During much of the Mesozoic Era, the air was dominated by prehistoric reptiles of the order Pterosauria. These archosaurs (Table 2) appeared during the Upper Triassic period, about 228 mya. This was some 70 million years before the first known birds (Archaeopteryx spp.). The pterosaurs flourished until the end of the Cretaceous, 155 million years later. Two suborders of pterosaurs are recognised: Rhamphorhynchoidea which had long, stiffened tails, and Ptero-dactyloidea with short tails. The rhamphorhynchoids evolved first but died out during the Upper Jurassic, somewhat after the pterodactyloids came onto the scene. Fossil pterosaurs have been found throughout most of the world, mainly concentrated in marine deposits.

Peter Wellnhofer (1991) and Norman and Wellnhofer (2000) have not only outlined the probable lines of descent of the pterosaurs, but have also provided illustrations indicating the way in which they may have evolved their wings and powers of flight. Benton (1985) regarded them as archosaurs and a close sister group to the Dinosauria, as had Padian (1984) and many others. According to Wild (1984), the small size, enlarged hooked claws and elongated fourth finger could have been acquired only during a scansorial stage in their ancestry. In his hypothetical prepterosaur, the body surface tended to become enlarged from a lateral band of skin that extended from the front limb and fourth finger to the flanks of the body and the upper parts of the legs. The formation of a flight membrane was beneficial as it checked the descent of these reptiles if they fell from the trees they were climbing. The ability to glide improved as the fourth digit lengthened, the fifth disappeared and the forequarters and pectoral girdle were strengthened. The sternum, too, became ossified and afforded an area for the attachment of powerful flight muscles. This hypothetical prepterosaur has not been found - it probably never will be, because transitional 'missing links' naturally occurred over relatively brief periods of geological history and were quickly replaced by their better adapted descendants.

The earliest pterosaurs, such as Eudimorphodon (Fig. 45) from Europe, showed all the unique features of the taxon: viz. a short body, reduced and fused hip bones, five long toes - of which the fifth was divergent - and a long neck under a large but lightweight head with pointed jaws. The reduction in weight was achieved by the two pairs of temporal openings in the diapsid skull

■ Fig. 45. Eudimorphodon (Rhamphorhynchoidea; Upper Triassic; wingspan ca. 1 m)

(Fig. 2). The membranous wing consisted of flaps of skin attached to the enormously elongated fourth fingers of each hand; the first three fingers were short, with strong claws for grasping rocks and vegetation. The fourth finger was connected to the wrist by an elongated metacarpal bone, and the wing membranes were attached to the body on each side of the thighs. A second membrane ran from the bones of each wrist to the animal's neck (Palmer 1999;

■ Fig. 46. Quetzalcoatlus (Pterodactyloidea; Upper Cretaceous; wingspan ca. 12 m)

Benton 2004). Eudimorphodon is well known from fossilised remains preserved in Late Triassic rocks of northern Italy. It was evidently an active flier and flapped its wings as a bird does. Although the head was long compared with that of modern birds, the sternum had evolved into a broad, flattened plate to which the powerful wing muscles were attached. Many of the early pterosaurs of both suborders were fairly small - about the same size as pigeons - but some of the Upper Cretaceous pterodactyloids were enormous. Quet-zalcoatlus (Fig. 46) from the Late Cretaceous of North America - appropriately in Texas - had an extended wingspan of about 1.2 m. This is comparable with that of an 'executive jet'; and Quetzalcoatlus was the largest flying animal ever known (Lawson 1975). Before discussing the pterosaurs, however, brief mention should be made of other aerial Mesozoic diapsid reptiles.

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