Take a look at almost any illustration of a pterosaur, be it in a research piece or a life reconstruction and the wing finger is generally depicted as being some kind of straight spar. Each of the four wing finger bones is a dead straight element and the leading edge is therefore basically just a line drawn with a ruler). However, take a look at the actual specimens of pterosaurs and it’s actually quite clear that for lots of them, the last (distal) element is often curved, if only a little, but sometimes quite a lot.
This is really obvious in something like Pteranodon for example (and indeed it’s been noted before that this genus has curved distal phalanges) and yet illustrations of this animal, even in the technical literature, will give it a straight distal phalanx. I’d noted for a while that actually there were quite a few pterosaurs with curved phalanges in particular having looked at Bellubrunnus and its bizarre forward swept wingtips. I’d realised that even the posterior curve might actually have some major flight implications – the shape and position of the very distal part of the wing can have a big impact on vortex shedding and other issues even in static glides and anything like a twist or elevation to the tip can make a huge difference to how it performs.
Knowing this would be an issue and working out what it would be and why are two very different areas and I know enough mechanics for the first and not enough to even begin to think about the second. Enter, somewhat inevitably, Mike Habib and he started looking at this issue and working towards what such a curve would mean both in Bellubrunnus but also those pterosaurs with posterior curves on the distal phalanges. We still needed a good dataset and some actual numbers though and so while I trawled the literature and my photographic archives for examples, any I found I passed onto Matt Van Rooijen who had volunteered to produce both the figures for the paper but also do the detailed digital measuring of the curvature of the phalanges.
The resultant paper is rather light on in depth analysis and numbers because there are potentially some severe issues of taphonomy that can distort the apparent curvature of these bones (in particular reducing a curved bone to look straight) but given the strong consistency of at least some results, there do appear to be some major and genuine signals in the data. There’s some fair consistency within and between clades therefore (and to a degree within and between species of a single genus) so despite the taphonomic issue, it’s perhaps not too bad (though still very hard to estimate or account for).
A number of specimens of multiple genera show that scaphognathines and tapejarids have relatively strong curvature to the distal phalanges and so to do various pteranodontids. In other words, two groups often considered to be highly terrestrial, and another than is highly pelagic both seem to go more for this curvature and others show lesser or no curvature. This might seem rather odd with the two extremes of flying environment / style coming together in morphology but it actually makes a fair bit of sense.
Curvature in the pteranodontids would potentially correspond to an expanded wingtip which aligns with existing hypotheses of the forward swept wing position of these animals in flight. A curved wingtip can also increase the chord of the wing which would be good for terrestrial-based fliers, and also might help protect the wingtip from damage from impact which could be important for animals flying in cluttered environments.
An additional issue comes in here of compliance, a compliant phalanx could potentially also help reduce injuries from impact with things like twigs or even the ground when taking off. Bat phalanges are highly compliant (i.e. bendy) under loads but eyeballing bat fossils at least, there’s no obvious difference between the bones of the phalanges and other elements of the skeleton that are less compliant, so perhaps at least some pterosaur phalanges were highly compliant. In that case under loading in flight they could be considerably more curved, and those of Bellubrunnus might actually be straight in flight!
Overall then this paper has a bit of something for everyone (hopefully). There is likely to be some kind of taxonomic and systematic signal in the presence of curved wingtips though it would have to be treated with caution as a potential character, but that’s also true of lots of other things too, it should not be overlooked. Second, there really does seem to be an ecological signal there which helps potentially restore the ecological habits and habitats of various taxa. There is very much some aerodynamic ideas in here which can be explore further in terms of wingtip shape, and the implications for thing like chord, stall speeds and how this might relate to wing position in flight. Out hypothesis about compliant bone can potentially be tested with histological sampling and finally this should provide a bit more information for those of the artistic persuasion who like drawing pterosaurs. Enjoy!
Dave Hone's Archosaur Musings 
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