CHAPTER XII
EVIDENCES OF THE ANIMAL'S HABITS FROM ITS REMAINS
Such are the more remarkable characters of the bones in a type of animal life which was more anomalous than any other which peopled the earth in the Secondary Epoch of geological time. Its skeleton in different parts resembles Reptiles, Birds, and Mammals; with modifications and combinations so singular that they might have been deemed impossible if Nature's power of varying the skeleton could be limited. Since Ornithosaurs were provided with wings, we may believe the animals to some extent to have resembled birds in habit. Their modes of progression were more varied, for the structures indicate an equal capacity for movement on land as a biped, or as a quadruped, with movement in the air. There is little evidence to support the idea that they were usually aquatic animals. The majority of birds which frequent the water have their bodies stored with fat and the bones of their extremities filled with marrow. And a bird's marrow bones are stouter and stronger than those which are filled with air. There are few, if any, bones of Pterodactyles so thick as to suggest the conclusion that they contained marrow, and the bones of the extremities appear to have been constructed on the lightest type found among terrestrial birds. Their thinness, except in a few specimens from the Wealden rocks, is marvellous; and all the later Pterodactyles show the arrangement, as in birds, by which air from the lungs is conveyed to the principal bones. No Pterodactyle has shown any trace of the web-footed condition seen in birds which swim on the water, unless the diverging bones of the hind foot in Rhamphorhynchus supports that inference. The bones of the hind foot are relatively small, and if it were not that a bird stands easily upon one foot, might be considered scarcely adequate to support the animal in the position which terrestrial birds usually occupy. Yet, as compared with the length and breadth of the foot in an Ostrich, the toes of an Ornithosaur are seen to be ample for support. These facts appear to discourage the idea that the animals were equally at home on land and water, and in air.
Some light may be thrown upon the animal's habits by the geological circumstances under which the remains are found. The Pterodactyle named Dimorphodon, from the Lias of the south of England, is associated with evidences of terrestrial land animals, the best known of which is Scelidosaurus, an armoured Dinosaur adapted by its limbs for progression on land. And the Pterodactyle Campylognathus, from the Lias of Whitby, is associated with trunks of coniferous trees and remains of Insects. So that the occurrence of Pterodactyles in a marine stratum is not inconsistent with their having been transported by streams from off the old land surface of the Lias, on which coniferous trees grew and Dinosaurs lived.
Similar considerations apply to the occurrence of the Rhamphocephalus in the Stonesfield Slate of England. The deposit is not only formed in shallow water, but contains terrestrial Insects, a variety of land plants, and many Reptiles and other animals which lived upon land. The specimens from the Purbeck beds, again, are in strata which yield a multitude of the spoils of a nearly adjacent land surface; while the numerous remains found in the marine Solenhofen Slate in Germany are similarly associated with abundant evidences of varied types of terrestrial life. The evidence grows in force from its cumulative character. The Wealden beds, which yield many terrestrial reptiles and so much evidence of terrestrial vegetation, and shallow-water conditions of disposition, have afforded important Pterodactyle remains from the Isle of Wight and Sussex.
The chief English deposit in which these fossils are found, the Upper Greensand, has afforded thousands of bones, battered and broken on a shore, where they have lain in little associated groups of remains, often becoming overgrown with small marine shells. Side by side with them are found bones of true terrestrial Lizards and Crocodiles of the type of the Gavial of the Indian rivers, many terrestrial Dinosaurs, and other evidences of land life, including fossil resins, such as are met with in the form of amber or copal at the present day.
The great bones of Pterodactyles found in the Chalk of Kent, near Rochester, became entombed, beyond question, far from a land surface. There is nothing to show whether the animals died on land and were drifted out to sea like the timber which is found water-logged and sunken after being drilled by the ship-worm (Teredo) of that epoch. Seeing the power of flight which the animal possessed, storms may have struck down travellers from time to time, when far from land.
Evidence of habit of another kind may be found in their teeth. They are brightly enamelled, sharp, formidable; and are frequently long, overlapping the sides of the jaws. They are organs which are often better adapted for grasping than for tearing, as may be seen in the inclined teeth of Rhamphocephalus of the Stonesfield Slate; and better adapted for killing than tearing, from their piercing forms and cutting edges, in genera like Ornithocheirus of the Greensand. The manner in which the teeth were implanted and carried is better paralleled by the fish-eating crocodile of Indian rivers than by the flesh-eating crocodiles, or Muggers, which live indifferently in rivers and the sea. As the Kingfisher finds its food (see [Fig. 20]) from the surface of the water without being in the common sense of the term a water bird, so some Pterodactyles may have fed on fish, for which their teeth are well adapted, both in the stream and by the shore.
A Pterodactyle's teeth vary a good deal in appearance. The few large teeth in the front of the jaw in Dimorphodon, associated with the many small vertical teeth placed further backward, suggest that the taking of food may have been a process requiring leisure, since the hinder teeth adapted to mincing the animal's meat are extremely small. The way in which the teeth are shaped and arranged differs with the genera. In Pterodactylus they are short and broad and few, placed for the most part towards the front of the jaws. Their lancet-shaped form indicates a shear-like action adapted to dividing flesh. In the associated genus Rhamphorhynchus the teeth are absent from the extremity of the jaw, are slender, pointed, spaced far apart, and extend far backward. When the jaws of the Rhamphorhynchus are brought together there is always a gap between them in front, which has led to belief that the teeth were replaced by some kind of horny armature which has perished. In the long-nosed English type of Ornithocheirus the jaws are compressed together, so that the teeth of the opposite sides are parallel to each other, with the margins well filled with teeth, which are never in close contact, though occasionally closer and larger in front, in some of the forms with thick truncated snouts.
It is not the least interesting circumstance of the dentition of Pterodactyles that, associated in the same deposits with these most recent genera with teeth powerfully developed, there is a genus named Ornithostoma from the resemblance of its mouth to that of a bird in being entirely devoid of teeth. It is scarcely possible to distinguish the remains of the toothed and toothless skeletons except in the dentary character of the jaws. There is no evidence that the toothless types ever possessed a tooth of any sort. They were first found in fragments in England in the Cambridge Greensand, but were afterwards met with in great abundance in the Chalk of Kansas, where the same animals were named Pteranodon. A jaw so entirely bird-like suggests that the digestive organs of Pterodactyles may in such toothless forms at least have been characterised by a gizzard, which is so distinctive of Birds. The absence of teeth in the Great Ant-eater and some other allied Mammals has transferred the function which teeth usually perform to the stomach, one part of which becomes greatly thickened and muscular, adapting itself to the work which it has to perform. It is probable that the gizzard may be developed in relation to the necessities which food creates, since even Trout, feeding on the shell-fish in some Irish lochs, acquire such a thickened muscular stomach, and a like modification is recorded in other fishes as produced by food.