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 space 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 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.
Sometimes a horny 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 interspace had originally been occupied by organic substance like a horny beak.
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.
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—Archæopteryx, 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 association 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.
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 classes of vertebrate animals than either the head or limbs. It is divided into neck bones, the cervical vertebræ; back bones, the dorsal vertebræ; loin bones, the lumbar vertebræ; the sacrum, or sacral vertebræ, 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 vertebræ. 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 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 Tanystrophœus. 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 vertebræ, 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 vertebræ 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 vertebræ. 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 vertebræ are longer, but the length visibly shortens in the sixth and seventh.
Sometimes the vertebræ 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 massive. The vertebræ 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 vertebræ 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 vertebræ. 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 vertebræ 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 vertebræ have no cervical ribs.
The articular surfaces between the bodies of the vertebræ, 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 vertebræ; 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 vertebræ 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 vertebræ, so that for what it is worth the character now described in Ornithosaurs is reptilian, when judged by comparison with existing animals.
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 vertebræ 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 interspace between the foramina is one-half the width of the vertebræ, 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 crassirostris 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 vertebræ 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 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 vertebræ, 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 vertebræ between the neck and girdle of hip bones number from twelve to sixteen. They have a general resemblance in form to the dorsal vertebræ in birds. The greatest number of such vertebræ in birds is eleven. The number is small because some of the later vertebræ 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 vertebræ 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 vertebræ, though still rather wider than deep, are much less depressed than in the region of the neck. The neural canal 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 canal, at the sides of vertebræ 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 associated 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 associated with either.
The upper figures show the side and back of a dorsal vertebra of Ornithocheirus compared with corresponding views of the side and back of a dorsal vertebra of a Crocodile
There are two remarkable modifications of the early dorsal vertebræ in some of the Cretaceous Pterodactyles. First, in the genus Ornithodesmus from the Weald the early dorsal vertebræ are blended together into a continuous mass, like that which is found in the corresponding region of the living Frigate-bird, only more consolidated, and similar to that consolidated structure found behind the dorsal vertebræ, known as the sacrum, made by the blending of the vertebræ into a solid mass which supports the hip bones. Secondly, in some of the Cretaceous genera of Pterodactyles of Europe and America the vertebræ in the front part of the back are similarly blended, but their union is less complete; and in genera Ornithocheirus and Ornithostoma—the former chiefly English, the latter chiefly American—the sides of the neural spines are flattened to form an oval articular surface on each side, which gives attachment to the flattened ends of their shoulder-blade bones named the scapulæ. This condition is found in no other animals. Three vertebræ appear to have their neural arches thus united together. The structure so formed may be named the notarium to distinguish it from the sacrum.
For some mysterious reason the part of the backbone which lies between the bones of the hips and supports them is termed the sacrum. Among living reptiles the number of vertebræ in this region is usually two, as in lizards and crocodiles. There are other groups of fossil reptiles in which the number of sacral vertebræ is in some cases less and in other cases more. There is, perhaps, no group in which the sacrum makes a nearer approach to that of birds than is found among these Pterodactyles, although there are more sacral vertebræ in some Dinosaurs. In birds the sacral vertebræ number from five to twenty-two. In bats the number is from five to six. In some Solenhofen species, such as Pterodactylus dubius and P. Kochi and P. grandipelvis, the number is usually five or six. The vertebræ are completely blended. The pneumatic foramina in the sacrum, so far as they have been observed, are on the under sides of the transverse processes; while in the corresponding notarial structure in the shoulder girdle the foramina are in front of the transverse processes. Almost any placental mammal in which the vertebræ of the sacral region are anchylosed together has a similar sacrum, which differs from that of birds in the more complete individuality of the constituent bones remaining evident. The transverse processes in front of the sacrum are wider than in its hinder part; so that the pelvic bones which are attached to it converge as they extend backward, as among mammals. The bodies of the vertebræ forming the sacrum are similar in length to those of the back. Each transverse process is given off opposite the body of its own vertebra, but from a lower lateral position than in the region of the back, in which the vertebræ are free.
Showing the complete blending of the vertebræ and ribs as in a bird, with the well-defined Iliac bones, produced chiefly in front of the acetabulum for the head of the femur.
The hip bones are closely united with the sacrum by bony union, and rarely appear to come away from the sacral vertebræ, as among mammals and reptiles, though this happens with the Lias Pterodactyles. In the Stonesfield Slate and Solenhofen Slate the slender transverse processes from the vertebræ blend with the ilium of the hip girdle, and form a series of transverse foramina on each side of the bodies of the vertebræ. In the Cambridge Greensand genera the part of the ilium above the acetabulum for the articular head of the femur appears to be always broken away, so that the relation of the sacrum to the pelvis has not been observed. This character is no mark of affinity, but only shows that ossification obliterated sutures among these animals in the same way as among birds.
