Dragons of the Air Part 12

We have to determine whether the Ornithosauria incline towards the Sauropsidan or Bird-Reptile alliance, or to the Mammal-Reptile or Theropsidan alliance. There can be no doubt that the predominant tendency is to the former, with a minor affinity towards the latter.

The Ornithosauria are one of a series of groups of animals, living and extinct, which have been combined in an alliance named the Ornith.o.m.orpha. That group includes at least five great divisions of animals, which circle about birds, known as Ornithosauria, Crocodilia, Saurischia, Aves, Ornithischia, and Aristosuchia. Their relations to each other are not evident in an enumeration, but may be shown in some degree in a diagram (see p. 190).

THE ORNITh.o.m.oRPHA

The Ornith.o.m.orpha arranged in this way show that the three middle groups--carnivorous Saurischia, Aristosuchia, herbivorous Ornithischia--which are usually united as Dinosauria, intervene between Birds and Ornithosaurs; and that the Crocodilia and Ornithosauria are parallel groups which are connected with Birds, by the group of Dinosaurs, which resembles Birds most closely.

The Ornith.o.m.orpha is only one of a series of large natural groups of animals into which living and extinct terrestrial vertebrata may be arranged. And the succeeding diagram may contribute to make evident the relations of Ornithosauria to the other terrestrial vertebrata (see p.

191).

Herein it is seen that while the Ornith.o.m.orpha approach towards Mammalia through the Ornithosauria, and less distinctly through the Crocodilia, they approach more directly to the Sauromorpha, through the Plesiosaurs and Hatteria; while that group also approaches more directly to the Mammals through the Plesiosaurs and Anomodonts.

[Ill.u.s.tration: DIAGRAM OF THE AFFINITIES OF THE ORDERS OF ANIMALS COMPRISED IN THE ORNITh.o.m.oRPHA.

After a diagram in the _Philosophical Transactions of the Royal Society_, 1892.]

The Aristosuchia is imperfectly known, and therefore to some extent a provisional group. It is a small group of animals.

[Ill.u.s.tration: DIAGRAM SHOWING THE RELATIONS OF THE ORNITh.o.m.oRPHA TO THE CHIEF LARGE GROUPS OF TERRESTRIAL VERTEBRATA, AND THEIR AFFINITIES WITH EACH OTHER.

After a diagram in the _Philosophical Transactions of the Royal Society_, 1892.]

Cordylomorpha are Ichthyosaurs and the Labyrinthodont group.

Herpetomorpha include Lacertilia, h.o.m.oeosauria, Dolichosauria, Chameleonoidea, Ophidia, Pythonomorpha.

The Sauromorpha comprises the groups of extinct and living Reptiles named Chelonia, Rhynchocephala, Sauropterygia, Anomodontia, Nothosauria, and Protorosauria. These details may help to explain the place which has been given to the Ornithosauria in the cla.s.sification of animals.

[Ill.u.s.tration: FIG. 75. COMPARISON OF SIX GENERA

The skulls are seen on the left side in the order of the names below them]

Turning to the Pterodactyles themselves, Von Meyer divided them naturally into short-tailed and long-tailed. The short-tailed indicated by the name Pterodactylus he further divided into long-nosed and short-nosed. The short-nosed genus has since been named Ptenodracon (Fig. 59, p. 167). The long-tailed group was divided into two types--the Rhamphorhynchus of the Solenhofen Slate (Fig. 56, p. 161) and the English form now known as Dimorphodon (Fig. 52, p. 150), which had been described from the Lias.

The Cretaceous Pterodactyles form a distinct family. So that, believing the tail to have been short in that group (Fig. 58), there are two long-tailed as well as two short-tailed families, which were defined from their typical genera Pterodactylus, Ornithocheirus, Rhamphorhynchus, and Dimorphodon.

The differences in structure which these animals present are, first: the big-headed forms from the Lias like Dimorphodon, agree with the Rhamphorhynchus type from Solenhofen in having a vacuity in the skull defined by bone, placed between the orbit of the eye and the nostril.

