters, not only resemblances to the other reptilia, but an appreciable number of mammalian and avian characters in their skeletons. The term crocodile, for example, indicates an animal in which the skeleton is dominated by one set of peculiar characters. Crocodiles retain enough of the characteristics of several other orders of reptiles to show that an animal sprung from the old crocodile stock might diverge widely from existing crocodiles by intensifying what might be termed its dormant characters in the crocodile skeleton. Comparing animals together, bone by bone, it is possible to value the modifications of form which they put on, and the resemblances between them so as to separate the inherited wealth of an animal's affinities with ancestors or collateral groups from the peculiar characters which have been acquired as an increase based upon its typical form. There is no part of the pterodactyl skeleton which is more distinctly modified than the head of the upper arm bone which fits into the socket between the coracoid bone and the shoulder blade. The head of the humerus, as the articular part is named, is somewhat crescent shaped, convex on its inner border and a little concave on its outer border, and therefore unlike the ball shaped head of the upper arm bone in man and the higher mammals. It is much more nearly paralleled in the little group of monotromata allied to the living ornithorhynchus. In that sense, the head of the humerus and pterodactyl has some affinity with the lowest mammalia which approach nearest to reptiles. The character might pass unregarded if it were not found in more striking development in fossil reptiles from Cape Colony which from having teeth like mammals are named Theriodontia. In several of those South African reptiles, the upper arm bone approaches closer to the humerus in ornithosars than to ornithorhynchus. Such coincidences of structure are sometimes dismissed from consideration and placed beyond investigation by being termed adaptive modifications. But there can be no hope of finding community of habit between the burrowing monotreme, the short-limbed Theriodont, and the flying pterodactyl which might have caused this articular part of the upper arm bone to acquire a form so similar in animals constructed so differently. If the resemblance in the humerus to monotremes in this respect is not to be attributed to burrowing, neither can the crescent form of its upper articulation be attributed to flight. For in birds the head of the bone is compressed but always convex and bats fly without any approach to the pterodactyl form in the head of the humerus. This apparently trivial character may from such comparisons be inferred to be something which the way of life of the animal does not sufficiently account for. These deepest seated parts of the limbs are slow to adapt themselves to changing circumstances of existence and retain their characters with moderate variation of the bones in each of the orders or classes of animals. It therefore is safer to regard mammalian characters as well as the resemblances which pterodactyls show to other kinds of animals as due to inheritance from a time when there was a common stock from which none of these animals which have been considered had been distinctly elaborated. A few characters of ornithosars are regarded as having been acquired because they are not found in any other animals or have been developed only in a portion of the group. The most obvious of these is the elongated wing finger. But in some genera like dimorphodon there is also a less elongation of the fifth digit of the foot and perhaps in all genera there is a backward development of the first digit of the hand which is without a claw and therefore unlike the clawed digit of a bat. An acquired character of another kind which is limited to the Cretaceous genera is seen in the shoulder blade being directed transversely outward so that its truncated end articulates by a true joint with the early vertebrae of the back and defended the cavity enclosed by the ribs by a strong bony external arch. And finally as the animals later in time acquire short tails and relatively longer limbs the bones of the back of the hand termed metacarpals acquire greater and distinctive length which is not seen in the long tailed types like rampphyrincus. These and such like acquired characters distinguish the class of animals from all groups with which it may be compared and mark the possible limits of variation of the skeleton within the boundary of the order. But no further variation of these parts of the skeleton could make a transition to another order of animals or explain how the pterodactyls came into existence because the characters which separate orders and classes of animals from each other differ in kind from those which separate smaller groups named genera and species of which the order is made up. The accumulation of the characters of genera will not sum up into the characters of an order or class. In making the division of vertebrate animals into classes the skeleton is often almost ignored. Its value is entirely empirical and based upon the observed association of the various forms of bones with the more important characters of the brain and other vital organs. What is understood as a mammalian or avian character in the skeleton is the form of bone which is found in association with the soft vital organs which constitute an animal, a mammal or a bird. The characters which theoretically define a mammal appear to be the enormous overgrowth of the cerebral hemispheres of the brain by which the cerebrum comes into contact with the cerebellum as among birds. This character distinguishes both groups of animals from