These illustrations may be accepted as demonstrating a relationship 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, Cœlurus, and Thecospondylus, the bone texture is as thin as in Pterodactyles, and the vertebræ 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 Cœlurus 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 vertebræ.
According to the late Professor Marsh, the limbs of Cœlurus 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 anticipated. But the skull of Cœlurus 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 constitute 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 class, 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 scapulæ, 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 Archæopteryx 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 Archæopteryx 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 Archæopteryx 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 Archæopteryx 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 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 vertebræ, 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 identity, 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.