Dragons of the Air Part 5

Mr. E. T. Newton, F.R.S., has shown that in the English skull from the Lias of Whitby, the forms of the bones are similar to the palate in birds and unlike the conditions in reptiles. There is one feature, however, which may indicate a resemblance to Dicynodon and other fossil reptiles from South Africa. A slender bone extends from the base of the brain case, named the basi-sphenoid bone, outward and forward to the inner margin of the quadrate bone (Fig. 22). A bone is found thus placed in those South African Reptiles, which show many resemblances to the Monotreme and Marsupial Mammals. It is not an ordinary element of the skeleton and is unknown in living animals of any kind in that position.

It has been thought possible that it may represent one of the bones which among mammals are diminutive and are included in the internal ear.

The resemblance may have some interest hereafter, as helping to show that certain affinities of the Ornithosaurs may lie outside the groups of existing reptiles. Instead of being directed transversely outward, as in the palatal region of _Dicynodon lacerticeps_, they diverge outward and forward to the inner border of the articulation for the lower jaw which is upon the quadrate bone.

[Ill.u.s.tration: FIG. 22]

BONES OF THE PALATE

There is a pair of bones which extend forward from these inner articular borders of the quadrate bones, and converge in a long #V#-shape till they merge in the hard palate formed by the bones of the front of the beak, named intermaxillary and maxillary bones. The limits of the bones of the palate are not distinct, but there can be no doubt that the front of the #V# is the bone named vomer, that the palatine bones are at its sides, and that its hinder parts are the pterygoid bones as in birds. There is a long, wide, four-sided, open s.p.a.ce in the middle of the palate, between the vomer and the basi-sphenoid bone, unlike anything in birds or other animals.

Professor Marsh, in a figure of the palate in the great skull of the toothless Pterodactyle named Ornithostoma (Pteranodon), from the Chalk of Kansas, found a large oval vacuity in this region of the palate. In that genus the pterygoid bones meet each other between the quadrate bones as in Dicynodon (Fig. 73, p. 182). Hence the great palatal vacuity here seen in the Ornithosaur is paralleled by the small vacuity in the South African reptile, which is sometimes distinct and sometimes partly separated from the anterior part of the vacuity which forms the openings of the nostrils on the palate.

The Solenhofen skulls which give any evidence of the palate are exposed in side view only, and the bones, imperfectly seen through the lateral vacuities, are displaced by crushing. They include long strips like the vomerine bones in the Lias fossil, and they diverge in the same way as they extend back to the quadrate bones. The oblique division into vomer in front and pterygoid bone behind is shown by Goldfuss in his original figure of Scaphognathus. Thus there is some reason for believing that all Ornithosaurs have the palate formed upon the same general plan, which is on the whole peculiar to the group, especially in not having the palatal openings of the nares divided in the middle line. It would appear probable that the short-tailed animals have the pterygoid bones meeting in the middle line and triangular; and that they are slender rods entirely separate from each other in the long-tailed genera.

THE TEETH

The teeth are all of pointed, elongated shape, without distinction into the kinds seen in most mammals and named incisors, canines, and grinders. They are organs for grasping, like the teeth of the fish-eating Crocodile of India, and are not unlike the simple teeth of some Porpoises. They are often implanted in oblique oval sockets with raised borders, usually at some distance apart from each other, and have the crown pointed, flattened more on the outer side than on the inner side, usually directed forward and curved inward. As in many extinct animals allied to existing reptiles, the teeth are reproduced by germs, which originate on the inner side of the root and grow till they gradually absorb the substance of the old tooth, forming a new one in its place. Frequently in Solenhofen genera, like Scaphognathus and Pterodactylus, the successional tooth is seen in the jaw on the hinder border of the tooth in use. There is some variation in the character of bluntness or sharpness of the crowns in the different genera, and in their size.

