Dragons of the Air Part 7

[Ill.u.s.tration: FIG. 34. COMPARISON OF THE TIBIA AND FIBULA IN ORNITHOSAUR AND VULTURE]

TIBIA AND FIBULA

In _Pterodactylus longirostris_ the tibia is slender, more than a fifth longer than the femur. A crest is never developed at the proximal end, like that seen in the Guillemot and Diver and other water birds. The bone is of comparatively uniform thickness down the shaft in most of the Solenhofen specimens, as in most birds. At the distal end the shin bone commonly has a rounded, articular termination, like that seen in birds.

This is conspicuous in the _Pterodactylus grandis_. In other specimens the tarsal bones, which form this pulley, remain distinct from the tibia; and the upper row of these bones appears to consist of two bones, like those which in many Dinosaurs combine to form the pulley-like end of the tibia which represents the bird's drum-stick bone. They correspond with the ankle bones in man named astragalus and os calcis.

Complete English specimens of tibia and fibula are found in the genus Dimorphodon from the Lias, in which the terminal pulley of the distal end has some expansion, and is placed forward towards the front of the tibia, as in some birds. The rounded surface of the pulley is rather better marked than in birds. The proximal end of the shaft is relatively stout, and is modified by the well-developed fibula, which is a short external splint bone limited to the upper half of the tibia, as in birds; but contributing with it to form the articular surface for the support of the lower end of the femur, taking a larger share in that work than in birds. Frequently there is no trace of the fibula visible in Solenhofen specimens as preserved; or it is extremely slender and bird-like, as in _Pterodactylus longirostris_. In Rhamphorhynchus it appears to extend the entire length of the tibia, as in Dinosaurs. In the specimens from the Cambridge Greensand there is indication of a small proximal crest to the tibia with a slight ridge, but no evidence that this is due to a separate ossification. The patella, or knee-cap, is not recognised in any fossil of the group. There is no indication of a fibula in the specimens thus far known from the Chalk rocks either of Kansas in America, or in England.

The region of the tarsus varies from the circ.u.mstance that in many specimens the tibia terminates downward in a rounded pulley, like the drum-stick of a bird; while in other specimens this union of the proximal row of the tarsal bones with the tibia does not take place, and then there are two rows of separate tarsal bones, usually with two bones in each row. When the upper row is united with the tibia the lower row remains distinct from the metatarsus, though no one has examined these separate tarsal bones so as to define them.

THE FOOT

[Ill.u.s.tration: FIG. 35. METATARSUS AND DIGITS IN THREE TYPES OF ORNITHOSAURS]

The foot sometimes has four toes, and sometimes five. There are four somewhat elongated, slender metatarsal bones, which are separate from each other and never blended together, as in birds. There has been a suspicion that the metatarsal bones were separate in the young Archaeopteryx. In the young of many birds the row of tarsal bones at the proximal end of the metatarsus comes away, and there is a partial division between the metatarsal bones, though they remain united in the middle. And among Penguins, in which the foot bones are applied to the ground instead of being carried in the erect position of ordinary birds, there is always a partial separation between the metatarsal bones, though they become blended together. The Pterodactyle is therefore different from birds in preserving the bones distinct through life, and this character is more like Reptiles than Mammals. The individual bones are not like those of Dinosaurs, and diverge in Rhamphorhynchus as though the animals were web-footed. There is commonly a rudimentary fifth metatarsal. It is sometimes only a claw-shaped appendage, like that seen in the Crocodile. It is sometimes a short bone, completely formed, and carrying two phalanges in Solenhofen specimens: though no trace of these phalanges is seen in the large toothless Pterodactyles from the Cretaceous rocks of North America. In the _Pterodactylus longirostris_ the number of foot bones on the ordinary digits is two, three, four, five, as in lizards; but the short fifth metatarsal has only two toe bones. In Dimorphodon the fifth digit was bent upward, and supported a membrane for flight. There are slight variations in the number of foot bones. In the species _Pterodactylus scolopaciceps_ the number of bones in the toes follows the formula two, three, three, four.

