On the size and flight diversity of giant pterosaurs, the use of birds as pterosaur analogues and comments on pterosaur flightlessness

Abstract

The size and flight mechanics of giant pterosaurs have received considerable research interest for the last century but are confused by conflicting interpretations of pterosaur biology and flight capabilities. Avian biomechanical parameters have often been applied to pterosaurs in such research but, due to considerable differences in avian and pterosaur anatomy, have lead to systematic errors interpreting pterosaur flight mechanics. Such assumptions have lead to assertions that giant pterosaurs were extremely lightweight to facilitate flight or, if more realistic masses are assumed, were flightless. Reappraisal of the proportions, scaling and morphology of giant pterosaur fossils suggests that bird and pterosaur wing structure, gross anatomy and launch kinematics are too different to be considered mechanically interchangeable. Conclusions assuming such interchangeability--including those indicating that giant pterosaurs were flightless--are found to be based on inaccurate and poorly supported assumptions of structural scaling and launch kinematics. Pterosaur bone strength and flap-gliding performance demonstrate that giant pterosaur anatomy was capable of generating sufficient lift and thrust for powered flight as well as resisting flight loading stresses. The retention of flight characteristics across giant pterosaur skeletons and their considerable robustness compared to similarly-massed terrestrial animals suggest that giant pterosaurs were not flightless. Moreover, the term 'giant pterosaur' includes at least two radically different forms with very distinct palaeoecological signatures and, accordingly, all but the most basic sweeping conclusions about giant pterosaur flight should be treated with caution. Reappraisal of giant pterosaur material also reveals that the size of the largest pterosaurs, previously suggested to have wingspans up to 13 m and masses up to 544 kg, have been overestimated. Scaling of fragmentary giant pterosaur remains have been misled by distorted fossils or used inappropriate scaling techniques, indicating that 10-11 m wingspans and masses of 200-250 kg are the most reliable upper estimates of known pterosaur size.

Figure 1. Azhdarchid humeri.

A, left Hatzegopteryx humerus in ventral view; B, distal view; C, right Quetzalcoatlus sp. humerus in proximal view. Note the distorted diaphysis of the Hatzegopteryx humerus compared to the undistorted profile of Quetzalcoatlus sp.. C, from Padian and Smith . Scale bar represents 100 mm.

Figure 2. Pterosaur and soaring bird wing ecomorphospace compared using principal component analyses from Norberg and Rayner and Rayner .

Blue shading, wing ecomorphospace of modern birds (from [64]); grey shading, modern bats (from [63]); orange shading; dynamically-soaring birds (tropic birds, petrels, albatrosses); purple shading, statically-soaring birds (condors, vultures, storks; cranes); purple dashed line, extent of pterosaur wing ecomorphology found in Witton ; blue dashed line, pterosaur wing ecomorphology of Brower and Venius ; green dashed line, broad-winged pterosaur wing ecomorphology of Hazlehurst and Rayner ; red dashed line, pterosaur wing ecomorphology of Chatterjee and Templin .

Figure 3. Dorsal views of giant and tiny pterosaur humeri.

A, Quetzalcoatlus northropi (10–11 m wingspan); B, Pteranodon (7 m wingspan); C, Pterodactylus (45 cm wingspan). Note that each bears a large deltopectoral crest (dp) and robust extremities. Scale bars represent 100 mm (A and B) and 10 mm (C). A and C, from Witton et al. ; B, modified from Bennett .

Figure 4. Albatross, azhdarchid and pteranodontian skeletons compared.

A, wandering albatross, Diomedea exulans; B, the azhdarchid Hatzegopteryx; C, the pteranodontian Pteranodon; D, functional wing region dimensions compared across a standard wing length. A, based on Paul ; B, based on Buffetaut et al. –, Kellner and Langston , Cai and Wei and Pereda Suberbiola ; C, based on Bennett ; D, functional regions taken from Prondvai and Hone . Images not to scale.

Figure 5. Lateral view of the forelimb musculature in Anhanguera santanae.

Note that the forelimb musculature is extensive, and that the major muscle base used for flight is more distributed than that of birds. Unlike avian taxa, pterosaurs derived substantial flapping power from several groups of muscles around the chest and back (rather than the two primary muscles in birds), as well as the antebrachium and manus. Illustration by Julia Molnar, used with permission.

Figure 6. Skeletal reconstruction of a quadrupedally launching Pteranodon.

Skeletal proportions based on Bennett ; kinematics from Habib .

Figure 7. Soaring animal planforms compared.

A, wandering albatross Diomedea exulans; B, the giant ornithocheiroid Pteranodon; the giant azhdarchid Quetzalcoatlus; D, shown to scale. See for details of pterosaur wing planform reconstruction.