The name Dimorphodon, given to the animal from the Lias of Lyme Regis, expresses the fact that the teeth are of two kinds. In the front of the jaw three or four large long teeth are found in the intermaxillary bone on each side, as in some Plesiosaurs, while the teeth found further back in the maxillary bone are smaller, and directed more vertically downward. This difference is more marked in the lower jaw than in the upper jaw. In Rhamphorhynchus the teeth are all relatively long and large, and directed obliquely forward, but absent from the extremities of the beak, as in the German genus from the Lias named Dorygnathus, in which the bone of the lower jaw (which alone is known) terminates in a compressed spear. In Scaphognathus the teeth are few, more vertical, and do not extend backward so far as in Rhamphorhynchus, but are carried forward to the extremity of the blunt, deep jaw.

In the short-tailed Pterodactyles the teeth are smaller, shorter, wider at the base of the crown, closer together, and do not extend so far backward in the jaw. In Ornithocheirus two teeth always project forward from the front of the jaw. Ornithostoma is toothless.

SUPPOSED h.o.r.n.y BEAK

Sometimes a h.o.r.n.y covering has been suggested for the beak, like that seen in birds or turtles, but no such structure has been preserved, even in the Solenhofen Slate, in which such a structure would seem as likely to be preserved as a wing membrane, though there is one doubtful exception. There are marks of fine blood vessels on some of the jaws, indicating a tough covering to the bone. In Rhamphorhynchus the jaws appear to gape towards their extremities as though the inters.p.a.ce had originally been occupied by organic substance like a h.o.r.n.y beak.

LOWER JAW

The lower jaw varies in relative length with the vertical or horizontal position of the quadrate bone in the skull. In Dimorphodon the jaw is as long as the skull; but in the genera from the Oolitic rocks the mandible is somewhat shorter, and in Ornithostoma the discrepancy reaches its maximum. The hinder part of the jaw is never prolonged backward much beyond the articulation, differing in this respect from Crocodiles and Plesiosaurs.

The depth of the jaw varies. It is slender in Pterodactylus, and is probably stronger relatively to the skull in Scaphognathus than in any other form. It fits between the teeth and bones of the alveolar border in the skull, in all the genera. In Dimorphodon its hinder border is partly covered by the descending edge of the malar process which these animals develop in common with some Dinosaurs, and some Anomodont reptiles, and many of the lower mammals. In this hinder region the lower jaw is sometimes perforated, in the same way as in Crocodiles. That condition is observed in Dimorphodon, but is not found in Pterodactylus.

The lower jaw is always composite, being formed by several bones, as among reptiles and birds. The teeth are in the dentary bone or bones, and these bones are almost always blended as in most birds and Turtles, and not separate from each other as among Crocodiles, Lizards, and Serpents.

An interesting contour for the lower border of the jaw is seen in Ornithostoma, as made known in figures of American examples by Professors Marsh and Williston. It deepens as it extends backwards for two-thirds its length, stops at an angle, and then the depth diminishes to the articulation with the skull. This angle of the lower jaw is a characteristic feature of the jaws of Mammals. It is seen in the monotreme Echidna, and is characteristic of some Theriodont Reptiles from South Africa, which in many ways resemble Mammals. The character is not seen in the jaws of specimens from the Oolitic rocks, but is developed in the toothed Ornithocheirus from the Cambridge Greensand, and is absent from the jaws of existing reptiles and birds.

[Ill.u.s.tration: FIG. 23 COMPARISON OF THE LOWER JAW IN ECHIDNA AND ORNITHOSTOMA]

SUMMARY OF CHARACTERS OF THE HEAD

Taken as a whole, the head differs from other types of animals in a blending of characters which at the present day are found among Birds and Reptiles, with some structures which occur in extinct groups of animals with similar affinities, and perhaps a slight indication of features common to the lowest mammals. It is chiefly upon the head that the diverse views of earlier writers have been based. Cuvier was impressed with the reptilian aspect of the teeth; but in later times discoveries were made of Birds with teeth--Archaeopteryx, Ichthyornis, Hesperornis. The teeth are quite reptilian, being not unlike miniature teeth of Mosasaurus. If those birds had been found prior to the discovery of Pterodactyles, the teeth might have been regarded as a link with the more ancient birds, rather than a crucial difference between birds and reptiles.