In _Pterodactylus micronyx_ the number is two, three, three, three. The terminal claws are much less developed than is usual with Birds; and there is a difference from Bats in the unequal length of the digits.

Taken as a whole, the foot is perhaps more reptilian than avian, and in some genera is crocodilian.

The foot is the light foot of an active animal. Von Meyer thought that the hind legs were too slender to enable the animal to walk on land; and Professor Williston, of the University of Kansas, remarks that the rudimentary claws and weak toes indicate that the animal could not have used the feet effectively for grasping, while the exceedingly free movement of the femur indicates great freedom of movement of the hind legs; and he concludes that the function of the legs was chiefly for guidance in flight through their control over the movements, and expresses his belief that the animal could not have stood upon the ground with its feet. There may be evidence to sustain other views. If the limb bones are reconstructed, they form limbs not wanting in elegance or length. If it is true, as Professor Williston suggests, that the weight of his largest animals with the head three feet long, and a stretch of wing of eighteen or nineteen feet, did not exceed twenty pounds, there can be no objection to regarding these animals as quadrupeds, or even as bipeds, on the ground of the limbs lacking the strength necessary to support the body. The slender toes of many birds, and even the two toes of the ostrich, may be thought to give less adequate support for those animals than the metatarsals and digits of Pterodactyles.

CHAPTER XI

SHOULDER-GIRDLE AND FORE LIMB

STERNUM

The sternum is always a distinguishing part of the bony structure of the breast. In Crocodiles it is a cartilage to which the sternal ribs unite; and upon its front portion a flat knife-like bone called the interclavicle is placed. In lizards like the Chameleon, it is a lozenge-shaped structure of thin bony texture, also bearing a long interclavicle, which supports the clavicular bones, named collar bones in man, which extend outward to the shoulder blades. Among mammals the sternum is usually narrow and flat, and often consists of many successive pieces in the middle line, on the under side of the body.

Among Bats the anterior part is somewhat widened from side to side, to give attachment to the collar bones, but the sternum still remains a narrow bone, much narrower than in Dolphins, and not differing in character from many other Mammals, notwithstanding the Bat's power of flight. The bone develops a median keel for the attachment of the muscles of the breast, but something similar is seen in burrowing Insectivorous mammals like the Moles. So that, as Von Meyer remarked, the presence of a keel on the sternum is not in itself sufficient evidence to prove flight.

Among birds the sternum is greatly developed. Broad and short in the Ostrich tribe, it is devoid of a keel; and therefore the keel, if present in a bird, is suggestive of flight. The keel is differently developed according to the mode of attachment of the several pectoral muscles which cover a bird's breast. In several water birds the keel is strongly developed in front, and dies away towards the hinder part of the sternum, as in the Cormorant and its allies. The sternum in German Pterodactyles is most nearly comparable to these birds.

[Ill.u.s.tration: FIG. 36. COMPARISON OF THE STERNUM]

In the Solenhofen Slate the sternum is fairly well preserved in many Ornithosaurs. It is relatively shorter than in birds, and is broader than long; but not very like the sternum of reptile or mammal in form.

The keel is limited to the anterior part of the shield of the sternum, as in Merganser and the Cormorant, and is prolonged forward for some distance in advance of it. Von Meyer noticed the resemblance of this anterior process to the interclavicle of the Crocodile in position; but it is more like the keel of a bird's sternum, and is not a separate bone as in Reptiles. In Pterodactyles from the Cretaceous rocks, the side bones, called coracoids, are articulated to saddle-shaped surfaces at the hinder part of the base of this keel, which are parallel in Ornithocheirus, as in most birds, but overlap in Ornithodesmus, as in Herons and wading birds.

