During the course of evolution, the oxygen content of the atmosphere has increased and decreased many times. When there was double the amount found today, this might possibly have allowed for the growth of giant plants and reptiles during the Mesozoic Era and also have contributed to the evolution of aerial reptiles. Some of these were very large indeed compared with modern birds.
Three types of flight, which are by no means mutually exclusive, have evolved in the animal kingdom. These are gliding, soaring and flapping flight. For a long while it was assumed that the pterosaurs were merely gliding and soaring animals, and would have been incapable of flapping flight, since their sternum lacked a prominent keel like that of birds. Recent studies, however, have shown that this was not the case. The pterosaurs were almost as well adapted to flapping flight as modern birds are (Wellnhofer 1975,1991; Padian 1984; Padian and Rayner 1993; see also Benton 2004.)
Pterosaurs had hollow, lightweight bones, streamlined heads, and other aerodynamic adaptations. They may also have possessed a system of air-sacs like that of birds. Moreover, they were probably endothermal, and their bodies covered with insulating hair. Only endotherms have external insulation, and endothermy would have enabled the pterosaurs to maintain the high, sustained metabolic rate necessary for powered flight. Even in the 19th century, it was conjectured by T.H Huxley that the pterosaurs might have been warmblooded; and as long ago as 1870 H.G. Seeley suggested that they might have had a covering of hair, like that of bats. Sordes pilosus (Fig. 52), of which the first specimen was discovered in 1971 among the Late Jurassic deposits of Kazakhstan, seems to have been covered with dense insulating hair except on the tail and legs (de Ricqles 1975).The exact nature of this hair has yet to be de-
termined although, like feathers, it probably evolved from reptilian body scales. Microscope examination of the wing membranes of several species of pterosaurs shows that they were reinforced with stiff fibres, especially distally. These probably prevented the outer parts of the wings from bending. Much of the pelt of Sordes has been shown to consist of similar fibres, although true hairs have been detached from areas that were not concerned with flight (Padian and Rayner 1993; Unwin and Bakhurina 1994). Probably all the pterosaurs had hairy coats (see Naish and Martill 2003).
The power stroke in pterosaur flight was directed down and forward, while the recovery stroke was upwards and backwards. When viewed from the side, the path of the wing tip would have described a figure-of-eight. The power stroke was generated by massive pectoral muscles, the upstroke by muscles which ran from the sternum over pulley arrangements at the shoulder joints to the dorsal sides of the humerus bones. Although situated below the wings, these muscles actually pulled them up. A similar system is found in birds.
Pterosaurs had relatively large wings and, in consequence, flew rather slowly. Nevertheless, they were highly manoeuverable: their wings were comparable with those of soaring birds such as vultures and albatrosses, or with aerial predators such as falcons and gulls. They probably took off from trees and cliffs, or jumped into the air after a short run into the wind - as storks and vultures do. Even the larger pterosaurs, such as Pteranodon spp. (Fig. 53), could probably become airborne at very low air speeds. Landing must have been
awkward for larger species, as it is for large birds, so that the reinforced pelvis and sacrum would, from time to time, have had to withstand quite powerful blows on impact (Benton 2004).
Unwin and Bakhurina (1994) emphasised the bird-like construction of the narrow, stiff wings of Sordes which were either free from the legs or else - and most people agree with this - the flight membranes were attached to the thighs, and the legs were intimately involved in the flight apparatus. Unique among flying vertebrates, Sordes and other pterosaurs had a structurally non-homogeneous flight surface,with a stiffened outer half and a softer, more extensible, inner region. In Pterodactylus and probably other pterodactyloids, a small 'uropatagium' stretched between the inner sides of the legs and the body. The femora were held almost perpendicular to the spinal column - resulting from their association with the uropatagium - and, in consequence, the flight membrane would have been highly manipulatable and the wing loading low. However, the uropatagium must have seriously impeded movement on the ground (Sect. 6.4). At least some fossils reveal the presence of webbing between the three free fingers of the hand supporting a 'propatagium' even larger than is often depicted. Webbing between the toes implies use of the feet as paddles or air-brakes, as reviewed by Naish and Martill (2003).
The suggestion has been made that the cranial crests of Pteranodon ingens (Fig. 63g) and P. sternbergi (Fig. 63h) might have compensated for air pressure on the beak when the animals turned their heads sideways to the wind. However, not all species of Pteranodon had large cranial crests, so clearly these were not essential. Still, they may have contributed to the success of those species that did possess them. Langston (1981) and others have suggested that the function of cranial crests might have been to compensate for the lack of a tail in pterodac-tyloids. But again, by no means all species had them. Some years earlier, Bram-well and Whitfield (1974) carried out an experimental investigation of the bio-mechanics of Pteranodon (Fig. 53). By means of wind-tunnel experiments on model heads, they showed that the large and long cranial crest was primarily a weight-saving device. By balancing the aerodynamic load on the beak, it allowed the neck muscles to be reduced, thereby saving much more than its own weight. Previously, as already mentioned, the function of the sagittal crest had been thought to counteract the twisting effect of wind pressure on the large, toothless beak. Perhaps both functions were served simultaneously. On the other hand, the crests may have had nothing to do with aeronautics. They could have served in sexual display or for thermoregulation, as with the plates on the backs of stegosaurs (Sect. 7.5.2; Alexander 1989). Even the basal groups of pterosaurs included crested species. Crests were extremely common among the later forms, and were often composed mainly of soft tissues. In Tapejara there was a posteriorly directed spike at the back of the skull and a long rod growing up from the tip of the beak. Soft tissue spanned the space between the two spikes, producing a crest larger than the rest of the skull. Nyctosaurus had a bizarre rod-like crest even larger than the skull and body combined. Moreover, the rod bifurcated near its base to product a pseudo-antler! (Bennett 2003).
With a sinking speed of 0.42 m s-1, and a flying speed of 8 m s-1, Pteranodon would have been able to soar on weak thermals or by hill lift at very light wind speeds. Out to sea, it would have used weak thermals generated by convection; dynamic soaring or slope-soaring over the waves would not have been possible for such a low-speed glider. Its low stalling speed would, however, have enabled it normally to land very gently. The Cretaceous climate was more uniform than that in present times, and light winds prevailed. Although primarily a glider, Pteranodon was nevertheless capable of level flight. It could have taken off either by dropping from the cliff on which it roosted or, in emergencies, by facing the wind and just spreading its wings while on a small elevation or on the crest of a wave.
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