All the specimens show a lateral temporal hole in the bones behind the eye, and this is found in no bird or mammal, and is typical of such reptiles as Hatteria. The quadrate bone may not be so decisive as Cuvier thought it to be, for its form is not unlike the quadrate of a bird, and different, so far as I have seen, from that of living reptiles. This region of the head is reptilian, and if it occurred in a bird the character would be as astonishing as was the discovery of teeth in extinct birds. These characters of the head are also found in fossil animals named Dinosaurs, in a.s.sociation with many resemblances to birds in their bones.

The palate might conceivably be derived from that of Hatteria by enlarging the small opening in the middle line in that reptile till it extended forward between the vomera; but it is more easily compared with a bird, which the animal resembles in its beak, and in the position of the nares. Excepting certain Lizards, all true existing Reptiles have the nostrils far forward and bordered by two premaxillary bones instead of one intermaxillary, as in Birds and Ornithosaurs. If nothing were known of the animal but its head bones, it would be placed between Reptiles and Birds.

CHAPTER IX

THE BACKBONE, OR VERTEBRAL COLUMN

The backbone is a more deep-seated part of the skeleton than the head.

It is more protected by its position, and has less varied functions to perform. Therefore it varies less in distinctive character within the limits of each of the cla.s.ses of vertebrate animals than either the head or limbs. It is divided into neck bones, the cervical vertebrae; back bones, the dorsal vertebrae; loin bones, the lumbar vertebrae; the sacrum, or sacral vertebrae, which support the hind limbs; and the tail. Of these parts the tail is the least important, though it reaches a length in existing reptiles which sometimes exceeds the whole of the remainder of the body, and includes hundreds of vertebrae. It attains its maximum among serpents and lizards. In frogs it is practically absent. In some of the higher mammals it is a rudiment, which does not extend beyond the soft parts of the body.

THE NECK

The neck is more liable to vary than the back, with the habit of life of the animal. And although mammals almost always preserve the same number of seven bones in the neck, the bones vary in length between the short condition of the porpoise, in which the neck is almost lost, and the long bones which form the neck of the Llama, though even these may be exceeded by some fossil reptiles like Tanystrophoeus. In many mammals the neck bones do not differ in length or size from those of the back.

In others, like the Horse and Ox, they are much broader and larger.

There is the same sort of variation in the bones of the neck among birds, some being slender like the Heron, others broad like the Swan.

But there is also a singular variation in number of vertebral bones in a bird's neck. At fewest there are nine, which equals the exceptionally large number found among mammals in the neck of one of the Sloths.

Usually birds have ten to fifteen cervical vertebrae, and in the Swan there are twenty-three. Most of the neck bones of birds are relatively long, and the length of the neck is often greater than the remainder of the vertebral column.

Reptiles usually have short necks. The common Turtle has eight bones in the neck, ten in the back. The two regions are sharply defined by the dorsal shield. Their articular ends are sometimes cupped in front, in the neck, sometimes cupped behind, or convex at both ends, or even flattened, or the articulation may be made exceptionally by the neural arch alone. Nine is the largest number of neck bones in existing Lizards, and there are usually nine in Crocodiles; so that reptiles closely approach mammals in number of the neck bones. It is remarkable that the maximum number in a mammal and in living reptiles should coincide with the minimum number in birds. Therefore the number of cervical vertebrae as an attribute of Mammal, Bird, or Reptile, can only be important from its constancy.

German naturalists affirm on clear evidence that the Solenhofen Pterodactyles have seven cervical vertebrae. In many specimens there can be no doubt about the number, because the neck bones are easily distinguished from those of the back by their size; but the number is not always easy to count.

