Dragons of the Air: An Account of Extinct Flying Reptiles

by H. G. Seeley

Showing how the scapulæ 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 scapulæ 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 scapulæ 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 circumstance 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 Archæopteryx. 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 associated 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.