The Fossil Evidence Only one specimen, from the Triassic of Fergana, Khirgizia

1. Aerodynamics and the reconstruction of the Triassic gliding reptile Sharovipteryx Jeremy M. V. Rayner1 & Gareth J. Dyke2 1School of Biology, University of Leeds, Leeds UK 2Department of Ornithology, American Museum of Natural History, New York, USA • Sharovipteryx: • an enigmatic Triassic gliding reptile • relationships: is it the pterosaur sister group? Is it a Prolacertiform or a basal Archosaur (Sereno 1991, Benton 1999, Unwin et al. 2000, Peters 2000)? • the only gliding tetrapod with a dominant hind-limb wing • hind-limb membrane contains pterosaur-like ‘actinofibrillae’ • the forelimbs and the pectoral region are missing (or tiny), so there has been debate about its reconstruction • it was small - M = 4.5 g, SVL = 6.5 cm - about the same size as the recent squamate Draco volans; but the wing area was up to 35% larger • The Fossil Evidence • Only one specimen, from the Triassic of Fergana, Khirgizia • Described originally by Sharov (1971) (see also Cowen 1981) • Reconsidered by Gans et al. (1987) and redescribed by Peters (2001) • The wing membrane was formed by a uropatagium, spanning tail to feet, and an further portion anterior to the hind limb • Forelimbs are either missing, or preserved as tiny elements on counter slab (Peters 2000: right) We use simple aerodynamic models to ask not how Sharovipteryx flew, but to determine how realistic flight performance sets limits on its aerodynamic design. Right: counter slab, with main part of specimenFar right: specimen as drawn by Sharov (1971); scale bar 20 mm Left: comparison of Sharovipteryx flight area (red) with Draco and Ptychozoon (after Russell et al. 2001).Centre: Sharovipteryx uropatagium (main slab) compared with pterosaur wing (Padian & Rayner 1993).Right: composite of main and counter (red) slabs showing possible location of forelimbs (after Peters 2000). Reconstructions Several alternatives have been proposed, but not all are consistent with the fossil or with aerodynamics: Sharov’s own reconstruction, imagining the forelimbs, and interpolating gular mem-branes. How can this be stable? How can the animal control wing incidence since the membrane attaches to the tail, and the femur cannot rotate? (from Sharov 1971 and 1998) The reconstruction of Gans et al. (1987) with a forelimb canard, and an unusual shape for the hindwing leading edge. The model (below) glided well, but how heavy was the model? It is now even harder to control hind wing incidence. (left from Wellnhofer 1991; right from Caley 1999; bottom from Gans et al. 1987) Our reconstruction, with an aero-dynamically realistic shape, retaining the forelimb canard, and incorporating the membranes anterior to the hindlimb. This is shown with maximum extension, with femur extended. By protracting the femur and flexing the knee the wingshape can be controlled. How largeshould the forewing be? Aerodynamics We used simple aerodynamic models of single wing and bi-wing animals (left). For Sharovipteryx we modelled lift and drag by delta wing theory (Polhamus 1971), with the wing generating leading edge vortices (top right), as used, tested and confirmed for isolated bird tails of similar Re by Maybury et al. (2001) (bottom right). We used conventional lifting line theory to model Draco, for comparison. We also considered pitching moments about the centre of mass, although this is limited because we do not know the position of the centre of drag. For the lift model, we reduced ideal delta wing lift by a factor of 0.9, because natural wings do not have a sharp leading edge. We used a drag coefficient of 0.05, as measured on a starling tail at 5 m/s (L. B. Couldrick, pers. comm.) • ACKNOWLEDGEMENTS • Thanks to David Peters for making available his photographs and drawings of Sharovipteryx. • BIBLIOGRAPHY AND REFERENCES • Bennett, S. C. 2000. Pterosaur flight: the role of actinofibrils in wing function. Historical Biology14, 255-284. • Benton, M. J. 1999. Scleromochlus taylori and the origin of dinosaurs and pterosaurs. Philosophical Transactions of the Royal Society of London, B 354, 1423-1446. • Caley, K. C. 1999 Reconstructions: Sharovipteryx. http://ibis.nott.ac.uk/~plzkjc/sharov.html • Colbert, E. H. 1967 Adaptations for gliding in the lizard Draco. American Museum Novitates2283. • Colbert, E. H. 1970 The Triassic gliding reptile Icarosaurus. Bulletin of the American Museum of Natural History143, 85–142. • Cowen, R. 1981 Homonyms of Podopterix. J. Paleontol. 