Locomotion on the Ground

Palaeontologists generally concur that pterosaurs were capable of active flight even though the leverage of their pectoral muscles,in the absence of a keel,was less than it is in birds. But there is somewhat less agreement as to how they managed to walk on land. Indeed, the problem of how they moved on the ground, and took off into the air, dates almost from the discovery of the first fossil remains of the animals. The argument has often been polarised with two contrary views: bipedal versus quadrupedal.

Bipedal Locomotion

Padian (1984), Bennett (1990), and Padian and Rayner (1993) argued that the pterosaurs could have walked well on fully erect hind legs. According to these authors, the pelvic girdle was fixed firmly beneath the body, and the limb movements must have been like those of small bipedal dinosaurs. When running, the wings were held horizontally beside the body (Fig. 54). This view had been proposed as long ago as 1922 by Carl Stieler, with respect to the long-tailed pterosaur Dorygnathus from the Liassic of Germany. Stieler concluded that Dorygnathus was capable of reaching the speed necessary for take off by running on its toes with short steps and its legs held wide apart. The central issue in the discussion of position has been the anatomy of the pelvis - whether the right and left halves were fused, as they were in dinosaurs or whether they became disjointed vertically. If they were fused, the hip sockets faced outward and lightly downward, then the posture could have been erect (Padian 1984). In Pteranodon (Fig. 53), the two halves were indisputably joined, and possibly in Anhanguera (Fig. 54) also.

McGowan (1991) considered that too much has been made of pelvic fusion. 'The equation that has been used is: no fusion = hip socket faces outward and upward = laterally splayed hind legs = no bipedal locomotion'. Yet birds lack a fused pelvis and may have hip sockets that face outward and upward 'though

■ Fig. 54. Left Rhamphorhynchus (Rhamphorhynchoidea; Upper Jurassic; wingspan ca. 1 m). Right Anhanguera (Pterodactyloidea; Lower Cretaceous; wingspan ca. 4 m). In both cases the reconstructions show possible quadrupedal locomotion above and bipedal locomotion below

admittedly, the head of the femur is somewhat different from that of pterosaurs'. He added that the functional reason why birds lack a vertically fused pelvis has to do with the evolution of balance, and there do not appear to be very good grounds for ruling out the possibility that pterosaurs may have been bipedal. Nevertheless, all the most recent evidence points to quadrupedal locomotion (Sect. 6.4.2).

Quadrupedal Locomotion

Among many others, Wellnhofer (1991) argued that the hind limbs of the pterosaurs pointed sideways in an awkward, sprawling posture. These creatures would, therefore, have scrambled about on all fours (Figs. 54,55), using their feet and the claws of their hands with the wing tips sticking upwards on either side of the head. This gait has been confirmed independently from several examples of fossilised pterosaur tracks. Some of these are open to question, but most are unequivocal. In one case, an apparent pterosaur track was later proved to have been made by a crocodilian, in another by a horseshoe crab (Xiphosura). Not only do some fossils from Brazil prove that a semi-sprawling posture was adopted, but this has been confirmed by biochemical modelling. Like Pterodactylus (Fig. 55) and Pteranodon (Fig. 53; Sect. 6.6), the

■ Fig. 55. Pterodactylus (Pterodactyloidea; Upper Jurassic; wingspan ca. 75 cm). Reconstructions showing: left quadrupedal locomotion. Centre Roosting beneath a branch. Right Climbing upside down among the branches of a tree. (After Wellnhofer 1991 from Abel 1925)

majority of species probably roosted hanging from the edges of cliffs, or clinging beneath the branches of trees.

Pterosaurs must have spent most of their lives airborne and were presumably rather helpless on the ground. They could probably have taken off from the ground, however, by standing on their hind legs, facing the wind, and stretching out their wings. Some earlier genera such as Dimorphodon (Fig. 57) as well as Pterodactylus (Fig. 55), which had relatively long hind legs and were small and light in weight, may well have raised themselves into the air by a jump and a simultaneous stroke of the wings (Wellnhofer 1991). The great pterodactyls of the Cretaceous, however, would not have been able to do this. They probably took off from a hanging position on the edges of cliffs and crags, as Bramwell and Whitfield (1974) postulated in the case of Pteranodon (Sect. 6.3). Whether they could also have hung downwards from branches and rocky protrusions, as bats do, seems improbable. Certainly the Rhamphorhyn-choidea, with their long stiff tails, would have found it very difficult to do so! Many questions about the behaviour of pterosaurs remain to be answered, but it does seem that parallel evolution has occurred between pterosaurs, birds, and bats. The problem is to determine in what ways the pterosaurs most resembled birds, and in what ways bats.

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