Dragons of the Air: An Account of Extinct Flying Reptiles

by H. G. Seeley

A Pterodactyle is shown between a carnivorous Dinosaur above and a herbivorous Dinosaur below

Another pelvic character of some interest is the blending of the pubis and ischium of the right and left sides in the middle line of the body. There are some genera of Dinosaurs like the English Aristosuchus from the Weald, and the American genera Cœlurus, Ceratosaurus, and others, in which the pubic bones, instead of uniting at their extremities, are pinched together from side to side, and unite down the lower part of their length, terminating in an expanded end like a shoe, which is seen to be a separate ossification, and probably formed by a pair of ossifications joined in the median line. This small bone, which is below the pubes, and in these animals becomes blended with them, we may regard as a pair of prepubic bones like those of Pterodactyles and Crocodiles, except that they have lost the stalk-like portions, which in those animals are developed to compensate for the diminished length of the pubic bones. The prepubic bones may also be developed in Iguanodon, in which a pair of bones of similar form remains throughout life in advance of the pubes, as in Pterodactyles. In those Dinosauria with the Bird-like type of pelvis the pubic bone is exceptionally developed, sending one process backward and another process forward, so that there is a great gap between these diverging limbs to the bone. In the region behind the sternum to which the ribs were attached, and in front of the pelvis, is a pair of bones in Iguanodon shaped like the prepubic bones of Dimorphodon. They have sometimes been interpreted as a hinder part of the sternum, but may more probably be regarded as a pair of prepubic bones articulating each with the anterior process of the pubis (see [Fig. 80]). The small bones found at the extremities of the pubes in such carnivorous Dinosaurs as Aristosuchus are blended by bony union with the pubes. The bones in Iguanodon are placed behind the sternal region without any attachment for sternal ribs, and the expanded processes converge forwards from the stalk and unite exactly like the prepubic bones of Ornithosaurs. While this character, on the one hand, may link Pterodactyles with the Dinosaurs, on the other hand it may be a link between both those groups and the Crocodiles, in which the front pair of bones of the pelvis has also appeared to be representative of the prepubic bones of Flying Reptiles (see [Fig. 32, p. 98]).

FIG. 80. DIAGRAM OF THE PELVIS SEEN FROM BELOW IN AN ORNITHOSAUR AND A DINOSAUR

The resemblances between Pterodactyles and Dinosaurs in the hind limb are not of less interest, though it is rather in the older Pterodactyles such as Dimorphodon, Pterodactylus, and Rhamphorhynchus that the resemblance is closest with the slender carnivorous Dinosaurs. They never have the head of the thigh bone, femur, separated from its shaft by a constricted neck, as in the Pterodactyles from the Chalk. In many ways the thigh bone of Dinosaurs tends towards being Avian; while that of Pterodactyles inclines towards being Mammalian, but with a tendency to be Bird-like in the older types, and to be Mammal-like in the most recent representatives of the group in the Chalk.

The bones of the leg in Ornithosaurs, known as tibia and fibula, are remarkable for the circumstance first that they resemble Birds in the fibula being slender and only developed in its upper part towards the femur, and secondly that in a genus like Dimorphodon this drum-stick bone has the two upper bones of the ankle blended with the tibia, so as to form a rounded pulley joint which is indistinguishable from that of a Bird (see [p. 102]). There is a large number of Dinosaurs in which this remarkable distinctive character of Birds is also found. Only, Dinosaurs like Iguanodon, for instance, have the slender fibula as long as the tibia, and contributing to unite with the separate ankle bones of the similarly rounded pulley at the lower end. There are no Birds in which the tarsal bones remain separated and distinct throughout life. But in Pterodactylus from Solenhofen, as in a number of Dinosaurs, especially the carnivorous genera, the bones of the tarsus remain distinct throughout life, and never acquired such forms as would have enabled the ankle bone, termed astragalus, to embrace the extremity of the tibia, as it does in Iguanodon. Thus the resemblance of the Ornithosaur drum-stick is almost as close to Dinosaurs as to Birds.

There is great similarity between Dinosaurs and Pterodactyles seen in the region of the instep, known as the metatarsus. These bones are usually four in number, parallel to each other, and similar in form. They are commonly longer than in Dinosaurs; but among some of the carnivorous Dinosaurs their length approximates to that seen in Pterodactyles. In neither group are the bones blended together by bony union, while they are always united in Birds, as in Oxen and similar even-hoofed mammals. Dinosaurs agree with Pterodactyles in maintaining the metatarsal bones separate, but they differ from them and agree with Birds frequently, in having the number of metatarsal bones reduced to three, as in Iguanodon, though Dinosaurs often have as many as five digits developed.

The toe bones, the phalanges of these digits of the hind limb, are usually longer in Pterodactyles than in Dinosaurs, but they resemble carnivorous Dinosaurs in the forms of their sharp terminal bones for the claws, which are similarly compressed from side to side.

So diverse are the functions of the fore limb in Dinosaurs and Pterodactyles, and so remarkably does the length of the metacarpal region of the back of the hand vary in the long-tailed and short-tailed Ornithosaurs, that there is necessarily a less close correspondence in that region of the skeleton between these two groups of animals; for the Pterodactyle fore limb is modified in relation to a function which can only be paralleled among Birds and Bats; and yet neither of those groups of animals approximates closely in this region of the skeleton to the Flying Reptile. Under all the modifications of structure which may be attributed to differences of function, some resemblance to Dinosaurs may be detected, which is best evident in the upper arm bone, humerus; is slight in the fore-arm bones, ulna and radius; and becomes lost towards the extremity of the limb.

If the tendency of the thigh bone to resemble a Mammalian type of femur ([p. 100]) is a fundamental, deep-seated character of the skeleton, it might be anticipated that a trace of Mammalian character would also be found in the humerus. For what the character is worth, the head of the humerus does show a closer approximation to a Monotreme Mammal than is seen in Birds, and is to some extent paralleled in those South African reptiles which approximate to Mammals most closely. Not the least remarkable of the many astonishing resemblances of these light aerial creatures to the more heavy bodied Dinosaurs is the circumstance that the humerus in both groups makes a not dissimilar approach to that of certain Mammals.