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.

Figure 1.

Competing reconstructions of pterosaur wing outline and pteroid orientations: (a) anteriorly orientated pteroid (e.g. Unwin et al. 1996; Wilkinson et al. 2006); (b) medially orientated pteroid (e.g. Bennett 2007); (c) the membrane shape implied by an anteriorly orientated pteroid (Wilkinson 2008) and the pteroid-supported triangle of the propatagium.

Figure 2.

Pteroid morphology and lift distribution: (a) tracings of the well-preserved pteroid of Coloborhynchus (Staatliches Museum für Naturkunde, Karlsruhe (SMNK 1133PAL)) redrawn from Unwin et al. (1996, fig. 2). Top tracing shows the pteroid outline in the ventral view, the lower tracing is in the medial view. Dotted lines show extension of the pteroid to a likely pointed shape; (b) wing section (black outline, above) and pressure distribution (grey outline, below) used for analysis.

Figure 3.

Results: (a) load distribution of pteroid and location of tissue required to react to bending moment produced by aerodynamic lift forces in an anterior orientation, based on Anhanguera but scaled to a 5.8 m wingspan Coloborhynchus (after Wilkinson 2008); (b) predicted membrane strain as pteroid flexion increases.