Dragons of the Air: An Account of Extinct Flying Reptiles

by H. G. Seeley

The metacarpus consists of bones which correspond to the back of the hand. The first digit of the hand in clawed animals has the metacarpal bone short, or shorter than the others. Among mammals metacarpal bones are sometimes greatly elongated; and a similar condition is found in Pterodactyles, in which the metacarpal bone may be much longer than the phalange which is attached to it. Two metacarpal bones appear to be singularly stouter than the others. The first bone of the first digit, if rightly determined, is much shorter than the others, and is, in fact, no longer than the carpus ([Fig. 43]). It is a flat oblong bone, attached to the inner side of the lower carpal, and instead of being prolonged distally in the same direction as the other metacarpal bones, is turned round and directed upward, so that its upper edge is flush with the base of the radius, and gives attachment to a bone which resembles a terminal phalange of the wing finger. According to this interpretation it is the first and only phalange in the first digit. The bone is often about half as long as the fore-arm, terminates upward in a point, is sometimes curved, and frequently diverges outward from the bones of the fore-arm, as preserved in the associated skeleton, being stretched towards the radial crest of the humerus. This mode of attachment of the supposed first metacarpal, which is true for all Cretaceous pterodactyles, has not been shown to be the same for all those from the Solenhofen Slate. There is no greater anomaly in this metacarpal and phalange on the inner side being bent backward, than there is in the wing finger being bent backward on the outer side. The three slender intervening digits extend forward between them, as though they were applied to the ground for walking.

FIG. 44. METACARPUS IN TWO ORNITHOSAURS

The bone which is usually known as the wing metacarpal is frequently stouter at the proximal end towards the carpus than towards the phalange. At the carpal end it is oblong and truncated, with a short middle process, which may have extended into the pit in the base of the carpal bone; while the distal terminal end is rounded exactly like a pulley. There is great difference in the length of the metacarpus. In the American genus Ornithostoma it is much longer than the fore-arm. In Rhamphorhynchus it is remarkably short, though perhaps scarcely so short as in Dimorphodon or in Scaphognathus. The largest Cretaceous examples are about two inches wide where they join the carpus. The bone is sometimes a little curved.

Between the first and fifth or wing metacarpal are the three slender metacarpal bones which give attachment to the clawed digits. They bear much the same relation to the wing metacarpal that the large metatarsal of a Kangaroo has to the slender bones of the instep which are parallel to it.

The facet for the wing metacarpal on the carpus is clearly recognised, but as a rule there is no surface with which the small metacarpals can be separately articulated. One or two exceptional specimens from the Cambridge Greensand appear to have not only surfaces for the wing metacarpal, but two much smaller articular surfaces, giving attachment to smaller metacarpals; while in one case there appears to be only one of these additional impressions. It is certain that all the animals from the Lias and Oolites have three clawed digits, but at present I have seen no evidence that there were three in the Cretaceous genera, though Professor Williston's statements and restoration appear to show that the toothless Pterodactyles have three. Another difference from the Oolitic types, according to Professor Williston, is in the length of the slender metacarpals of the clawed phalanges being about one-third that of the wing metacarpal, but this is probably due to imperfect ossification at the proximal end; for at the distal end the bones all terminated on the same level, showing that the four outer digits were applied to the ground to support the weight of the body. The corresponding bone in the Horse and Oxen is carried erect, so as to be in a vertical line with the bones of the fore-arm; and the same position prevails usually, though not invariably, with the corresponding bone in the hind limb, while in many clawed mammals the metacarpus and metatarsus are both applied upon the ground. In Pterodactyles the metatarsal bones are preserved in the rock in the same straight line with the smaller bones of the foot, or make an angle with the shin bone, leading to the conviction that the bones of the foot were applied to the ground as in Man, and sometimes as in the Dog, and were thus modified for leaping. Just as the human metacarpus is extended in the same line with the bones of the fore-arm, and the movement of jointing occurs where the fingers join the metacarpus, so Pterodactyles also had these bones differently modified in the fore and hind limbs for the functions of life. The result is to lengthen the fore limb as compared with the hind limb by introducing into it an elevation above the ground which corresponds to the length of the metacarpus, always supposing that the animal commonly assumed the position of a quadruped when upon the earth's surface.

This position of the metacarpus is a remarkable difference from Birds, because when the bird's wing is at rest it is folded into three portions. The upper arm bone extends backward, the bones of the fore-arm are bent upon it so as to extend forward, and then at the wrist the third portion, which includes the metacarpus and finger bones, is bent backward. So that the metacarpus in the Pterodactyle differs from birds in being in the same line as the bones of the fore-arm, whereas in birds it is in the same line with the digit bones of the hand. It is worthy of remark that in Bats, which are so suggestive of Pterodactyles in some features of the hand, the metacarpals and phalanges are in the same straight line; so that in this respect the bat is more like the bird. But Pterodactyles in the relation of these bones to flight are quite unlike any other animal, and have nothing in common with the existing animals named Reptiles.

THE HAND

From what has just been said it follows that the construction of the hand is unique. It may be contrasted with the foot of a bird. The bone which is called, in the language of anatomists, the tarso-metatarsus, and is usually free from feathers and covered with skin, is commonly carried erect in birds, so that the whole body is supported upon it; and from it the toes diverge outward. It is formed in birds of three separate bones blended together. In the fore limb of the Pterodactyle the metacarpus has the same relation to the bones of the fore-arm that the metatarsus has to the corresponding bones of the leg in a bird. But the three metacarpal bones in the Pterodactyle remain distinct from each other, perhaps because the main work of that region of the skeleton has devolved upon the digit called the wing finger, which is not recognised in the bird. In the Pterodactyles from the Solenhofen Slate there is a progressive number of phalanges in the three small digits of the hand, which were applied to the ground. This number in the great majority of species follows the formula of two bones in the first, three bones in second, and four in the third; so that in the innermost of the clawed digits only one bone intervenes between the metacarpal and the claw. The fingers slightly increase in length with increase in number of bones which form them.

FIG. 45. CLAW PHALANGE FROM THE HAND IN ORNITHOCHEIRUS.
(Half natural size)