Part 13 (1/2)

The bones of the leg in Ornithosaurs, known as tibia and fibula, are remarkable for the circ.u.mstance first that they resemble Birds in the fibula being slender and only developed in its upper part towards the femur, and secondly that in a genus like Dimorphodon this drum-stick bone has the two upper bones of the ankle blended with the tibia, so as to form a rounded pulley joint which is indistinguishable from that of a Bird (see p. 102). There is a large number of Dinosaurs in which this remarkable distinctive character of Birds is also found. Only, Dinosaurs like Iguanodon, for instance, have the slender fibula as long as the tibia, and contributing to unite with the separate ankle bones of the similarly rounded pulley at the lower end. There are no Birds in which the tarsal bones remain separated and distinct throughout life. But in Pterodactylus from Solenhofen, as in a number of Dinosaurs, especially the carnivorous genera, the bones of the tarsus remain distinct throughout life, and never acquired such forms as would have enabled the ankle bone, termed astragalus, to embrace the extremity of the tibia, as it does in Iguanodon. Thus the resemblance of the Ornithosaur drum-stick is almost as close to Dinosaurs as to Birds.

There is great similarity between Dinosaurs and Pterodactyles seen in the region of the instep, known as the metatarsus. These bones are usually four in number, parallel to each other, and similar in form.

They are commonly longer than in Dinosaurs; but among some of the carnivorous Dinosaurs their length approximates to that seen in Pterodactyles. In neither group are the bones blended together by bony union, while they are always united in Birds, as in Oxen and similar even-hoofed mammals. Dinosaurs agree with Pterodactyles in maintaining the metatarsal bones separate, but they differ from them and agree with Birds frequently, in having the number of metatarsal bones reduced to three, as in Iguanodon, though Dinosaurs often have as many as five digits developed.

The toe bones, the phalanges of these digits of the hind limb, are usually longer in Pterodactyles than in Dinosaurs, but they resemble carnivorous Dinosaurs in the forms of their sharp terminal bones for the claws, which are similarly compressed from side to side.

So diverse are the functions of the fore limb in Dinosaurs and Pterodactyles, and so remarkably does the length of the metacarpal region of the back of the hand vary in the long-tailed and short-tailed Ornithosaurs, that there is necessarily a less close correspondence in that region of the skeleton between these two groups of animals; for the Pterodactyle fore limb is modified in relation to a function which can only be paralleled among Birds and Bats; and yet neither of those groups of animals approximates closely in this region of the skeleton to the Flying Reptile. Under all the modifications of structure which may be attributed to differences of function, some resemblance to Dinosaurs may be detected, which is best evident in the upper arm bone, humerus; is slight in the fore-arm bones, ulna and radius; and becomes lost towards the extremity of the limb.

If the tendency of the thigh bone to resemble a Mammalian type of femur (p. 100) is a fundamental, deep-seated character of the skeleton, it might be antic.i.p.ated that a trace of Mammalian character would also be found in the humerus. For what the character is worth, the head of the humerus does show a closer approximation to a Monotreme Mammal than is seen in Birds, and is to some extent paralleled in those South African reptiles which approximate to Mammals most closely. Not the least remarkable of the many astonis.h.i.+ng resemblances of these light aerial creatures to the more heavy bodied Dinosaurs is the circ.u.mstance that the humerus in both groups makes a not dissimilar approach to that of certain Mammals.

These ill.u.s.trations may be accepted as demonstrating a relations.h.i.+p between the Ornithosaurs and Dinosaurs now compared, which can only be explained as results of influence of a common parentage upon the forms of the bones. But more interesting than resemblances of that kind is the similarity that may be traced in the way in which air is introduced into cavities in the bones in both groups. In some of the imperfectly known Dinosaurs, like Aristosuchus, Coelurus, and Thecospondylus, the bone texture is as thin as in Pterodactyles, and the vertebrae are excavated by pneumatic cavities, which are amazing in size when compared with the corresponding structures in birds, for the vertebra is often hollowed out so that nothing remains but a thin external film like paper for its thickness. In the Dinosaurian genus Coelurus this condition is as well marked in the tail and back as it is in the neck. The essential difference from Birds appears to be that in the larger carnivorous Dinosaurs the pneumatic condition of the bones is confined to the vertebral column; while Birds and Pterodactyles have the pneumatic condition more conspicuously developed in the limb bones. The pneumatic skeleton, however, appears to be absent from the herbivorous types like Iguanodon and all Dinosaurs which have the Bird-like form of pelvis, and are most Bird-like in the forms of bones of the hind limb. It is possible that some of the carnivorous Dinosaurs also possessed limb bones with pneumatic cavities. Many of those bones are hollow with very thin walls. If their cavities were connected with the lungs the foramina are inconspicuous and unlike the immense holes seen in the sides of the vertebrae.

According to the late Professor Marsh, the limbs of Coelurus and its allies, which at present are imperfectly known, are in some cases pneumatic. Therefore there is a closer fundamental resemblance between some carnivorous Dinosaurs and Pterodactyles than might have been antic.i.p.ated. But the skull of Coelurus is unknown, and the fragments of the skeleton hitherto published are insufficient to do more than show that the two types were near in kindred, though distinct in habit. Each has elaborated a skeleton which owes much to the common stock which transmitted the vital organs, and the tendency of the bones to take special forms; but which also owes more than can be accurately measured to the action of muscles in shaping the bones and the influence of the mechanical conditions of daily life upon the growth of the bones in both of these orders of animals. Enough is known to prove that all Dinosaurs cannot be regarded as Ornithosaurs which have not acquired the power of flight; though the evidence would lead us to believe that the primitive Ornithosaur was a four-footed animal, before the wing finger became developed in the fore limb as a means of extending a patagial membrane, like the membrane which in the hind limb of Dimorphodon has bent the outermost digit of the foot upward and outward to support the corresponding organ of flight extending down the hind legs.