[Ill.u.s.tration: FIG. 37. STERNUM IN ORNITHOCHEIRUS FROM THE CAMBRIDGE GREENSAND

Showing the strong keel and the facets for the coracoid bones on its hinder border above the lateral constrictions]

The keel was pneumatic, and when broken is seen to be hollow, and appears to have been exceptionally high in Rhamphorhynchus, a genus in which the wing bones are greatly elongated. Von Meyer found in Rhamphorhynchus on each side of the sternum a separate lateral plate with six pairs of sternal ribs, which unite the sternum with the dorsal ribs, as in the young of some birds. The hinder surface of the sternum is imperfectly preserved in the toothless Pterodactyles of Kansas.

Professor Williston states that the bone is extremely thin and pentagonal in outline, projecting in front of the coracoids, in a stout, blunt, keel-like process, similar to that seen in the Pterodactyles of the Cambridge Greensand. American specimens have not the same notch behind the articulation for the coracoid to separate it from the transverse lateral expansion of the sternal shield. The lateral margin in the Cambridge Greensand specimens figured by Professor Owen and myself is broken; but Professor Williston had the good fortune to find on the margin of the sternum the articular surfaces which gave attachment to the sternal ribs. The margin of the sternal bone thickens at these facets, four of which are preserved. The sternum in Ornithostoma was about four and a half inches long by less than five and a half inches wide. The median keel extends forward for rather less than two inches, while in the smaller Cambridge species of Ornithocheirus it extends forward for less than an inch and a half.

A sternum of this kind is unlike that of any other animal, but has most in common with a bird; and may be regarded as indicating considerable power of flight. The bone cannot be entirely attributed to the effect of flight, since there is no such expanded sternal shield in Bats. The small number of sternal ribs is even more characteristic of birds than mammals or reptiles.

THE SHOULDER-GIRDLE

The bones which support the fore limb are one of the distinctive regions of the skeleton defining the animal's place in nature. Among most of the lower vertebrata, such as Amphibians and Reptiles, the girdle is a double arch--the arch of the collar bone or clavicles in front, and the arch of the shoulder-blade or scapula behind. The clavicular arch, when it exists, is formed of three or five parts--a medium bar named the interclavicle, external to which is a pair of bones called clavicles, reaching to the front of the scapulae when they are present; and occasionally there is a second pair of bones called supraclavicles, extending from the clavicles up the front margins of the scapulae. Thus the clavicular arch is placed in front of the scapular arch. The supraclavicles are absent from all living Reptiles, and the clavicles are absent from Crocodiles. The interclavicle is absent from all mammals except Echidna and Ornithorhynchus. Clavicles also may be absent in some orders of mammals. Hence the clavicular arch may be lost, though the collar bones are retained in man.

The scapular arch also is more complicated and more important in the lower than in the higher vertebrata. It may include three bones on each side named coracoid, precoracoid, and scapula. But in most vertebrates the coracoid and precoracoid appear never to have been segmented so as to be separated from each other; and it is only among extinct types of reptiles, which appear to approximate to the Monotreme mammals, that separate precoracoid bones are found; though among most mammals, probably, there are stages of early development in which precoracoids are represented by small cartilages, though few mammals except Edentata like the Sloths and Ant-eaters, retain even the coracoids as distinct bones. Therefore, excepting the Edentata and the Monotremes, the distinctive feature of the mammalian shoulder-girdle appears to be that the limbs are supported by the shoulder-blades, termed the scapulae.

Among reptiles there are several distinct types of shoulder-girdle.

Chelonians possess a pair of bones termed coracoids which have no connexion with a sternum; and their scapulae are formed of two widely divergent bars, divided by a deeper notch than is found in any fossil reptiles. Among Lizards both scapula and coracoid are widely expanded, and the coracoid is always attached to the sternum. Chameleons have the blade of the scapula long and slender, but the coracoid is always as broad as it is long. Crocodiles have the bone more elongated, so that it has somewhat the aspect of a very strong first sternal rib when seen on the ventral face of the animal. The bone is perforated by a foramen, which would probably lie in the line of separation from the precoracoid if any such separation had ever taken place. The scapula, or shoulder-blade, of Crocodiles is a similar flat bone, very much shorter than the scapula of a Chameleon, and more like that of the New Zealand Hatteria. Thus there is very little in common between the several reptilian types of shoulder-girdle.