With those characters are correlated the comparatively short bones which correspond to the back of the hand termed metacarpals, and the tail is long, and stiffened down its length with ossified tendons. These characters separate Ornithosaurs with long tails from those with short tails.

The short-tailed types represented by Pterodactylus and Ornithocheirus have no distinct antorbital vacuity in the skull defined by bone. The metacarpal bones of the middle hand are exceptionally elongated, and the tail, which was flexible in both, appears to have been short. These differences in the skeleton warrant a primary division of flying reptiles into two princ.i.p.al groups.

The short-tailed group, which was recognised by De Blainville as intermediate between Birds and Reptiles, may take the name Pterodactylia, which he suggested as a convenient, distinctive name. It may probably be inconvenient to enlarge its significance to comprise not only the true Pterodactyles originally defined as Pterosauria, but the newer Ornithostoma and Ornithocheirus which have been grouped as Ornithocheiroidea.

The second order, in which the wing membrane appears to have had a much greater extent, in being carried down the hind limbs, where the outermost digit and metatarsal are modified for its support, has been named Pterodermata, to include the types which are arranged around Rhamphorhynchus and Dimorphodon.

Both these princ.i.p.al groups admit of subdivision by many characters in the skeleton, the most remarkable of which is afforded by the pair of bones carried in front of the p.u.b.es, and termed prepubic bones. In the Pterodactyle family the bones in front of the p.u.b.es are always separate from each other, always directed forward, and have a peculiar fan-shaped form with concave sides like the bone which holds a similar position in a Crocodile. In the Ornithocheirus family the prepubic bones appear to have been originally triangular, but were afterwards united so as to form a strong continuous bar which extends transversely across the abdomen in advance of the pubic bones. This at least is the distinctive character in the genus Ornithostoma according to Professor Williston, which in many ways closely resembles Ornithocheirus.

The two families in the long-tailed order named Pterodermata are separated from each other by a similar difference in their prepubic bones. In Dimorphodon those bones are separate from each other, and remain distinct through life, meeting in the middle line of the body in a wide plate. On the other hand, in Rhamphorhynchus the prepubic bones, which are at first triangular and always slender, become blended together into a slight transverse bar, which only differs from that attributed to Ornithostoma in its more slender bow-shaped form.

[Ill.u.s.tration: FIG. 76. LEFT SIDE OF PELVIS OF ORNITHOSTOMA (After Williston)]

Thus if other characters of the skeleton are ignored and a cla.s.sification based upon the structure of the pelvis and prepubic bones, there would be some ground for a.s.sociating the long-tailed Rhamphorhynchus from the Upper Oolites which is losing the teeth in the front of its jaw with the Cretaceous Ornithostoma, which has the teeth completely wanting; while the long-tailed Dimorphodon would come into closer a.s.sociation with the short-tailed Pterodactylus. The drum-stick bone or tibia in Dimorphodon, with its slender fibula, like that of a Bird, also resembles a Bird in the rounded and pulley-shaped terminal end which makes the joint corresponding to the middle of the ankle bones in man. The same condition of a terminal pulley joint is found in the Cretaceous Pterodactyles. But in the true Pterodactyles and in Rhamphorhynchus there usually is no pulley-shaped termination to the lower end of the drum-stick, for the tarsal bones remain separate from each other, and form two rows of ossifications, showing the same differences as separate Dinosaurs into the divisions which have been referred to, from their Bird-like pelvis and tibio-tarsus, as Ornithischia in the one case, and Saurischia in the other from their bones being more like those of living Lizards.

CHAPTER XVII

FAMILY RELATIONS OF PTERODACTYLES TO ANIMALS WHICH LIVED WITH THEM

Enough has been said of the general structure of Pterodactyles and the chief forms which they a.s.sumed while the Secondary rocks were acc.u.mulating, to convey a clear idea of their relations to the types of vertebrate animals which still survive on the earth. We may be unable to explain the reasons for their existence, and for their departure from the plan of organisation of Reptiles and Birds. But the evidence has not been exhausted which may elucidate their existence. Sometimes, in problems of this kind, which involve comparison of the details of the skeleton in different animals, it is convenient to imagine the possibility of changes and transitions which are not yet supported by the discovery of fossil remains. If, for example, the Pterodactyle be conceived of as divested of the wing finger, which is its most distinctive character, or that finger is supposed to be replaced by an ordinary digit, like the three-clawed digits of the hand which we have regarded as applied to the ground, where, it may be asked, would the animal type be found which approximates most closely to a Pterodactyle which had been thus modified? There are two possible replies to such a question, suggested by the form of the foot. For the old Bird Archaeopteryx has three such clawed digits, but no wing finger. And some Dinosaurs also have the hand with three digits terminating in claws, which are quite comparable to the clawed digits of Pterodactyles.

