Pterosaur wing folding

Yes, it’s another picture from Stuttgart (hey, they have some fantastic models) but with a specific purpose this time out, to talk about the way the wings of pterosaurs fold. We have already covered on here the shape and structure of the pterosaurian wing, and the bony structure, and also the mechanism by which the wing finger might extend and retract but here I want to talk about what this might mean in practical terms for a living animal, or at least how things might look.

While understandably both research and discussion on the wing have focused on flight and how the attachment point of the wing to the body / leg might affect terrestrial locomotion, there is a practical consideration of just where the wing ‘goes’ so to speak. Obviously we have a pretty good idea of how the supporting arm bones fold (i.e. which joints could bend and how much and in which direction) and indeed the legs, but what of the actual membrane itself. If the arm folds up then all that membrane won’t just disappear – it has to go somewhere (just as with folding a fan or umbrella) so how might it behave?

ptero wing482This model, I think, shows the concept off quite well. The wing was elastic (though to exactly what degree is questionable) but also had some rigidity to it (thanks to a combination of muscle fibres and stiffening rods called actinofibrils – more on them at some point though here is a good start). As a result the wing is likely to buckle as it were with these folds and wrinkles appearing as the wing bones start to fold up. As the wing retracts fully I’d expect many of these to form into a single large fold (try making several non-creased folds in a piece of paper then pushing the edges together and the folds soon join into a single large one). The obvious conclusion is that this would flap ‘up’ as opposed to ‘down’ and the final folded wing membrane (with the arm pretty tight to the body and the wing finger retracted) would lie in an arch in the space between the body and arm as supported by the actinofibrils.

I’m really not sure much has been said about this in the literature as it’s pretty self evident that the wing membrane would have to go somewhere when the arm retracts and really it has only got one place to go. Added to its properties of stiffness and elasticity and the situation is hardly controversial. However, it has if anything received less attention in the popular literature with pretty much every model or drawing of a pterosaur showing them in full flight or walking with a retracted wing rather than capturing them in mid-fold as this one does, so I think it’s well worth showing off.

2 Responses to “Pterosaur wing folding”

  1. 1 Zach Miller 09/09/2009 at 12:53 am

    This is a concept I’ve struggled many times with while restoring pterosaurs. I suppose it would easy enough to make a model. I’ve always been told that the wing-finger is exceedingly stiff, and the individual joints would not bend much (if at all).

    • 2 David Hone 09/09/2009 at 9:22 am

      Well that is one question that has been going back and fourth a bit of late. The facts are as follows:

      1. Almost all pterosaurs, unless the wing finger is properly disarticulated) preserve with the wing phalanges aligned (the anurognathids and embryos are the excpetions).

      2. Given how the other bones fold in the wing, it should not be necessary to flex the phalanx joints to get the wing out of the way for landing / walking etc.

      3. However unlike most other joints in the wing there is no obvious mechancism to held the finger bones together. They do disartciualte in some fossils, there are no ‘locks’ on the bones, no obvious places for big tendons to attach, or muscles scars.

      This means that it is possible at least that these joints could flex quite a bit and possible under control (they must have flexed a bit, a very rigid spar would be very vulnerable to breakage in a storm etc. and we do have pterosaurs with broken wings – it did happen). The question really is did they flex that much? My guess is probably not, and that the wing was largely straight the whole time, but this model is not necessarily wrong. I forgot to mention this point in the text as I was more referring to how the membrane folds as a sheet than the bones, thanks for bringing this up.

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