Biomechanics of the unique pterosaur pteroid

Abstract

Pterosaurs, flying reptiles from the Mesozoic, had wing membranes that were supported by their arm bones and a super-elongate fourth finger. Associated with the wing, pterosaurs also possessed a unique wrist bone-the pteroid-that functioned to support the forward part of the membrane in front of the leading edge, the propatagium. Pteroid shape varies across pterosaurs and reconstructions of its orientation vary (projecting anteriorly to the wing leading edge or medially, lying alongside it) and imply differences in the way that pterosaurs controlled their wings. Here we show, using biomechanical analysis and considerations of aerodynamic efficiency of a representative ornithocheirid pterosaur, that an anteriorly orientated pteroid is highly unlikely. Unless these pterosaurs only flew steadily and had very low body masses, their pteroids would have been likely to break if orientated anteriorly; the degree of movement required for a forward orientation would have introduced extreme membrane strains and required impractical tensioning in the propatagium membrane. This result can be generalized for other pterodactyloid pterosaurs because the resultant geometry of an anteriorly orientated pteroid would have reduced the aerodynamic performance of all wings and required the same impractical properties in the propatagium membrane. We demonstrate quantitatively that the more traditional reconstruction of a medially orientated pteroid was much more stable both structurally and aerodynamically, reflecting likely life position.

Flight in slow motion: aerodynamics of the pterosaur wing

“…Generalized shape of the wing of a large ornithocheirid pterosaur. Redrawn from recent reconstructions [16,17,46], and showing the locations of the wing sections that were tested and the extent of the wing membrane assumed (cross-hatched area). Figure 2.…”

Neck biomechanics indicate that giant Transylvanian azhdarchid pterosaurs were short-necked arch predators

“…Calculating bone strength requires some assumptions about the Young’s modulus of pterosaur bone. We follow Palmer & Dyke (2010) in using 22 GPa—a value agreeing with several avian long bones—which seems a reasonable proxy for pterosaur bones. Following Currey (2004) and Palmer & Dyke (2010), we used the relationship between Young’s modulus and yield stress in tension of 162 MPa.…”

“…We follow Palmer & Dyke (2010) in using 22 GPa—a value agreeing with several avian long bones—which seems a reasonable proxy for pterosaur bones. Following Currey (2004) and Palmer & Dyke (2010), we used the relationship between Young’s modulus and yield stress in tension of 162 MPa. We modelled a range of values reflecting different upper limits for giant pterosaur body mass (180–250 kg) for W to demonstrate the sensitivity of our results and calculate Relative Failure Force (RFF; Witton & Habib, 2010) for each model.…”

Constraints on the wing morphology of pterosaurs

“…While actual wing bone loading cannot be estimated, the possible maximum bone curvature can be calculated on the basis of acceptable strain ( §2). Using a yield strain (e yield ) of 0.0075 [11] and a safety factor of 3 [32], calculated average deflections are 88 for the first and second wing phalanges (WP1 and WP2), 108 for WP3 and 128 for WP4. It is also apparent from many fossil specimens [1,4,33] that WP4 frequently had natural curvature, typically around 158.…”