The truth expressed in the saying that no man by taking thought can add a cubit to his stature is of universal application in the animal world, in relation to the result upon the skeleton of the exercise of a function by the individual. Yet such is the relation in proportions of the different parts of the animal to the work which it performs, so marked is the evidence that growth has extended in direct relation to use of organs and active life, and that structures have become dwarfed from overwork, or have wasted away from disuse--seen throughout all vertebrate animals, that we may fairly attribute to the wing finger some correlated influence upon the proportions of the animal, as a consequence of the dependence of the entire economy upon each of its parts. Therefore if an allied animal did not possess a wing finger, and did not fly, it might not have developed the lightness of bone, or the length of limb which Pterodactyles possess.

The mere expansion of the parachute membrane seen in so-called flying animals, both Mammals and Reptiles, which are devoid of wings, is absolutely without effect in modifying the skeleton. But when in the Bat a wing structure is met with which may be compared to a gigantic extension of the web foot of the so-called Flying Frog, the bones of the fingers and the back of the hand elongate and extend under the stimulus of the function of flight in the same way as the legs elongate in the more active hoofed animals, with the function of running. Therefore it is not improbable that the limbs shared to some extent in growth under stimulus of exercise which developed the wing finger. And if an animal can be found among fossils so far allied as to indicate a possible representative of the race from which these Flying Dragons arose, it might be expected to be at least shorter legged, and possibly more distinctly Reptilian in the bones of the shoulder-girdle which support the muscles used in flight. It may readily be understood that the kinds of life which were most nearly allied to Pterodactyles are likely to have existed upon the earth with them, and that flight was only one of the modes of progression which became developed in relation to their conditions of existence. The princ.i.p.al a.s.semblage of terrestrial animals available for such comparison is the Dinosauria. They may differ from Pterodactyles as widely as the Insectivora among Mammals differ from Bats, but not in a more marked way. Comparisons will show that there are resemblances between the two extinct groups which appeal to both reason and imagination.

Dinosaurs are conveniently divided by characters of the pelvis first into the order Saurischia, which includes the carnivorous Megalosaurus and the Cetiosaurus, with the pelvis on the Reptile plan; and secondly the order Ornithischia, represented by Iguanodon, with the pelvis on the Bird plan. It may be only a coincidence, but nevertheless an interesting one, that the characters of those two great groups of reptiles, which also extend throughout the Secondary rocks, are to some extent paralleled in parts of the skeleton of the two divisions of Pterodactyles. This may be ill.u.s.trated by reference to the skull, pelvis, hind limb, and the pneumatic condition of the bones.

[Ill.u.s.tration: FIG. 77. COMPARISON OF THE SKULL OF THE DINOSAUR ANCHISAURUS WITH THE ORNITHOSAUR DIMORPHODON]

The Saurischian Dinosauria have an antorbital vacuity in the side of the skull between the nasal opening and the eye, as in the long-tailed Ornithosaurs named Pterodermata. In some of the older genera of these carnivorous Dinosaurs of the Trias, the lateral vacuities of the head are as large as in Dimorphodon. But in some at least of the Iguanodont, or Ornithischian Dinosaurs, there is no antorbital vacuity, and the side of the face in that respect resembles the short-tailed Pterodactylia.

