The Cretaceous Pterodactyles form a distinct family. So that, believing the tail to have been short in that group ([Fig. 58]), there are two long-tailed as well as two short-tailed families, which were defined from their typical genera Pterodactylus, Ornithocheirus, Rhamphorhynchus, and Dimorphodon.
The differences in structure which these animals present are, first: the big-headed forms from the Lias like Dimorphodon, agree with the Rhamphorhynchus type from Solenhofen in having a vacuity in the skull defined by bone, placed between the orbit of the eye and the nostril. With those characters are correlated the comparatively short bones which correspond to the back of the hand termed metacarpals, and the tail is long, and stiffened down its length with ossified tendons. These characters separate Ornithosaurs with long tails from those with short tails.
The short-tailed types represented by Pterodactylus and Ornithocheirus have no distinct antorbital vacuity in the skull defined by bone. The metacarpal bones of the middle hand are exceptionally elongated, and the tail, which was flexible in both, appears to have been short. These differences in the skeleton warrant a primary division of flying reptiles into two principal groups.
The short-tailed group, which was recognised by De Blainville as intermediate between Birds and Reptiles, may take the name Pterodactylia, which he suggested as a convenient, distinctive name. It may probably be inconvenient to enlarge its significance to comprise not only the true Pterodactyles originally defined as Pterosauria, but the newer Ornithostoma and Ornithocheirus which have been grouped as Ornithocheiroidea.
The second order, in which the wing membrane appears to have had a much greater extent, in being carried down the hind limbs, where the outermost digit and metatarsal are modified for its support, has been named Pterodermata, to include the types which are arranged around Rhamphorhynchus and Dimorphodon.
Both these principal groups admit of subdivision by many characters in the skeleton, the most remarkable of which is afforded by the pair of bones carried in front of the pubes, and termed prepubic bones. In the Pterodactyle family the bones in front of the pubes are always separate from each other, always directed forward, and have a peculiar fan-shaped form with concave sides like the bone which holds a similar position in a Crocodile. In the Ornithocheirus family the prepubic bones appear to have been originally triangular, but were afterwards united so as to form a strong continuous bar which extends transversely across the abdomen in advance of the pubic bones. This at least is the distinctive character in the genus Ornithostoma according to Professor Williston, which in many ways closely resembles Ornithocheirus.
The two families in the long-tailed order named Pterodermata are separated from each other by a similar difference in their prepubic bones. In Dimorphodon those bones are separate from each other, and remain distinct through life, meeting in the middle line of the body in a wide plate. On the other hand, in Rhamphorhynchus the prepubic bones, which are at first triangular and always slender, become blended together into a slight transverse bar, which only differs from that attributed to Ornithostoma in its more slender bow-shaped form.
Thus if other characters of the skeleton are ignored and a classification based upon the structure of the pelvis and prepubic bones, there would be some ground for associating the long-tailed Rhamphorhynchus from the Upper Oolites which is losing the teeth in the front of its jaw with the Cretaceous Ornithostoma, which has the teeth completely wanting; while the long-tailed Dimorphodon would come into closer association with the short-tailed Pterodactylus. The drum-stick bone or tibia in Dimorphodon, with its slender fibula, like that of a Bird, also resembles a Bird in the rounded and pulley-shaped terminal end which makes the joint corresponding to the middle of the ankle bones in man. The same condition of a terminal pulley joint is found in the Cretaceous Pterodactyles. But in the true Pterodactyles and in Rhamphorhynchus there usually is no pulley-shaped termination to the lower end of the drum-stick, for the tarsal bones remain separate from each other, and form two rows of ossifications, showing the same differences as separate Dinosaurs into the divisions which have been referred to, from their Bird-like pelvis and tibio-tarsus, as Ornithischia in the one case, and Saurischia in the other from their bones being more like those of living Lizards.
FIG. 76. LEFT SIDE OF PELVIS OF ORNITHOSTOMA
(After Williston)