all reptiles, recent and fossil. But in examining the mold of the interior of the brain case it is rare to have the bones fitting so closely to the brain as to prove that the lateral expansion below the cerebrum and cerebellum is formed by the optic lobes of the brain. Otherwise the brain of a pterodactyl might be as like to the brain of ornitho rancous as it is like that of a bird, figure 19. But it is precisely in this condition of arrangement of the parts of the brain that the specimens appear to be most clear. The lateral mass of brain in specimens of ornithosaurs from the lower secondary rocks appears to be transversely divided into back and front parts which may be thought to correspond to the structures in a mammal brain named corpora quadragemina. But to be placed as the optic lobes are placed in birds and to have relatively greater dimensions than in mammals. No evidence has been observed of this transverse division of the optic lobes of the brain in pterodactyls from the chalk and cretaceous rocks and so far as the evidence goes this part of the brain was shaped as in birds but rather smaller. The brain is the only soft organ in which a mammalian character could be evidenced. The uniformity and character of the brain throughout the group in mammals is remarkable in reference to the circumstance that the reproduction varies in type. The lowest or monotreme division being oviparous. If there is no necessary connection between the mammalian brain and the prevalent condition under which the young are produced alive it may be affirmed also that there is no necessary connection between the form of the brain and the form of the bones since the brain cavity in pterodont reptiles shows no resemblance to that of a mammal. While the bones are in so many respects only paralleled among monotremata and mammalia. The variety of forms which the existing mammalian orders of animals assume shows the astonishing range of structure of the skeleton which may coexist with the mammalian brain. And therefore we are led to the conclusion that any other fundamental modification of brain such as distinguishes the class of birds might also be associated with forms and structures of the skeleton which would vary in similar ways. In other words if for convenience we define a mammal by its form of brain structure of the heart and lungs and provision for nutrition of the young without regard to the covering of the skin which varies between the scales of a pangolin and the practically naked skin of the whale. A bird might be also defined by its peculiar conditions of brain and lungs without reference to the feathered condition of the skin though the feathered condition extends backward in time to the upper secondary rocks as seen in the Archaeopteryx. The avian characters of pterodactyls are the predominant parts of their organization for the conditions of the brain and lungs shown by the molds of the brain case and the thin hollow bones with conspicuous pneumatic foramina give evidence of a community of vital structures with birds which is supported by characters of the skeleton. If any classificational value can be associated with the distribution of the pneumatic foramina as tending to establish membership of the same class for animals fashioned on the same plan of soft organs. The evidence is not weakened when a community of structures is found to extend among the bones to such distinctive parts of the skeleton as the sternum shoulder girdle bones of the forearm and foreleg. For in all these regions the pterodactyl bones are practically indistinguishable from those of birds. This is the more remarkable because other parts of the skeleton such as the humerus and pelvis show a partial resemblance to birds while the parts which are least avian like the neck bones have no tendency to vary the number of the vertebrae in the way which is common among birds following more closely the formula of the seven cervical vertebrae of mammals. It would therefore appear from the vital community of structures with birds that pterodactyls and birds are two parallel groups which may be regarded as ancient divergent forks of the same branch of animal life which became distinguished from each other by acquiring the different condition of the skin and the structures which were developed in consequence of the bony skeleton ministering to flight in different ways. And with different habit of terrestrial progression this extinct group of animals acquired some modifications of the skeleton which birds have not shown. There is nothing to suggest that pterodactyls are a branch from birds but their relation to birds is much closer so far as the skeleton goes than is their relation with the flightless dinosaurs with which birds and pterodactyls have many characters in common. On the theory of elimination of character which I have used to account for the disappearance of some mammalian characters from the pterodactyl that loss is seen chiefly in the removal of the parts which have left a reptilian articulation of the lower jaw with the skull and the articulation of the vertebrae throughout the vertebral column by a modified cup and ball form of joint. The furcule of the bird is always absent from the pterodactyl. No specimen has shown recognizable clavicles or collar bones. Judged by the standard of existing life pterodactyls belong to the same group as birds on the evidence of brain and lungs but they belong to a different group on account of the dissimilar modifications of the skeleton and apparent absence of feathers from the skin. The most impressive facts in the pterodactyl skeleton in view of these affinities are the structures which it has in common with reptiles. Some structures