It may thus be seen that the characters of Ornithosaurs which have already been spoken of as Reptilian, as distinguished from the resemblances to Birds, may now with more accuracy be regarded as Dinosaurian. The Dinosaurs, like Pterodactyles, must be regarded as intermediate in some respects between Reptiles and Birds. The resemblances enumerated would alone const.i.tute a partial transition from the Reptile to the Bird, although no Dinosaurs have organs of flight; many are heavily armoured with plates of bone, and few, if any, approximate in the technical parts of the skeleton to the Bird cla.s.s, except in the hind limbs. Yet Dinosaurs have sometimes been regarded as standing to Birds in the relation of ancestors, or as parallel to an ancestral stock.

Before an attempt can be made to estimate the mutual relation of the Flying Reptiles to Dinosaurs on the one hand, and to Birds on the other, it may be well to remember that the resemblance of such a Dinosaur as Iguanodon to a Bird in its pelvis and hind limb is not more remarkable than that of Pterodactyles to Birds in the shoulder-girdle and bones of the fore limb. The keeled sternum, the long, slender coracoid bones and scapulae, are absolutely Bird-like in most Ornithosaurs; and that region of the skeleton only differs from Birds in the absence of a furculum which represents the clavicles, and is commonly named the ”merry-thought.” The elongated bones of the fore-arm and the hand, terminating in three sharp claws, are characters in which the fossil bird Archaeopteryx resembles the Pterodactyle Rhamphorhynchus, a resemblance which extends to a similar elongation of the tail. It is remarkable that the resemblance should be so close, since Archaeopteryx affords the only bird's skeleton known to be contemporary which can be compared with the Solenhofen Flying Reptiles. The resemblance may possibly be closer than has been imagined. The back of the head of Archaeopteryx is imperfectly preserved in the region of the quadrate bone, malar arch, and temporal vacuity. And till these are better known it cannot be affirmed that the back of the head is more Reptilian in Pterodactyles than in the oldest Birds. The side of the head in Archaeopteryx is distinguished by the nostril being far forward, the vacuity in front of the orbit being as large as in the Pterodactyle Scaphognathus from Solenhofen and other long-tailed Pterodactyles.

CHAPTER XVIII

HOW PTERODACTYLES MAY HAVE ORIGINATED

Ornithosauria have many characters inseparably blended together which are otherwise distinctive of Reptiles, Birds, and Mammals, and a.s.sociated 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 Pterodactyles are sometimes Mammalian in having the head of the thigh bone moulded as in carnivorous Mammals and Man, the corresponding bone in a Dinosaur is more like that of a Bird. And while the Pterodactyle 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 Vertebrata, demonstrating that a Mammal may lay eggs like a Reptile or Bird, that the skull may include the reptilian characters of the malar arch and pre-frontal and post-frontal 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 Pterodactyles, Mammal Bird and Reptile lose their ident.i.ty, as three colours 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 Ornithosaur, 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 organisation is modified, by one strain of characters being eliminated so that another becomes predominant, while a distinctive set of structures is elaborated in each cla.s.s of animals. The earlier geological history of the higher Vertebrata is very imperfectly known, but the evidence tends to the inference that the older representatives of the several cla.s.ses 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 recognisable. The relation of the great groups of animals to each other, among Vertebrata, is essentially a parallel relation, like the colours 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 realised, it was imagined that Nature, by slow summing up and acc.u.mulation of differences which were observed, would so modify one animal type that it would pa.s.s 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, a.n.a.logous to those which survive, and distinct from them in the same way as surviving groups are distinct from each other. The circ.u.mstance that Mammals, Birds, and Reptiles are all known low down in the Secondary epoch of geological time, is favourable 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 Pterodactyles 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 Theriodonts, towards the Monotremata 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 a 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 bony possessions or osteological capital. There is no part of the Pterodactyle 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 Monotremata allied to the living Ornithorhynchus. In that sense the head of the humerus in a Pterodactyle has some affinity with the lowest Mammalia, which approach nearest to Reptiles. The character might pa.s.s 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 Ornithosaurs 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 Pterodactyle 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 Pterodactyle 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 circ.u.mstances of existence, and retain their characters with moderate variation of the bones in each of the orders or cla.s.ses of animals. It therefore is safer to regard Mammalian characters, as well as the resemblances which Pterodactyles 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 Ornithosaurs 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 inclosed 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 Rhamphorhynchus.

These and such-like acquired characters distinguish the cla.s.s 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 Pterodactyles came into existence, because the characters which separate orders and cla.s.ses 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 acc.u.mulation of the characters of genera will not sum up into the characters of an order or cla.s.s.

In making the division of Vertebrate animals into cla.s.ses the skeleton is often almost ignored. Its value is entirely empirical and based upon the observed a.s.sociation 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 a.s.sociation with the soft vital organs which const.i.tute 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 mould 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 Pterodactyle might be as like to the brain of Ornithorhynchus as it is like that of a Bird (Fig. 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 ma.s.s 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 quadrigemina_, 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 Pterodactyles 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 in character of the brain throughout the group in Mammals is remarkable, in reference to the circ.u.mstance that the reproduction varies in type; the lowest, or Monotreme division, being oviparous. If there is no necessary connexion 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 connexion between the form of the brain and the form of the bones, since the brain cavity in Theriodont 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 a.s.sume, shows the astonis.h.i.+ng range of structure of the skeleton which may coexist with the Mammalian brain.