The skull of a carnivorous Dinosaur possesses teeth which, though easily distinguished from those of Pterodactyles, can be best compared with them. The most striking difference is in the fact that in the Dinosaur the nostrils are nearly terminal, while in the Pterodactyle they are removed some distance backward. This result is brought about by growth taking place, in the one case at the front margin of the maxillary bone so as to carry the nostril forward, and in the other case at the back margin of the premaxillary bone. Thus an elongated part of the jaw is extended in front of the nostril. Hence there is a different proportion between the premaxillary and maxillary bones in the two groups of animals, which corresponds to the presence of a beak in a bird, and its absence in living reptiles. It is not known whether the extremity of the Pterodactyle's beak is a single bone, the intermaxillary bone, such as forms the corresponding toothless part of the jaw in the South African reptile Dicynodon, or whether it is made by the pair of bones called premaxillaries which form the extremity of the jaw in most Dinosaurs.

Too much importance may perhaps be attached to such differences which are partly hypothetical, because the extinct Ichthyosaurus, which has an exceptionally long snout, has the two premaxillary bones elongated so as to extend backward to the nostrils. A similar elongation of those bones is seen in Porpoises, which also have a long snout; and the bones are carried back from the front of the head to the nostrils, which are sometimes known as blowholes. But the Porpoise has those premaxillary bones not so much in advance of the bones which carry teeth named maxillary, as placed in the inters.p.a.ce between them. The nostrils, however, are not limited to the extremity of the head in all Dinosaurs.

If this region of the beak in Dimorphodon be compared with the corresponding part of a Dinosaur from the Permian rocks, or Trias, the relation of the nostril to the bones forming the beak may be better understood.

[Ill.u.s.tration: FIG. 78. COMPARISON OF THE SKULL OF THE DINOSAUR ORNITHOSUCHUS WITH THE ORNITHOSAUR DIMORPHODON]

In the sandstone of Elgin, usually named Trias, a small Dinosaur is found, which has been named Ornithosuchus, from the resemblance of its head to that of a Bird. Seen from above, the head has a remarkable resemblance to the condition in Rhamphorhynchus, in the sharp-pointed beak and positions of the orbits and other openings. In side view the orbits have the triangular form seen in Dimorphodon, and the preorbital vacuities are large, as in that genus, while the lateral nostrils, which are smaller, are further forward in the Dinosaur. The differences from Dimorphodon are in the articulation for the jaw being carried a little backward, instead of being vertical as in the Pterodactyle, and the bone in front of the nose is smaller. Notwithstanding probable differences in the palate, the approximation, which extends to the Crocodile-like vacuity in the lower jaw, is such that by slight modification in the skull the differences would be substantially obliterated by which the skull of such an Ornithosaur is technically distinguished from such a Dinosaur.

The back of the skull is clearly seen in the Whitby Pterodactyle, and its structure is similar to the corresponding part of such Dinosaurs as Anchisaurus or Atlantosaurus, without the resemblance quite amounting to ident.i.ty, but still far closer than is the resemblance between the same region in the heads of Crocodiles, Lizards, Serpents, Chelonians. Few of these fossil Dinosaur skulls are available for comparison, and those differ among themselves. The coincidences rather suggest a close collateral relation than prove the elaboration of one type from the other. They may have had a common ancestor.

The Trias rocks near Stuttgart have yielded Dinosaurs as unlike Pterodactyles as could be imagined, resembling heavily armoured Crocodiles, in such types as the genus Belodon. Its jaws are compressed from side to side, as in many Pterodactyles, and the nostrils are at least as far backward as in Rhamphorhynchus. Belodon has preorbital vacuities and pos...o...b..tal vacuities, but the orbit of the eye is never large, as in Pterodactyles. It might not be worth while dwelling on such points in the skull if it were not that the pelvis in Belodon is a basin formed by the blending of the expanded plates of the ischium and the pubis, into a sheet of bone which more nearly resembles the same region in Pterodactyles than does the ischio-pubic region in other Dinosaurian animals like Cetiosaurus.

The backbone in a few Dinosaurs is suggestive of Pterodactyles. In such genera as have been named Coelurus and Calamospondylus, in which the skeleton is only partially known, the neck vertebrae become elongated, so as to compare with the long-necked Pterodactyles. The cervical rib is often very similar to that type, and blended with the vertebra, as in Pterodactyles and Birds. The early dorsal vertebrae of Pterodactyles might almost be mistaken for those of Dinosaurs. The tail vertebrae of a Pterodactyle are usually longer than in long-tailed Dinosauria.