As in Birds, the first vertebra, or atlas, in Pterodactyles is extremely short, and is generally--if not always--blended with the much longer second vertebra, named the axis. The front of the atlas forms a small rounded cup to articulate with the rounded ball of the basioccipital bone at the back of the skull. The third and fourth vertebrae are longer, but the length visibly shortens in the sixth and seventh.

Sometimes the vertebrae are slender and devoid of strong spinous processes. This is the condition in the little _Pterodactylus longirostris_ and in the comparatively large _Cycnorhamphus Fraasii_, in which there is a slight median ridge along the upper surface of the arch of the vertebra. This condition is paralleled in birds with long necks, especially wading birds such as the Heron. Other Ornithosaurs, such as Ornithocheirus from the Cretaceous rocks, have the neck much more ma.s.sive. The vertebrae are flattened on the under side. The arch above the nervous matter of the spinal cord has a more or less considerable transverse expansion, and may even be as wide as long. These vertebrae have proportions and form such as may be seen in Vultures or in the Swan. In either case the form of the neck bones is more or less bird-like, and the neural spine may be elevated, especially in Pterodactyles with long tails.

One of the most distinctive features of the neck bones of a bird is the way in which the cervical ribs are blended with the vertebrae. They are small, and each is often prolonged in a needle-like rod at the side of the neck bone.

In Ornithocheirus the cervical rib similarly blends with the vertebra by two articulations, as in mammals, so that it might escape notice but for the channel of a blood vessel which is thus inclosed. In several of the older Pterodactyles from Solenhofen the ribs of the neck vertebrae remain separated, as in a Crocodile, though still bird-like in their form, anterior position, and mode of attachment. In Terrapins and Tortoises the long neck vertebrae have no cervical ribs.

[Ill.u.s.tration: FIG. 24 UNITED ATLAS AND AXIS OF ORNITHOCHEIRUS (Cambridge Greensand)]

The articular surfaces between the bodies of the vertebrae, in the neck, are transversely oval. The middle part of this articular joint is made by the body of the vertebra; its outer parts are in the neural arch. In front this surface is a hollow channel, often more depressed than in any other animals. The corresponding surface behind is convex, with a process on each side at its lower outer angles (Fig. 25). It is a modification of the cup-and-ball form of vertebral articulation, which at the present day is eminently reptilian. Serpents and Crocodiles have the articulations similarly vertical, but in both the form of the articulation is a circle. In Lizards the articular cup is usually rather wider than deep, when the cup and ball are developed in the vertebrae; it differs from the vertical condition in pterodactyles in being oblique and much narrower from side to side. Only among Crocodiles and Hatteria is there a double articulation for the cervical rib, though in neither order have rib or vertebra in the neck the bird-like proportions which are usual in these animals. Pterodactyles show no resemblance to birds in this vertebral articulation. A Bird has the corresponding surface concave from side to side in front, but it is also convex from above downward, producing what is known as the saddle-shaped form which is peculiarly avian, being found in existing birds except in part of the back in Penguins. It is faintly approximated to in one or two neck vertebrae in man. Professor Williston remarks that in the toothless Pterodactyles of Kansas the hinder ball of the vertebral articulation is continued downward and outward as a concave articulation upon the processes at its outer corners. There are no mammals with a cup-and-ball articulation between the vertebrae, so that for what it is worth the character now described in Ornithosaurs is reptilian, when judged by comparison with existing animals.

[Ill.u.s.tration: FIG. 25. CERVICAL VERTEBRA OF ORNITHOCHEIRUS From the Cambridge Greensand]