are fundamental like the cup and ball articulation of the vertebrae which is never found in birds or mammals. Although not quite identical with the condition in any reptile, this structure is approximately lizard-like or crocodile-like in the cup and ball character. It shows that the deepest-seated part of the skeleton is reptile-like, though it may not be more reptilian than is the vertebral column of a mammal if comparison is made between mammals and extinct groups of animals known as reptiles such as dinosaurs and pterodontia. The orders of animals which have been included under the name reptilia comprise such different structural conditions of the parts of the skeleton which may be termed reptilian in ornithosaurs that there is good reason for regarding the cup and ball articulation as quite a distinctive reptilian specialization in the same sense that the saddle-shaped articulation between the bodies of adjacent vertebrae in a bird is an avian specialization. From the theoretical point of view, the ornithosaur acquired its reptilian characters simultaneously with its avian and mammalian characters. There is nothing in the structure of the skeleton of the dinosauria to which ornithosaurs approximate in several parts of the body which would help to explain the cup and ball articulation of the backbone if the flying reptile were supposed to be an offshoot from the carnivorous dinosaurs. The elimination of reptile characters from so much of the skeleton and the substitution for them of the characters of birds and mammals would be of exceptional interest if there had been any ground for regarding the flying animal as more nearly related to a reptile than to a bird. But if the evidence from the form of the brain and nature of the pneumatic organs seen in the limb bones accounts for the avian features of the skeleton, the reptilian condition of the vertebral column helps to show a capacity for variation, and that the fixity of type and structure which the skeleton of the modern bird has attained is not necessarily limited to or associated with the vital organs of birds. The variation of the cup and ball articulation in the neck of a colonion, which makes the third vertebra cup behind, the fourth biconvex, the fifth cup in front, and the sixth flatten behind, shows that too much importance may be attached to the mode of union of these bones in serpents, crocodiles, and those lizards which have the cup in front. For while in lizards the anterior cup, oblique and depressed, is found in most of its groups. The geckos show no trace of the cup and ball structure, and in that respect resemble the Hatteria of New Zealand. If, therefore, the cup and ball articulation of vertebrae in ornithosaria has any significance as a mark of affinity to reptiles, it could only be an approximation to those living reptiles which possess the same character, and would have it on the hypothesis that both have preserved the structure by descent from an earlier type of animal. This hypothesis is negative by the fact that the cup and ball articulation is unknown in the older fossil reptiles. Although the articulation for the lower jaw with the skull in ornithosars is only to be paralleled among reptiles, the structure is adapted to a brain case which is practically indistinguishable from that of a bird, except for the post orbital arch. The hypothesis of descent, therefore, becomes impossible in any intelligible form in explanation of distinctive character of the skeleton. The hypothesis of elimination may also seem to be insufficient unless the potential capacity for new development be recognized as concurrent and as capable of modifying each region of the skeleton or hard parts of the animal in the same way that the soft organs may be modified. From which we infer that all structures which distinguish the several grades of organization in modern classifications, soft parts and hard parts alike may come into existence together insofar as they are compatible with each other in any class or ordinal division of animals. Although the young mammal passes through a stage of growth in which the brain may be said to be reptilian, there is no good ground for inferring that mammal or bird type of skeleton was developed later in time than that of reptiles. The various types of fishes have the brains in general so similar to those of reptiles that it is more intelligible for all the vertebrate forms of brain to have differentiated at the same time under the law of elimination of characters than that there should be any other bond of union between the classes of animals. If we ask what started the ornithosaria into existence and created the plan of construction of that animal type, I think science has justified in boldly affirming that the initial cause can only be sought under the development of patageal membranes such as have been seen in various animals ministering to flight. Such membranes in an animal which was potentially a bird in its vital organs have owed development to the absence of quill feathers. Thus the wing membrane may be the cause for the chief differences of the skeleton by which ornithosars are separated from birds. For the stretch of wing in one case is made by the skin attached to the bones and in the other case by feathers on the skin so attached as to necessitate that the wing bones have different proportions from ornithosars. It is a well-known observation that each great epic of geological time has had its dominant forms of animal life which so far as the earth's history is known now came into existence, lived their time, and were seen no more. In the same way, the smaller groups of species in genera included in an ordinal group of animals or class have