55, 483. • Gans, C., Darevski, I. & Tatarinov, L. P. 1987 Sharovipteryx, a reptilian glider? Paleobiology13, 415–426. • Padian, K. 1984. The origin of pterosaurs. In Proceedings of the Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers, ed. W. E. Reif & F. Westphal, pp. 163–168. Tübingen, Attempto Verlag. • Padian, K. & Rayner, J. M. V. 1993 The wings of pterosaurs. American Journal of Science 293A, 91–166. • Polhamus, E. C. 1971. Predictions of vortex-lift characteristics by a leading-edge suction analogy. Journal of Aircraft8, 193–199. • Maybury, W. J., Rayner, J. M. V. & Couldrick, L. B. 2001 Lift generation by the avian tail. Proceedings of the Royal Society of London B. • Russell, A. P., Dijkstra, L. D. & Powell, G. L. 2001 Structural characteristics of the patagium of Ptychozoon kuhli (Reptilia: Gekkonidae) in relation to parachuting locomotion. Journal of Morphology 247, 252–263. • Sereno, P. C. 1991. Basal Archosaurs: phylogenetic relationships and functional implications. Society of Vertebrate Paleontology Memoir 2, Journal of Vertebrate Paleontology11, 1-53. • Sharov, A. G. 1970 [Unusual reptile from the Lower Triassic of Fergana.] Paleontologicheskii Zhurnal, pp. 127–131. • Sharov, A. G. 1971 Novyiye lyetayushchiye reptili iz myezozoya Kazakhstana i Kirgizii. [New flying reptiles from the Mesozoic of Kazakhstan and Kirghizia.] Trudy paleontologicheskii InstitutaMoskva130, 104–113. • Sharov, A. 1998 Wings on Hind Legs. http://www.ento.vt.edu/~sharov/reptiles/reptiles.html • Tatarinov, L. P. 1989 The systematic position and way of life of the problematic upper Triassic reptile Sharovipteryx mirabilis. Paleontologicheskii Zhurnal, pp. 107–110. • Unwin, D. M. 2000 Sharovipteryx and its significance for the origin of the pterosaur flight apparatus. In Abstracts 5th European Workshop on Vertebrate Palaeontology, p. 83. • Unwin, D. M., Alifanov, V. R. & Benton, M. J. 2000 Enigmatic small reptiles from the Middle-Late Triassic of Kirgizstan. In The Age of Dinosaurs in Russia and Mongolia, ed. M. J. Benton, E. N. Kurochkin, M. A. Shishkin & D. M. Unwin, pp. 177–186. Cambridge University Press. • Wellnhofer, P. 1991 The Illustrated Encyclopaedia of Pterosaurs. London: Hamlyn. Results A: Sharov’s model as a singledelta wing reaching to the neck B: Our model as a single delta wing reaching to just behind the wings B: Our model as a delta wing reaching to just behind the wings with a forewing canard (of area = 10% maximum tail area) • Conclusions • As reconstructed by Sharov, the animal is ineffective: the glide angle is steep, and the animal lacks control and is unstable in pitch (Results A). • Gans’ reconstruction also lacks a control mechanism, and because of the low wing area flies fast. Could the forelimb have been big enough to support the wings required for this model? Was this hind wing design aerodynamically effective? • Sharovipteryx probably had a triangular or delta wing, extending from the pectoral girdle to the feet and tail. It could control flight speed by varying wing spread, and therefore did not need to be able to depress the hindwing or rotate the femur to adjust wing angle of incidence. • With the hindwing alone, the glide angle is rather steep, because the lift-drag ratio of a delta wing (= tan angle of incidence) can be unfavourable (Results A, B). But flexing the hindwing gives good penetration at higher speed without excessive glide angle. • The triangular single wing model is unstable in pitch as the animal cannot control the position of the centre of lift, which lies posterior to the femoral acetabulum and to the centre of mass. • With forelimb wings (or speculatively gular membranes) introduced to balance pitch the glide speed is dramatically reduced, but the animal can still obtain good penetration from hindlimb flexure (Results C). The predicted aerodynamic performance and flight envelope are better than those predicted for recent gliding tetrapods. Further questions • What selective pressures encouraged the evolution of this mechanism? • What factor in the life-history of Sharovipteryx favoured such fast but shallow glides? • Why is this design unique, given the richness of gliding diapsids both in the Permian and Triassic and the Recent? • If it is accepted as the pterosaur sister group, can Sharovipteryx tell us anything about the origin and evolution pterosaur flight, given that its tail morphology is so distinct and selection for flight performance need not encourage forelimb development? Results are shown as glide polars for the whole animal, plotting sinking speed against ground speed, for different apex angles. Regions beyond the left-hand end of the curve are unavailable because of stall. A calculated polar for Draco of same mass and same assumptions is shown for comparison. Sharovipteryx should spread its tail to glide at lower speeds. Neither of the single wing reconstructions (A, B) is stable in pitch, but Sharov’s model (possibly inconsistent with the fossil evidence) has slower, shallower glides with maximum tail spread owing to its larger area. The bi-winged glider (C) designed to be stable in pitch can obtain much shallower, and rather slower, glides with forewings only 10% of tail area: this is equivalent to a forewing length in the range 1.5-2.5 cm, which is consistent with the size of the minimal forelimb skeleton possibly identified by Peters (2000). If the animal used gular membranes as a canard (cf. Peters 2000) the area required would be smaller still since the lifting surface is more anterior.