Dragons of the Air Part 6

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 vertebrae in some of the Cretaceous Pterodactyles. First, in the genus Ornithodesmus from the Weald the early dorsal vertebrae are blended together into a continuous ma.s.s, 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 vertebrae, known as the sacrum, made by the blending of the vertebrae into a solid ma.s.s which supports the hip bones. Secondly, in some of the Cretaceous genera of Pterodactyles of Europe and America the vertebrae 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 scapulae. This condition is found in no other animals. Three vertebrae appear to have their neural arches thus united together. The structure so formed may be named the notarium to distinguish it from the sacrum.

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 vertebrae in this region is usually two, as in lizards and crocodiles. There are other groups of fossil reptiles in which the number of sacral vertebrae 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 vertebrae in some Dinosaurs. In birds the sacral vertebrae 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 vertebrae 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 vertebrae of the sacral region are anchylosed together has a similar sacrum, which differs from that of birds in the more complete individuality of the const.i.tuent 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 vertebrae 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 vertebrae are free.

[Ill.u.s.tration: FIG. 27. SACRUM OF RHAMPHORHYNCHUS

Showing the complete blending of the vertebrae 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 vertebrae, 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 vertebrae blend with the ilium of the hip girdle, and form a series of transverse foramina on each side of the bodies of the vertebrae. 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 vertebrae; 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 vertebrae. Similarly the five vertebrae which follow the true sacral vertebrae are originally part of the tail, and have been blended with the other vertebrae 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

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 vertebrae in _Pterodactylus grandipelvis_ and _P. Kochi_, and as many as fifteen vertebrae in _Pterodactylus longirostris_. The tails are more like those of mammals than existing birds, in which there are usually from six to ten vertebrae terminating in the ploughshare bone. But just as some fossil birds, like the Archaeopteryx, have about twenty long and slender vertebrae in the tail, so in the pterodactyle Rhamphorhynchus this region becomes greatly extended, and includes from thirty-eight to forty vertebrae. In Dimorphodon the tail vertebrae are slightly fewer. The earliest are very short, and then they become elongated to two or three times the length of the early tail vertebrae, and finally shorten again towards the extremity of the tail, where the bones are very slender. In all long-tailed Ornithosaurians the vertebrae 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 vertebrae 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 vertebrae 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.

[Ill.u.s.tration: FIG. 28. EXTREMITY OF THE TAIL OF _RHAMPHORHYNCHUS PHYLLURUS_ (MARSH)

Showing the processes on the upper and under sides of the vertebrae which make the terminal leaf-like expansion]

The tail vertebrae from the Cambridge Greensand are mostly found isolated or with not more than four joints in a.s.sociation. They are very like the slender type of neck vertebrae seen in long-necked pterodactyles, but are depressed, and though somewhat wider are not unlike the tail vertebrae 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 vertebrae 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 vertebrae are yoked together are small. There is nothing to suggest that the tail was long, except the circ.u.mstance that the slender caudal vertebrae are almost as long as the stout cervical vertebrae in the same animal. No small caudal vertebrae 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.

CHAPTER X

THE HIP-GIRDLE AND HIND LIMB

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--Muraenosaurus--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 p.u.b.es, 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 p.u.b.es, 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.

[Ill.u.s.tration: FIG. 29. COMPARISON OF THE LEFT SIDE OF THE PELVIS IN A BIRD AND A PTERODACTYLE]

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.

[Ill.u.s.tration: FIG. 30. LEFT PELVIC BONES WITH PREPUBIC BONE IN _PTERODACTYLUS LONGIROSTRIS_]

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.

[Ill.u.s.tration: FIG. 31 PELVIS AND PREPUBIC BONES OF RHAMPHORHYNCHUS

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.

[Ill.u.s.tration: FIG. 32. THE PELVIC BONES OF AN ALLIGATOR SEEN FROM BELOW

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

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 s.h.i.+n 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.

THE FEMUR

[Ill.u.s.tration: FIG. 33. THE FEMUR

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 ill.u.s.trate 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 a.s.sociated 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 vertebrae. The end next the s.h.i.+n 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 circ.u.mstance 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.