The great difference between the sacrum of a Pterodactyle and that of a bird has been rendered intelligible by the excellent discussion of the sacral region in birds made by Professor Huxley. He showed that it is only the middle part of the sacrum of a chicken which corresponds to the true sacrum of a reptile, and comprises the five shortest of the vertebræ; while the four in front correspond to those of the lower part of the back, which either bear no ribs or very short ribs, and are known as the lumbar region in mammals, so that the lower part of the back becomes blended with the sacrum, and thus reduces the number of dorsal vertebræ. Similarly the five vertebræ which follow the true sacral vertebræ are originally part of the tail, and have been blended with the other vertebræ in front, in consequence of the extension along them of the bird's hip bones. This interpretation helps to account for the great length of the sacrum in many birds, and also explains in part the singular shortness of the tail in existing birds. The Ornithosaur sacrum has neither the lumbar nor the caudal portions of the sacrum of a bird.
The tail is perhaps the least important part of the skeleton, since it varies in character and length in different genera. The short tails seen in typical pterodactyles include as few as ten vertebræ in Pterodactylus grandipelvis and P. Kochi, and as many as fifteen vertebræ in Pterodactylus longirostris. The tails are more like those of mammals than existing birds, in which there are usually from six to ten vertebræ terminating in the ploughshare bone. But just as some fossil birds, like the Archæopteryx, have about twenty long and slender vertebræ in the tail, so in the pterodactyle Rhamphorhynchus this region becomes greatly extended, and includes from thirty-eight to forty vertebræ. In Dimorphodon the tail vertebræ are slightly fewer. The earliest are very short, and then they become elongated to two or three times the length of the early tail vertebræ, and finally shorten again towards the extremity of the tail, where the bones are very slender. In all long-tailed Ornithosaurians the vertebræ are supported and bordered by slender ossified ligaments, which extend like threads down the tail, just as they do in Rats and many other mammals and in some lizards.
Professor Marsh was able to show that the extremity of the tail in Rhamphorhynchus sometimes expands into a strong terminal caudal membrane of four-sided somewhat rhomboidal shape. He regards this membrane as having been placed vertically. It is supported by delicate processes which represent the neural spines of the vertebræ prolonged upward. They are about fifteen in number. A corresponding series of spines on the lower border, termed chevron bones, equally long, were given off from the junctions of the vertebræ on their under sides, and produced downward. This vertical appendage is of some interest because its expansion is like the tail of a fish. It suggests the possibility of having been used in a similar way to the caudal fin as an organ for locomotion in water, though it is possible that it may have also formed an organ used in flight for steering in the air.
Showing the processes on the upper and under sides of the vertebræ which make the terminal leaf-like expansion
The tail vertebræ from the Cambridge Greensand are mostly found isolated or with not more than four joints in association. They are very like the slender type of neck vertebræ seen in long-necked pterodactyles, but are depressed, and though somewhat wider are not unlike the tail vertebræ of the Rhamphorhynchus. The pneumatic foramen in them is a mere puncture. They have no transverse processes or neural spines, nor indications of ribs, or chevron bones.
The hindermost specimens of tail vertebræ observed have the neural arch preserved to the end, as among reptiles; whereas in mammals this arch becomes lost towards the end of the tail. The processes by which the vertebræ are yoked together are small. There is nothing to suggest that the tail was long, except the circumstance that the slender caudal vertebræ are almost as long as the stout cervical vertebræ in the same animal. No small caudal vertebræ have ever been found in the Cambridge Greensand. The tail is very short, according to Professor Williston, in the toothless Ornithostoma in the Chalk of Kansas.
The bones of the hip-girdle form a basin which incloses and protects the abdominal vital organs. It consists on each side of a composite bone, the unnamed bones—ossa innominata of the older anatomists—which are each attached to the sacrum on their inner side, and on the outer side give attachment to the hind limbs. As a rule three bones enter into the borders of this cup, termed the acetabulum, in which the head of the thigh bone, named the Femur, moves with a more or less rotary motion.
There are a few exceptions in this division of the cup between three bones, chiefly among Salamanders and certain Frogs. In Crocodiles the bone below the acetabular cup is not divided into two parts. And in certain Plesiosaurs from the Oxford Clay—Murænosaurus—the actual articulation appears to be made by two bones—the ilium and ischium. The three bones which form each side of the pelvis are known as the ilium, or hip bone, sometimes termed the aitch bone; secondly, the ischium, or sitz bone, being the bone by which the body is supported in a sitting position; and thirdly the pubis, which is the bone in front of the acetabulum. The pubic bones meet in the middle line of the body on the under side of the pelvis in man, and on each side are partly separated from the ischia by a foramen, spoken of as the obturator foramen, which in Pterodactyles is minute and almost invisible, when it exists.
