Pterosaur wings 2: structure

Ok, so following on from part one now we have a ‘broad’ wing with an expanded tip – now to the nitty gritty. The pterosaur wing (as I have previously stressed) is not some sheet of tough leather, but an incredibly complex organ which in many ways is actually quite superior to the equivalent structure in bats (all this ‘pterosaurs as bad fliers’ junk can go too) and would have allowed them superb control over their wings during flight. The pterosaur wing is made up of at least 5 layers and probably more. It is hard to tell as obviously looking at this kind of microstrucutre is pretty difficult and we have to rely on comparing some very different fossils, preserved in very different ways for our information. In addition to an outer epidermis (top and bottom), there are three key features that we do know in quite good detail though and these are worth spending some time over. Some of these might be duplicated (i.e. there could be two muscle layers) and so five is a conservative figure as there could be more, or other layers might interact and be less clear-cut than we think.

Much of what we know comes from four key specimens – (to those who know them – Sordes, the dark wing, the Zittel wing, and the Rio azhdarchid). A still greater debt is owed to the photographic magician Helmut Tischlinger. Helmut is often credited as the ‘King of UV’ (copyright Ushi Goehlich) and is a title not undeserved (equally deserved was his recent honorary doctorate from the LRZ in Munich for his work on the fossils of the Solnhofen). Helmut has been taking photographs of fossils for decades and is a first rate example of an amateur in the field making a genuine and highly significant contribution to the field (especially with pterosaurs) both alone and with collaborators. Sadly, he is little known outside of Bavaria, let alone Germany as a whole but his work deserves far more recognition. His work in UV light allows us to see not only subtleties in far greater detail, but also hidden features that show up only as the varying phosphatised compounds that have formed from the different tissues fluoresce differently under UV light. Thanks to him, incredible details of pterosaur wings are revealed in beautiful colours (thanks to the filters he employs) and with amazing clarity.

In terms of describing the function it is easiest to go from the bottom up – so we will start with the ventral layer. So, from the bottom (after the epidermis of course) we have a layer that consists of small blood vessels. These form a network across the entire wing, with a large blood vessel running sub-parallel to the wing finger and then branches coming off of it. Obviously these would supply the wing with blood and by extension oxygen, fluids, ions etc. to keep it functioning. But why would an essentially broad piece of skin require so much blood? Well, as I have wearily pointed out before, these are hardly bits of leathery skin, but highly advanced tissues. The blood is needed to supply our next layer – that of muscle.

The muscle layer is not like what you might immediately imagine, it is not a thick block of traditional skeletal muscle, but a delicate network of muscle fibres that spread like an misshapen net across the whole wing. Their function might not immediately be obvious – but it should be. By contracting or relaxing the muscle fibres, the pterosaurs can change the shape of the wing! Yes, they can actively wing-warp, altering the camber and thus aerodynamic properties of the wing – high lift and low steering to high steering and low lift! Flatten out one wing, the lift will drop with a low camber and the pterosaur will roll to that side. Flatten them both and it will fall into a soft glide as the amount of lift is lowered. These guys were no slouches in the air and had tricks in their wings birds and bats could only dream of!

But how is the wing held together? What gives it it’s shape it, if it is a just a loose piece of stretchy muscles, skin and blood vessels? Well now we come onto the top and perhaps most important layer – that of the actinofibrils (sometimes aktinofibrils). If you have read the previous post on pterosaur wings you will recall that these help give the wing it’s shape at the distal tip. However, they also provide much more structural strength and shape to the wing as a whole. Pterosaurs don’t have fingers like bats, or split the wing into dozens of ‘parts’ like birds, so they need to have some structure to hold the wing taught and provide lift. If it was not attached securely to the body of the pterosaur, and given some shape, it would just fold up like an umbrella without any ribs.Thus these provide a level of stiffness to the wing and in combination with the muscle layer give the pterosaur control over the wing camber and its lift.

Actinofibrils are unusual structures and we are not sure exactly what they are composed of. The best guess is collagen, but it could also be cartilage or keratin. Determining this in fossils is obviously near impossible but all three are realistic possibilities, though of course collagen is the most likely given the position of the fibres inside the wing membrane (rather than on the surface) and they do not connect to the bones of the wing finger. They lie sub-parallel to the wing towards the wingtips and then sub-perpendicular as we move more proximally. There are no actinofibrils in the proximal wing close to the body, and they get more densely packed the further away you go.

In short they are more densely packed distally and lie in the same direction as the finger, and proximally are rare or missing and lie perpendicular to the long axis of the wing. Functionally this means that the wing is quite stiff in one plane, but flexible in another (the wing membrane can concertina up when the wing finger retracts). The fibrils themselves are slightly elastic and can slide across each other where they are packed tightly tighter, which in tandem with the elastic nature of the muscles fibres makes the wing as a whole highly elastic. This means it can be quite compact when at rest and contracted, but can unfurl to a large size and remain quite rigid when necessary.

That about wraps it up for pterosaur wings and how they are built. There is however, one last possibility – that there is a tendon of some form lying along the trailing edge of the wing. Mechanically it makes perfect sense – it would help enormously to keep the trailing edge stiff and prevent wing flutter. But although there is a tantalising evidence of something in the trailing edge of one Pterodactylus specimen, two obvious factors mitigate against it. If it was there we should see it far more often than one possible trace in one specimen – a tendon should preserve more easily than the rest of the wing, yet we have quite a few wings with no trace of tendons, and if it was there, why did the wing-tip bulge, and not get pulled flat by the tendon?

To try and bring this monster to some sort of conclusion, it is worth returning to the point about the sophistication of pterosaur wings. These were no clumsy fliers with big, ungainly, leathery wings but had highly derived wings with multiple layers of specialised tissues providing an integrated ability to control the shape and camber of the wing during flight. It would make them exceptionally competent aeronauts and easily the equal of birds and bats, and quite possibly, in the case of pterodactyloids at least, their superior.

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