[Ill.u.s.tration: FIG. 38. COMPARISON OF SCAPULA AND CORACOID IN THREE PTERODACTYLES AND A BIRD]

In birds the apparatus for the support of the wings has a far-off resemblance to the Crocodilian type. The coracoid bones, instead of being directed laterally outward and upward from the sternum, as among Crocodiles, are directed forward, so as to prolong the line of the breast bone, named the sternum. The bird's coracoid is sometimes flattened towards the breast bone among Swans and other birds; yet as a rule the coracoid is a slender bar, which combines with the still more slender and delicate blade of the scapula, which rests on the ribs, to make the articulation for the upper arm bone. Among reptiles the scapula and coracoid are more or less in the same straight line, as in the Ostrich, but in birds of flight they meet at an angle which is less than a right angle, and where they come in contact the external surface is thickened and excavated to make the articulation for the head of the humerus. There is nothing like this shoulder-girdle outside the cla.s.s of birds, until it is compared with the corresponding structure in these extinct animals called Pterodactyles. The resemblance between the two is surprising. It is not merely the ident.i.ty of form in the coracoid bone and the scapula, but the similar angle at which they meet and the similar position of the articulation for the humerus. Everything in the Pterodactyle's shoulder-girdle is bird-like, except the absence of the representative of the clavicles, that forked #V#-shaped bone of the bird which in scientific language is known as the furculum, and is popularly termed the "merry-thought." This kind of shoulder-girdle is found in the genera from the Lias and the Oolitic rocks, both of this country and Germany.

In the Cretaceous rocks the scapula presents, in most cases, a different appearance. The coracoid is an elongated, somewhat triangular bone, compressed on the outer margin as in birds, but differing alike from birds and other Pterodactyles in not being prolonged forward beyond the articulation for the humerus. In these Cretaceous genera, toothed and toothless alike, the articulation for the upper arm bone truncates the extremity of the coracoid, so that the bone is less like that of a bird in this feature. Perhaps it shows a modification towards the crocodilian direction. The scapula, which unites with the coracoid at about a right angle, is similarly truncated by the articular surface for the humerus; but the bone is somewhat expanded immediately beyond the articulation, and compressed; and instead of being directed backward, it is directed inward over the ribs to articulate with the neural arches of the early dorsal vertebrae in the genera found in strata a.s.sociated with the Chalk.

As the bone approaches this articulation, it thickens and widens a little, becoming suddenly truncated by an ovate facet, which exactly corresponds to the transversely ovate impression, concave from front to back, which is seen in the neural arches of the dorsal vertebrae on which it fits. This condition is not present in all Cretaceous Pterodactyles.

It does not occur in the Kansas fossil, named by Professor Marsh, Nyctodactylus. And it appears to be absent from the Pterodactyles of the English Weald, named Ornithodesmus.

[Ill.u.s.tration: FIG. 39. THE NOTARIUM

An ossification which gives attachment to the scapulae seen in the early dorsal vertebra of Ornithocheirus

(From the Cambridge Greensand)]

[Ill.u.s.tration: FIG. 40. RESTORATION OF THE SHOULDER-GIRDLE IN THE CRETACEOUS ORNITHOCHEIRUS

Showing how the scapulae articulate with a vertebra and the articulation of the coracoids with the sternum. The humeral articulation with the coracoid is unlike the condition shown in other Ornithosaurs]

There is no approach to this transverse position of the scapulae among birds. And while the form of the bones in the older genera of Ornithosaurs is singularly bird-like, the angular arrangement in this Cretaceous genus is obtained by closely approximating the articulations on the sternum, so that the coracoids extend outward as in reptiles, instead of forward as in birds; and the extremities of the scapulae similarly approximate towards each other. This rather recalls the relative positions of scapula and coracoid among crocodiles. If crocodile and bird had been primitive types of animals instead of surviving types, it might almost seem as though there had been a cunning and harmonious blending of one with the other in evolving this form of shoulder-girdle.

