Pterosaur wings 1: shape

Flight is inherently really interesting and really complicated, especially for a flying animal where a single pair of wings have to produce all the necessary thrust *and* lift while also providing most of the steering. It is something humans have singularly failed to come even close to matching with machinery, yet pterosaurs were flying from at least 230 million years ago. For some reason people (and here I can include some researchers who should know far better) seem quite happy to assume that pterosaurs were that great in the air and just sort of glided about on some inferior proto-bat-wings. Oddly enough, I really don’t agree with that interpretation and I will hope to justify that a little here.

There is an enormous amount of interesting things I could say about pterosaur wings and flight surfaces (once I get started on the propatagium, the pteroid, ‘rudder’ feet, tail vanes and the rest) but I am going to have to resist. Not because I don’t want to write about them, but because I don’t want to write several more thousand-word posts without any pictures that deals with some really complex ideas. And also because I am involved in research projects that should develop my ideas (and those of my colleagues) about pterosaur flight, and I don’t want to give the game away early, much as I might like to.

First off: the wing tips. Anyone who knows much about aerodynamics will tell you that having a pointed wing tip is a recipe for disaster. Having a wing that finishes in a point will produce something called tip-stall which basically means that the whole wing can suddenly lose all of its lift as the shedding vortex collapses in the right (or wrong) conditions when banking. In short you stand a good chance of suddenly losing you lift and being left with just gravity as a force that is acting upon you, which can be a problem as a flying organism. Look at how the sails have changed in boats and wind-surfers as they have gone from a pointed tip to a rounded one in recent decades to prevent the same problem. Therefore you would expect to see pterosaurs with something similar (birds and bats don’t really have this problem thanks to their wing shape, but pterosaurs have just a spar [the finger] and sail [the wing membrane] so are always going to be at risk of this) but do you?

Well of course, but (rant alert) you never see that, even in reconstructions by pterosaur guys who I *know* know better. The tips finish in broad rounded flaps with very dense actinofibrils to help them keep their shape. We can see this in a number of pterosaur specimens, but anyone who knows the literature can check out the ‘Zittel Wing’ as it is perhaps the best example.

Already most of you should be thinking, ‘what are actinofibrils?’, well, sadly you will have to wait for the next post for details, but essentially they are stiffening fibres. Just to increase the tension a little further (good pun Dave) I will need to throw a few terms at you first and actually talk about the wing-shape before we delve deep into the structure next time.

The main pterosaur wing (that is the one that runs behind the wing finger, not the one in front – the propatagium, or between the legs – the cruropatagium) is called the brachiopatagium (the arm membrane) or occasionally the chieropatagium (finger membrane). The latter is perhaps more accurate in context, but obviously runs the risk of confusion with bats where the flight membranes go under the same name.

Much has been written on pterosaur wing-shape and sadly most of it by people who have clearly not studied the material carefully enough, or understand the principle or parsimony as applied to such situations. The very short version of this is that based on the fossil evidence (and once you start cobbling together all of the available bits we have, there is actually quite a lot) for the extent and shape of the wing we are left with a pretty simple conclusion. The main wing in most, if not all pterosaurs ran from (not surprisingly) the tip of the big 4th finger down to (more surprisingly) the ankle.

Here is where the parsimony comes in as basically we have several specimens where it is absolutely certain the wing reaches the ankle. We have several more where some postmortem contraction and dissassocaition has occurred and the wing has shrunk and folded up but still reaches at least to the thing. Finally we have some that have really folded up, or like the Zittel wing are completely separate from the body so we cannot be certain at all about how big it was in life. In short we have conclusive proof that the wing was broad in some, and nothing that could contradict that as other specimens are distorted or equivocal. The situation is a little more complex than this, but that is about it and so we can only follow the evidence and that points to a broad wing in all pterosaurs.

I would love to make a dozen drawings here to explain everything, but you will just have to bear with me sorry. Unfortunately the wing I have described is often termed ‘broad’ because of the broad attachment to the body (from the upper body to the ankle) which is misleading. This does not mean that it was broad along its whole width and was huge and square (or sub-triangular) like some enormous bed-sheet – it could actually be quite narrow and sweep in towards the body and thus the chord (the length from front to back) could be relatively narrow. In some of the big marine soarers like Pteranodon I suspect it was actually a pretty narrow chord, but would still have attached a long way down the leg.

That’s it for now as I don’t want to get in too deep. Next time we will get to grips with the remarkably complex and advanced structure of the pterosaurs wing.

This is a revised version of a Mk.1 post, to see the original with comments etc., go here.

1 Response to “Pterosaur wings 1: shape”

  1. 1 On a wing and an ankle attachment « Dave Hone's Archosaur Musings Trackback on 15/09/2010 at 3:00 pm
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