FIG. 41. COMPARISON OF THE HUMERUS IN PTERODACTYLE AND BIRD
ULNA AND RADIUS
The bones of the fore-arm are similar to each other in size, and if there be any difference between them the ulna is slightly the larger. There is some evidence that in Rhamphorhynchus the upper end of the ulna was placed behind the radius, probably in consequence of the mode of attachment of those bones to the humerus. The ulna abutted towards the inner and lower border, while the radius was towards the upper border, consequent upon the twist in the humerus. This condition corresponds substantially with the arrangement in birds, but differs from birds in the relatively more important part taken by the radius in making the articulation. The bones are compared in Dimorphodon with the Golden Eagle drawn of the same size ([Fig. 42]). In birds the ulna supports the great feathers of the wing, and this may account for the size of the bone. The ulna is best seen at its proximal end in the specimens from the Cambridge Greensand, where there is a terminal olecranon ossification forming an oblique articulation, which frequently comes away and is lost. It is sometimes well preserved, and indicated by a suture. The examples of ulna from the Lias show a slight expansion of the bone at both ends, and at the distal end toward the wrist the articulation is well defined, where the bone joins the carpus. The larger specimens of the bone are broken. The distal articular surface is only connected with the proximal end of the bone in small specimens: it always shows on the one margin a concavity, followed by a prominent boss, and an oblique articulation beyond the boss. On the side towards the radius, on the lower end of the shaft there is an angular ridge, which marks the line along which the ulna overlaps the radius. The lower end of the radius has a simple, slightly convex articulation, somewhat bean-shaped. No rotation of these bones on each other was possible as in man. There is a third bone in the fore-arm. This bone, named the pteroid, is commonly seen in skeletons from Solenhofen. It was regarded by Von Meyer as having supported the wing membrane in flight. Some writers have interpreted it as an essential part of the Pterodactyle skeleton, and Von Meyer thought that it might possibly indicate a fifth digit in the hand. The only existing structure at all like it is seen in the South African insectivorous mammal named Chrysochloris capensis, the golden mole, which also has three bones in the fore-arm, the third bone extending half-way up towards the humerus. In that animal the third bone appears to be behind the others and adjacent to the ulna. In the German fossils the pteroid articulated with a separate carpal or metacarpal bone, placed on the side of the arm adjacent to the radius, and the radius is always more inward than the ulna. If the view suggested by Von Meyer is adopted, this bone would be a first digit extending outward and backward towards the humerus. That view was adopted by Professor Marsh. It involves the interpretation of what has been termed the lateral carpal as the first metacarpal bone, which would be as short as that of a bird, but turned in the opposite direction backward. The first digit would then only carry one phalange, and would not terminate in a claw, but lie in the line of the tendon which supports the anterior wing membrane of a bird.
FIG. 42. COMPARISON OF THE BONES OF THE FORE-ARM IN BIRD AND ORNITHOSAUR
The third bone in the fore-arm of Chrysochloris does not appear to correspond to a digit. The bone is on the opposite side of the arm to the similar bone of a Pterodactyle, and therefore cannot be the same structure in the Golden Mole. The interpretation which makes the pteroid bone the first digit has the merit of accounting for the fifth digit of the hand. All the structures of the hand are consistent with this view. The circumstance that the bone is rarely found in contact with the radius, but diverging from it, shows that it plays the same part in stretching the membrane in advance of the arm, that the fifth digit holds in supporting the larger wing membrane behind the arm.
According to Professor Williston, the American toothless Pterodactyle Ornithostoma has but a single phalange on the corresponding first toe of the hind foot, and that bone he describes as long, cylindrical, gently curved, and bluntly pointed. There is some support for this interpretation; but I have not seen any English or German Pterodactyles with only one phalange in the first toe.
The wing in Pterodactyles would thus be stretched between two fingers which are bent backward, the three intermediate digits terminating in claws.