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Ask Mark Witton; Pterosaurs and more…

I contacted Mark Witton, author of Pterosaurs, which I’ve been reading with some artist-centric questions about his book. Mark’s been great in answering, but I think we’re both a bit too busy at the moment, so this is much more a dialog than an interview. My intention was to take advantage of Mark’s artistic abilities to ask questions visually… feel free to add questions or comments.

Interpreting skeletals…
dripWitton_shortening

How should your skeletals be interpreted? is there any fore-shortening or are they – as I suspect – constructed on flat plans? There are a few places where this makes for potentially large differences of interpretation… as in the hand digits, the humerus and the femur.

Mark Witton: The diagrams are on 2D planes. I think the takeoff pose used in them reduced most aspects of foreshortening, as the arm, for instance, is extended to the point where all the limb bones are more-or-less operating in the same plane. This, of course, wouldn’t be the case if the limbs were folded up tightly, because pterosaur limbs – the forelimbs in particular – don’t work entirely uniaxially.

 

The uropatagium…

dripWitton_uro

Your book gives a thorough idea of what parts of the pterosaur anatomy are open to interpretation… except the uropatagium. You draw it unattached to the tail on page 52 (left). I’m biased: it looks like a baggy squirrel suit for RedBull stunt divers. The tail-attached version feels like a real animal design. You point to papers talking about the toe and thigh attachment but not – despite ALL THAT SPACE on page 55 – anything about the tail. What’s the hard knowledge on this? a) Are there biases in the evolution (ie. probability of a membrane forming between along tail edge vs between tail and ass)? b) Are there aerodynamic considerations… wouldn’t the attached version allow for more finely-tuned flight control? The unattached version would require leg movements to alter the uropatagium’s position, but the legs are also defining the main wing surface.

Mark Witton: I guess I did forget to mention the relationship of the tail to the uropatagium. This is probably because – despite what you see in many reconstructions – there is no evidence that the tail is incorporated into the uropatagium in any pterosaur fossil. The only good uropatagium we have preserved in an early pterosaur (actually, perhaps in any pterosaur) belongs to the holotype of Sordes, in which the tail is markedly deflected laterally without any affect on the symmetry of the membrane. This is presumably because the tail was not attached to the membrane at all (this is reflected in the illustration of wing membrane distribution in chapter 5 of my book, showing a long tailed pterosaur with the tail skewed to the left). Indirect evidence for a lack of tail incorporation in early pterosaurs stems from anuronathids. These taxa have long fifth toes, which are almost certainly reflecting a broad uropatagium, but many anurognathids have very dumpy tails which cannot, for obvious reasons, be operating the posterior membrane. So yes, there may be good mechanical reasons to think the membrane was attached to the tail, but there’s no fossil evidence for it. Unwin and Bakhurina’s (1994) work on Sordes uropatagium remains the seminal work on this issue.

 As for the space on page 55, that’s up to the typesetters, not me! I could only work by my word counts and figure numbers (the former of which was already blown out of the water). If I’d have known there was free space, I’d certainly have made use of it.

The uropatagium (cont’d)…
dripWitton_uroM

I tried to model the uropatagium in a rough 3D sketch. I modeled a body and tail form from your skeletals, then tried three different uropatagium variations: attached, free and a partial attachment somewhere along the base of the tail. Posed as in your skeletal, there’s not much chance of getting the free version to work out (middle above). It would have to move way down to the dorsal surface, which would kill any aerodynamic function. I had to bend his tail up just make it look half-way functional. At the very least, the tail would be pressed against this u. surface much of the time. The other two versions seem more plausible imo. Sound right?

Mark Witton: I think the reason you were having trouble fitting the tail over the top of the uropatagium is that the tail base is far too thick. Check out the caudal myology reconstructions of Rhamphorhynchus by Scott Persons: http://pterosaur-net.blogspot.co.uk/2013/01/guest-post-dragon-tails-what-pterosaurs.html

There’s virtually no muscle on them whatsoever, even at the base. The characteristically big caudofemoralis of most archosaurs is all but gone. The tail is essentially a bony rod with a slither of soft-tissue around it, and would probably easily extend above the uropatagium without impacting on it. I also note that the membrane in your diagrams is does not connect along the midline of the leg, but is dorsally deflected. I don’t think any fossils show us exactly where the membrane attached with respect to the midline of the leg (some experts in animal flight may have some ideas, though), but this may also be influencing the intrusion of the tail into the uropatagium.

More soon! (‘soon’ paleontologically speaking)

There are 2 Comments to this article

David Peters says:
10/04/2013

The Sordes “uropatagium” is actually the dislocated wing membrane. The actual much smaller uropatagia (plural) are visible closer to the knees, identical in structure to those in Sharovipteryx and other pterosaurs. Bakhurina, Unwin and Witton have wrongly interpreted the specimen in this regard.

dmaas says:
10/05/2013

Hi David,
thanks for writing. I’d already pored over your article about this (for those interested: http://www.reptileevolution.com/pterosaur-wings.htm). It was actually one of the reasons I started this – next to the somewhat thin discussion of the uropatagium in Mark’s book. I agree that the proposed ‘ass-wing’ is at least lacking in details. I love hand-drawn reconstructions, but 3D can be particularly useful in maintaining volume and staying ‘on-model’. That, and being able to subsequently animate or repose a reconstruction using a consistent skeletal deformation. So… hoping to be able to contribute to the discussion.
I’m no biomechanical expert, (still working my way in on knowledge) but I don’t find your reconstruction convincing. Some of the tendon placement stuff looks great, but the wing shape / thigh-attachment seems like it would give only suboptimal functionality. Nature can come up with weird solutions, but to take that leap I feel more fossil interpretation is necessary than you outline on your site. I don’t see those lines that you pull out of the images.

I hope to get back to this in the near future. Depends on how a few job pitches go…

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