The Saurischian Dinosauria have an antorbital vacuity in the side of the skull between the nasal opening and the eye, as in the long-tailed Ornithosaurs named Pterodermata. In some of the older genera of these carnivorous Dinosaurs of the Trias, the lateral vacuities of the head are as large as in Dimorphodon. But in some at least of the Iguanodont, or Ornithischian Dinosaurs, there is no antorbital vacuity, and the side of the face in that respect resembles the short-tailed Pterodactylia. The skull of a carnivorous Dinosaur possesses teeth which, though easily distinguished from those of Pterodactyles, can be best compared with them. The most striking difference is in the fact that in the Dinosaur the nostrils are nearly terminal, while in the Pterodactyle they are removed some distance backward. This result is brought about by growth taking place, in the one case at the front margin of the maxillary bone so as to carry the nostril forward, and in the other case at the back margin of the premaxillary bone. Thus an elongated part of the jaw is extended in front of the nostril. Hence there is a different proportion between the premaxillary and maxillary bones in the two groups of animals, which corresponds to the presence of a beak in a bird, and its absence in living reptiles. It is not known whether the extremity of the Pterodactyle's beak is a single bone, the intermaxillary bone, such as forms the corresponding toothless part of the jaw in the South African reptile Dicynodon, or whether it is made by the pair of bones called premaxillaries which form the extremity of the jaw in most Dinosaurs. Too much importance may perhaps be attached to such differences which are partly hypothetical, because the extinct Ichthyosaurus, which has an exceptionally long snout, has the two premaxillary bones elongated so as to extend backward to the nostrils. A similar elongation of those bones is seen in Porpoises, which also have a long snout; and the bones are carried back from the front of the head to the nostrils, which are sometimes known as blowholes. But the Porpoise has those premaxillary bones not so much in advance of the bones which carry teeth named maxillary, as placed in the interspace between them. The nostrils, however, are not limited to the extremity of the head in all Dinosaurs. If this region of the beak in Dimorphodon be compared with the corresponding part of a Dinosaur from the Permian rocks, or Trias, the relation of the nostril to the bones forming the beak may be better understood.
FIG. 78. COMPARISON OF THE SKULL OF THE DINOSAUR ORNITHOSUCHUS WITH THE ORNITHOSAUR DIMORPHODON
In the sandstone of Elgin, usually named Trias, a small Dinosaur is found, which has been named Ornithosuchus, from the resemblance of its head to that of a Bird. Seen from above, the head has a remarkable resemblance to the condition in Rhamphorhynchus, in the sharp-pointed beak and positions of the orbits and other openings. In side view the orbits have the triangular form seen in Dimorphodon, and the preorbital vacuities are large, as in that genus, while the lateral nostrils, which are smaller, are further forward in the Dinosaur. The differences from Dimorphodon are in the articulation for the jaw being carried a little backward, instead of being vertical as in the Pterodactyle, and the bone in front of the nose is smaller. Notwithstanding probable differences in the palate, the approximation, which extends to the Crocodile-like vacuity in the lower jaw, is such that by slight modification in the skull the differences would be substantially obliterated by which the skull of such an Ornithosaur is technically distinguished from such a Dinosaur.
The back of the skull is clearly seen in the Whitby Pterodactyle, and its structure is similar to the corresponding part of such Dinosaurs as Anchisaurus or Atlantosaurus, without the resemblance quite amounting to identity, but still far closer than is the resemblance between the same region in the heads of Crocodiles, Lizards, Serpents, Chelonians. Few of these fossil Dinosaur skulls are available for comparison, and those differ among themselves. The coincidences rather suggest a close collateral relation than prove the elaboration of one type from the other. They may have had a common ancestor.
The Trias rocks near Stuttgart have yielded Dinosaurs as unlike Pterodactyles as could be imagined, resembling heavily armoured Crocodiles, in such types as the genus Belodon. Its jaws are compressed from side to side, as in many Pterodactyles, and the nostrils are at least as far backward as in Rhamphorhynchus. Belodon has preorbital vacuities and postorbital vacuities, but the orbit of the eye is never large, as in Pterodactyles. It might not be worth while dwelling on such points in the skull if it were not that the pelvis in Belodon is a basin formed by the blending of the expanded plates of the ischium and the pubis, into a sheet of bone which more nearly resembles the same region in Pterodactyles than does the ischio-pubic region in other Dinosaurian animals like Cetiosaurus.
The backbone in a few Dinosaurs is suggestive of Pterodactyles. In such genera as have been named Cœlurus and Calamospondylus, in which the skeleton is only partially known, the neck vertebræ become elongated, so as to compare with the long-necked Pterodactyles. The cervical rib is often very similar to that type, and blended with the vertebra, as in Pterodactyles and Birds. The early dorsal vertebræ of Pterodactyles might almost be mistaken for those of Dinosaurs. The tail vertebræ of a Pterodactyle are usually longer than in long-tailed Dinosauria.
In the limbs and the bony girdles which support them there is more resemblance between Pterodactyles and Dinosaurs than might have been anticipated, considering their manifest differences in habit. Thus all Dinosaurs have the hip bone named ilium prolonged in front of the articulation for the femur as well as behind it, almost exactly as in Pterodactyles and Birds (see [p. 95]). There is some difference in the pubis and ischium which is more conspicuous in form than in direction of the bones. There is a Pterodactyle imperfectly preserved, named Pterodactylus dubius, in which the ischium is directed backward and the pubis downward, and the bones unite below the acetabular cavity for the head of the femur to work in, but do not appear to be otherwise connected. In Rhamphorhynchus the connexion between these two thickened bars is made by a thin plate of bone. In such a Dinosaur as the American carnivorous Ceratosaurus the two bars of the pubis and ischium remain separate and diverging, and there is no film of bone extending over the interspace between them. The development of such a bony condition would make a close approximation between the Ornithosaurian pelvis and that of those Dinosaurs which closely resemble Pterodactyles in skull and teeth.