Low down on each side of the vertebra, at the junction of its body with the neural arch, is a large ovate foramen, transversely elongated, and often a little impressed at the border, which is the entrance of the air cell into the bone. These foramina are often one-third of the length of the neck vertebrae in specimens from the Cambridge Greensand, where the neck bones vary from three-quarters of an inch to about two and a half inches in length, and in extreme forms are as wide as long. The width of the inters.p.a.ce between the foramina is one-half the width of the vertebrae, though this character varies with different genera and species. Several species from the Solenhofen Slate have the neck long and slender, on the type of the Flamingo. In others the neck is thick and short--in the _Scaphognathus cra.s.sirostris_ and _Pterodactylus spectabilis_. Some genera with slender necks have the bones preserved with a curved contour, such as might suggest a neck carried like that of a Llama or a Camel. The neck is occasionally preserved in a curve like a capital #S#, as though about to be darted forward like that of a bird in the act of striking its prey. The genera of Pterodactyles with short necks may have had as great mobility of neck as is found among birds named Ducks and Divers; but those Pterodactyles with stout necks, such as Dimorphodon and Ornithocheirus, in which the vertebrae are large, appear to have been built more for strength than activity, and the neck bones have been chiefly concerned in the muscular effort to use the fighting power of the jaws in the best way.

THE BACK

The region of the back in a Pterodactyle is short as compared with the neck, and relatively is never longer than the corresponding region in a bird. The shortness results partly from the short length of the vertebrae, each of which is about as long as wide. There is also a moderate number of bones in the back. In most skeletons from Solenhofen these vertebrae between the neck and girdle of hip bones number from twelve to sixteen. They have a general resemblance in form to the dorsal vertebrae in birds. The greatest number of such vertebrae in birds is eleven. The number is small because some of the later vertebrae in birds are overlapped by the bones of the hip girdle, which extend forward and cover them at the sides, so that they become blended with the sacrum.

This region of the skeleton in the Dimorphodon from the Lias is remarkable for the length of the median process, named the neural spine, which is prolonged upward like the spines of the early dorsal vertebrae of Horses, Deer, and other mammals. In this character they differ from living reptiles, and parallel some Dinosaurs from the Weald. The bones of the back in Ornithocheirus from the Cambridge Greensand show the under side to be well rounded, so that the articular surfaces between the vertebrae, though still rather wider than deep, are much less depressed than in the region of the neck. The neural ca.n.a.l for the spinal cord has become larger and higher, and the sides of the bone are somewhat compressed. Strong transverse processes for the support of the ribs are elevated above the level of the neural ca.n.a.l, at the sides of vertebrae compressed on the under sides, and directed outward. Between these lateral horizontal platforms is the compressed median neural spine, which varies in vertical height. The articulation of the ribs is not seen clearly. Isolated ribs from the Stonesfield Slate have double-headed dorsal ribs, like those of birds. In some specimens from the Solenhofen Slate like the Scaphognathus, in the University Museum at Bonn, dorsal ribs appear to be attached by a notch in the transverse process of the dorsal vertebra, which resembles the condition in Crocodiles. Variations in the mode of attachment of ribs among mammals may show that character to be of subordinate importance. Von Meyer has described the first pair of ribs as frequently larger than the others, and there appear in Rhamphorhynchus to be examples preserved of the sternal ribs, which connect the dorsal ribs with the sternum. Six pairs have been counted. A more interesting feature in the ribs consists in the presence behind the sternum, which is shorter than the corresponding bone in most birds, of median sternal ribs. They are slender #V#-shaped bones in the middle line of the abdomen, which overlapped the ends of the dorsal ribs like the similar sternal bones of reptiles. Such structures are unknown among Birds and Mammals. There is no trace in the dorsal ribs of the claw-like process, which extends laterally from rib to rib as a marked feature in many birds. Its presence or absence may not be important, because it is represented by fibro-cartilage in the ribs of crocodiles, and may be a small cartilage near the head of the rib in serpents, and is only ossified in some ribs of the New Zealand reptile Hatteria. So that it might have been present in a fossil animal without being ossified and preserved. Although the structure is a.s.sociated with birds, it is possibly also represented by the great bony plates which cover the ribs in Chelonians, and combine to form the shield which covers the turtle's back. The structure is as characteristic of reptiles as of birds, but is not necessarily a.s.sociated with either.

[Ill.u.s.tration: FIG. 26