In the limbs and the bony girdles which support them there is more resemblance between Pterodactyles and Dinosaurs than might have been antic.i.p.ated, considering their manifest differences in habit. Thus all Dinosaurs have the hip bone named ilium prolonged in front of the articulation for the femur as well as behind it, almost exactly as in Pterodactyles and Birds (see p. 95). There is some difference in the pubis and ischium which is more conspicuous in form than in direction of the bones. There is a Pterodactyle imperfectly preserved, named _Pterodactylus dubius_, in which the ischium is directed backward and the pubis downward, and the bones unite below the acetabular cavity for the head of the femur to work in, but do not appear to be otherwise connected. In Rhamphorhynchus the connexion between these two thickened bars is made by a thin plate of bone. In such a Dinosaur as the American carnivorous Ceratosaurus the two bars of the pubis and ischium remain separate and diverging, and there is no film of bone extending over the inters.p.a.ce between them. The development of such a bony condition would make a close approximation between the Ornithosaurian pelvis and that of those Dinosaurs which closely resemble Pterodactyles in skull and teeth.

[Ill.u.s.tration: FIG. 79. LEFT SIDE OF PELVIS A Pterodactyle is shown between a carnivorous Dinosaur above and a herbivorous Dinosaur below]

Another pelvic character of some interest is the blending of the pubis and ischium of the right and left sides in the middle line of the body.

There are some genera of Dinosaurs like the English Aristosuchus from the Weald, and the American genera Coelurus, Ceratosaurus, and others, in which the pubic bones, instead of uniting at their extremities, are pinched together from side to side, and unite down the lower part of their length, terminating in an expanded end like a shoe, which is seen to be a separate ossification, and probably formed by a pair of ossifications joined in the median line. This small bone, which is below the p.u.b.es, and in these animals becomes blended with them, we may regard as a pair of prepubic bones like those of Pterodactyles and Crocodiles, except that they have lost the stalk-like portions, which in those animals are developed to compensate for the diminished length of the pubic bones. The prepubic bones may also be developed in Iguanodon, in which a pair of bones of similar form remains throughout life in advance of the p.u.b.es, as in Pterodactyles. In those Dinosauria with the Bird-like type of pelvis the pubic bone is exceptionally developed, sending one process backward and another process forward, so that there is a great gap between these diverging limbs to the bone. In the region behind the sternum to which the ribs were attached, and in front of the pelvis, is a pair of bones in Iguanodon shaped like the prepubic bones of Dimorphodon. They have sometimes been interpreted as a hinder part of the sternum, but may more probably be regarded as a pair of prepubic bones articulating each with the anterior process of the pubis (see Fig.

80). The small bones found at the extremities of the p.u.b.es in such carnivorous Dinosaurs as Aristosuchus are blended by bony union with the p.u.b.es. The bones in Iguanodon are placed behind the sternal region without any attachment for sternal ribs, and the expanded processes converge forwards from the stalk and unite exactly like the prepubic bones of Ornithosaurs. While this character, on the one hand, may link Pterodactyles with the Dinosaurs, on the other hand it may be a link between both those groups and the Crocodiles, in which the front pair of bones of the pelvis has also appeared to be representative of the prepubic bones of Flying Reptiles (see Fig. 32, p. 98).

[Ill.u.s.tration: FIG. 80. DIAGRAM OF THE PELVIS SEEN FROM BELOW IN AN ORNITHOSAUR AND A DINOSAUR]

The resemblances between Pterodactyles and Dinosaurs in the hind limb are not of less interest, though it is rather in the older Pterodactyles such as Dimorphodon, Pterodactylus, and Rhamphorhynchus that the resemblance is closest with the slender carnivorous Dinosaurs. They never have the head of the thigh bone, femur, separated from its shaft by a constricted neck, as in the Pterodactyles from the Chalk. In many ways the thigh bone of Dinosaurs tends towards being Avian; while that of Pterodactyles inclines towards being Mammalian, but with a tendency to be Bird-like in the older types, and to be Mammal-like in the most recent representatives of the group in the Chalk.