There is often a fourth bony element in the pelvis. In some Salamanders a single cartilage is directed forward, and forked in front. According to Professor Huxley something of this kind is seen in the Dog. The pair of bones which extend forward in front of the pelvis in Crocodiles may be of the same kind, in which case they should be called prepubic bones. But among the lower mammals named marsupials a pouch is developed for the protection of the young and supported by two slender bones attached to the pubes, and these bones have long been known as marsupial bones. In a still lower group of mammalia named monotremata, which lay eggs, and in many ways approximate to reptiles and birds, stronger bones are developed on the front edge of the pubes, and termed prepubic bones. They do not support a marsupium.
Naturalists have been uncertain as to the number of bones in the pelvis of Pterodactyles, because the bones blend together early in life, as in birds. Some follow the Amphibian nomenclature, and unite the ischium and pubis into one bone, which is then termed ischium, when the prepubis is termed the pubis, and regarded as removed from the acetabulum. There is no ground for this interpretation, for the sutures are clear between the three pelvic bones in the acetabulum in some specimens, like Cycnorhamphus Fraasii, from Solenhofen, and some examples of Ornithocheirus from the Cambridge Greensand. Pterodactyles all have prepubic bones, which are only known in Ornithorhynchus and Echidna among mammals, and are absent from the higher mammals and birds. They are unknown in any other existing animals, unless present in Crocodiles, in which ischium and pubis are always undivided. Therefore it is interesting to examine the characters of the Ornithosaurian pelvis.
The acetabulum for the head of the femur is imperforate, being a simple oval basin, as in Chelonian reptiles and the higher Mammals. It never shows the mark of the ligamentous attachment to the head of the femur, which is seen in Mammals. In Birds the acetabulum is perforated, as in many of the fossils named Dinosaurs, and in Monotremata.
Secondly, the ilium is elongated, and extends quite as much in front of the acetabulum as behind it. The bone is not very deep in this front process. Among existing animals this relation of the bone is nearer to birds than to any other type, since birds alone have the ilium extended from the acetabulum in both directions. The form of the Pterodactyle ilium is usually that of the embryo bird, and its slender processes compare in relative length better with those of the unhatched fowl and Apteryx of New Zealand than with the plate-like form in adult birds.
In mammals the ilium is directed forward, and even in the Cape ant-eater Orycteropus there is only an inappreciable production of the bone backward behind the acetabulum. Among reptiles the general position of the acetabulum is at the forward termination of the ilium, though the Crocodile has some extension of the bone in both directions, without forming distinct anterior and posterior processes. This anterior and posterior extension of the ilium is seen in the Theriodont reptiles of Russia and of South Africa, as well as in Dinosaurs.
Thirdly, in all pterodactyles the ischium and pubis are more or less completely blended into a sheet of bone, unbroken by perforation, though there is usually a minute vascular foramen; or the lower border may be notched between the ischium and the pubis, as in some of the Solenhofen species, and the pubis does not reach the median line of the body. But in Dimorphodon the pelvic sheet of bone is unbroken by any notch or perforation. The notch between the ischium and pubis is well marked in Pterodactylus longirostris, and better marked in Pterodactylus dubius, Cycnorhamphus Fraasii, and Rhamphorhynchus. The fossil animals which appear to come nearest to the Pterodactyles in the structure of the pelvis are Theriodonts from the Permian rocks of Russia. The type known as Rhopalodon has the ilium less prolonged front and back, and is much deeper than in any Pterodactyle; but the acetabulum is imperforate, and the ischium and pubis are not always completely separated from each other by suture. In the pelvis referred to the Theriodont Deuterosaurus there is some approximation to the pelvis of Rhamphorhynchus and of Pterodactylus dubius in the depth of the division between the pubis and ischium.
On the left-hand side the two prepubic bones are separate. On the right-hand they are united into a transverse bar which overlaps the front of pelvis seen from the under side
There are three modifications of the Ornithosaurian pelvis. First, the type of Rhamphorhynchus, in which the pubis and ischium are inclined somewhat backward, and in which the two prepubic bones are triangular, and are often united together to form a transverse bow in front of the pubic region.
Secondly, there is the ordinary form of pelvis in which the pubis and ischium usually unite with each other down their length, as in Dimorphodon, but sometimes, as in Pterodactylus dubius, divide immediately below the acetabulum. All these types possess the paddle-shaped prepubic bones, which are never united in the median line.