abounded, giving a tone to the life of each geological formation until the vitality of the animal is exhausted and the species becomes extinct or ceases to preponderate. This process is seen to be still modifying the life on the earth when some kinds of animals and plants are introduced to new conditions. Plants appear to wage successful war more easily than animals. The introduction of the cactus in some parts of Cape Colony has locally modified both the fauna and flora, just as the anacharis introduced into England spread from Cambridge over the whole country and became for many years the predominant form of plant life in the streams. The rabbit in Australia is a historic pest. Something similar to this physical fertility and increase appears to take place under new circumstances in certain organs within the bodies of animals by the development of structures previously unknown. A familiar example is seen in the internal anatomy of the trout introduced into New Zealand where the number of pyloric appendages about the stomach has become rapidly augmented while the size and the form of the animal have changed. The rapidity with which some of these changes have been brought about would appear to show that nature is capable of transforming animals more rapidly than might have been inferred from their uniform life under ordinary circumstances. Growth of the vital organs in this way may modify the distinctive form of any vital organ, brain or lungs, and thus as a consequence of modification of the internal structures due to changes of food and habit bring a new group of animals into existence. And just as the group of animals ceases to predominate after a time, so there comes a limit to the continued internal development of vital structures as their energy fails, for each organ behaves to some extent like an independent organism. Under such explanations of the mutual relations of the parts of animals and groups of animals, time ceases to be a factor of primary importance in their construction or elaboration. The supposed necessity for practically unlimited time to produce changes in the vital organs which separate animals into great orders or classes is a nightmare born of hypothesis and may be profitably dismissed. The geological evidence is too imperfect for dogmatism on speculative questions, but the nature of the affinities of ornithosaurs to other animals has been established on a basis of comparison which has no need of theory to justify the facts. It is not improbable that the primary epoch of time, even as known at present, may be sufficiently long to contain the parent races from which ornithosaurs and all their allies have arisen. In thus stating the relation of ornithosaurs to other animals, the flying reptile has been traced home to kindred, though not to its actual parents or birthplace. There is no geological history of the rapid or gradual development of the wing finger, and although the wing membrane may be accepted as its cause of existence, the wing finger is powerfully developed in the oldest known pterodactyls as in their latest representatives. Pterodactyls show singularly little variation in structure in their geological history. We chronicle the loss of the tail and loss of teeth. There is also the loss of the outermost wing digit from the hind foot as a supporter of the wing membrane, but the other variations are in the length of the metacarpus or of the neck or head. One of the fundamental laws of life necessitates that when an animal type ceases to adapt its organization and modify its structures to suit the altered circumstances forced upon it by revolutions of the earth's surface, its life's history becomes broken. It must bend or break. The final disappearance of these animals from the earth's history in the chalk may yet be modified by future discoveries, but the flying reptiles have vanished in the same way as so many other groups of animals which were contemporary with them in the secondary period of time. Such extinctions have been attributed to catastrophes like the submergence of land so that the habitations of animals became an area gradually decreasing in size which at last disappeared. It appears also to be a law of life illustrated by many extinct groups of animals that they endure for geological ages and having fought their battle in life's history grow old and unable to continue the fight and then disappear from the earth giving place to more vigorous types adapted to live under new conditions. The extinct pterodactyls hold a relation to birds in the scheme of life not unlike that which monotremata hold to other mammals. Both are remarkable for the variety of their affinities and resemblances to reptiles. The ornithosaria have long passed away. The monotremes are nearing extinction. Both appear to be supplanted by parallel groups which were their contemporaries. Birds now fill the earth in a way that flying reptiles never surpassed. But their flight is made in a different manner and the wing is extended to support the animal in the air chiefly by appendages to the skin. If these fossils have taught that ornithosars have a community of soft vital organs with dinosaurs and birds, they have also gone some way towards proving that causes similar to those which determined the structural peculiarities of their bony framework originated the special forms of respiratory organs and brain which lifted them out of association with existing reptiles. These old flying animals sleep through geological ages, not without honor, for the study of their story has illuminated the mode of origin of animals which survived them and in cleaving the rocks to display their bones we have opened a new page of the Book of Life.