THE FORE LIMB

The bones of the fore limb, generally, correspond in length with the similar parts of the hind limb. The upper arm bone corresponds with the upper leg bone, and the fore-arm bone is as long as the fore-leg bone; then differences begin. The bones which correspond to the back of the hand in man, termed the metacarpus, are variable in length in Pterodactyles--sometimes very long and sometimes short. The wing metacarpal bone is always stout, and the others are slender. The extremity of the metacarpus was applied to the ground. Three small digits of the hand are developed from the three small metacarpal bones, and terminate in large claws.

The great wing finger was bent backward, and only touched the ground where it fitted upon the wing metacarpal bone. It appears sometimes to have been as long as the entire vertebral column.

Owing to the circ.u.mstance that the joint in the arm in Pterodactyles was not at the wrist as among birds, but between the metacarpus and the phalanges, it follows that the fore limb was longer than the hind limb when the metacarpus was long; but the difference would not interfere with the movements of the animal, either upon four feet or on two feet, for in bats and birds the disproportion in length is greater.

HUMERUS OR UPPER ARM BONE

The first bone in the fore-arm, the humerus, is remarkable chiefly for the compressed crescent form of its upper articular end, which is never rounded like the head of the upper arm bone in man, and secondly for the great development of the external process of bone near that end, termed the radial crest. Sir Richard Owen compared the bone to the humerus of both birds and crocodiles, but in its upper articular end the crocodile bone may be said to be more like a bird than it is like the Pterodactyle. In flying reptiles the articular surface next to the shoulder-girdle is somewhat saddle-shaped, being concave from side to side above and convex vertically, while most animals with which it can be compared have the articular head of the bone convex in both directions. A remarkable exception to this general rule is found in some fossil animals from South Africa, which, from resemblance to mammals in their teeth, have been termed Theriodonts. They sometimes have the head of the bone concave from side to side and convex in the vertical direction. To this condition Ornithorhynchus makes a slight approximation. The singular expansion of the structure called the radial crest finds no close parallel in reptiles, though Crocodiles have a moderate crest on the humerus in the same position; and in Theriodonts the radial crest extends much further down the shaft of the humerus. No bird has a radial crest of a similar kind, though it is prolonged some way down the shaft in Archaeopteryx. In Pterodactyles it sometimes terminates outward in a smooth, rounded surface, which might have been articular if any structure could have articulated with it. There is also a moderate expansion of the bone on the ulnar side in some Pterodactyles, so that the proximal end often incloses nearly three-fourths of an ovate outline. The termination of the radial crest is at the opposite end of this oval to the wider articular part of the head of the bone, in some specimens from the Cambridge Greensand. The radial crest is more extended in Rhamphorhynchus. All specimens of the humerus show a twist in the length of the bone, so that the end towards the fore-arm, which is wider than the shaft, makes a right angle with the radial crest on the proximal end, which is not seen in birds. The shaft of the humerus is always stouter than that of the femur, though different genera differ in this respect.

The humerus in genera from rocks a.s.sociated with the Chalk presents two modifications, chiefly seen in the characters of the distal end of the bone. One of these is a stout bone with a curiously truncated end where it joins the two bones of the fore-arm; and the other is more or less remarkable for the rounded form of the distal condyles. Both types show distinct articular surfaces. The inner one is somewhat oblique and concave, the outer one rounded; the two being separated by a concave channel, so that the ulna makes an oblique articulation with the bone as in birds, and the radius articulates by a more or less truncated or concave surface.

[Ill.u.s.tration: FIG. 41. COMPARISON OF THE HUMERUS IN PTERODACTYLE AND BIRD]