Thirdly, there is the cretaceous form indicated by Ornithocheirus and Ornithostoma, in which the posterior half of the ilium is modified in a singular way, since it is more elevated towards the sacrum than the anterior half, suggesting the contour of the upper border of the ilium in a lizard. Without being reptilian—the anterior prolongation of the bone makes that impossible—it suggests the lizards. This type also possesses prepubic bones. They appear, according to Professor Williston, to be more like the paddle-shaped bones of Pterodactylus than like the angular bones in Rhamphorhynchus. The prepubic bones are united in the median line as in Rhamphorhynchus. But their median union in that genus favours the conclusion that the bones were united in the median line in all species, though they are only co-ossified in these two families.
The bones in front are here regarded as prepubic, but are commonly named pubic
This median union of the prepubic bones is a difference from those mammals like the Ornithorhynchus and Echidna, which approach nearest to the Reptilia. In them the prepubic bones have a long attachment to the front margin of the pubis, and extend their points forward without any tendency for the anterior extremities to approximate or unite. The marsupial mammals have the same character, keeping the marsupial bones completely distinct from each other at their free extremities. The only existing animals in which an approximation is found to the prepubic bones in Pterodactyles are Crocodiles, in bones which most writers term the pubic bones. This resemblance, without showing any strong affinity with the Crocodilia, indicates that Crocodiles have more in common with the fossil flying animals than any other group of existing reptiles; for other reptiles all want prepubic bones, or bones in front of the pubic region.
The hind limb is exceptionally long in proportion to the back. This is conspicuous in the skeletons of the short-tailed Pterodactyles, and is also seen in Dimorphodon. In Rhamphorhynchus the hind limb is relatively much shorter, so that the animal, when on all fours, may have had an appearance not unlike a Bat in similar position. The limb is exceptionally short in the little Ptenodracon brevirostris. The bones of the hind limb are exceptionally interesting. One remarkable feature common to all the specimens is the great elongation of the shin bones relatively to the thigh bones. The femur is sometimes little more than half the length of the tibia, and always shorter than that bone. The proportions are those of mammals and birds. Some mammals have the leg shorter than the thigh, but mammals and birds alone, among existing animals, have the proportions which characterise Pterodactyles. The foot appears to have been applied to the ground not always as in a bird, but more often in the manner of reptiles, or mammals in which the digits terminate in claws.
On the right is a front view of femur of a bear. In the middle are front and side views of the femur of Ornithocheirus. On the left is the femur of Echidna. These comparisons illustrate the mammalian characters of the Pterodactyle thigh bone
The thigh bone, on account of the small size of many of the specimens, is not always quite clear evidence as an indication of technical resemblance to other animals. The bone is always a little curved, has always a rounded, articular head, and rounded distal condyles. Its most remarkable features are shown in the large, well-preserved specimens from the Cambridge Greensand. The rounded, articular head is associated with a constricted neck to the bone, followed by a comparatively straight shaft with distal condyles, less thickened than in mammals. No bird is known, much less any reptile, with a femur like Ornithocheirus. Only among Mammals is a similar bone known with a distinct neck; and only a few mammals have the exceptional characters of the rounded head and constricted neck at all similar to the Cretaceous Pterodactyles. A few types, such as the higher apes, the Hyrax, and animals especially active in the hind limb, have a femur at all resembling the Pterodactyle in the pit for the obturator externus muscle, behind the trochanter major, such as is seen in a small femur from Ashwell. The femur varies in different genera, so as to suggest a number of mammalia rather than any particular animal for comparison. These approximations may be consequences of the ways in which the bones are used. When functional modifications of the skeleton are developed, so as to produce similar forms of bones, the muscles to which they give attachment, which act upon the bones, and determine their growth, are substantially the same. In the Pterodactylus longirostris the femur corresponds in length to about eleven dorsal vertebræ. The end next the shin bone is less expanded than is usual among Mammals, and rather suggests an approach to the condition in Crocodiles, in the moderate thickness and breadth of the articular end, and the slight development of the terminal pulley-joint. One striking feature of the femur is the circumstance that the articular head, as compared with the distal end, is directed forward and very slightly inward and upward. So that allowing for the outward divergence of the pelvic bones, as they extend forward, there must have been a tendency to a knock-kneed approximation of the lower ends of the thigh bones, as in Mammals and Birds, rather than the outward divergence seen in Reptiles.
Apparently the swing of the leg and foot, as it hung on the distal end of the femur, must have tended rather to an inward than to an outward direction, so that the feet might be put down upon the same straight line; this arrangement suggests rapid movement.