Sciencespeak: Pteroid

I’m going to teach you how to do a pterosaur impression.

Stick your arms out to your sides with your palms facing forward. Now, while keeping the rest of your fingers curled in a loose fist, stick out your fourth finger. It’s like you’re giving a New Jersey hello, but with your ring finger instead of your middle one. That’s really all there is to it. Flap and squawk a bit and the illusion will be complete.

Or maybe not. We have many of the same bones as pterosaurs – skull bones, vertebrae, ribs, limbs, and so on – but the thin-walled bones of the flying reptiles evolved into very different shapes from our own. Not to mention that they had a bone that we don’t – the pteroid.

The bone is easy to spot. It’s a curved splint on the pterosaur wrist, in between the body and the fingers. It acted as a skeletal support for the leading edge of the pterosaur flight membrane called the propatagium. The question paleontologists have struggled with, however, is how this important little bone was oriented in life.

Paleontologists have discovered dozens and dozens of pterosaurs, some of which are beautifully-preserved with wing membranes intact, but the proper position of the pteroid bone has remained controversial. Pterosaurs did not think to die in splayed-out, proper anatomical position to make it easy on us. So while some researchers have proposed that the splint of bone pointed straight forward, others have reconstructed the pteroid pointing towards the body at a low angle.

Differing ideas about pteroid orientation, showing the straight-forward (top and bottom) and medial hypotheses. From Palmer and Dyke,
Differing ideas about pterosaur pteroid orientation, showing the straight-forward (a, c) and medial (b) hypotheses. From Palmer and Dyke, 2009.

Which is correct? Perhaps a future find will clear up the matter once and for all. But, using what’s known about the requirements of flight, mechanical engineer Colin Palmer and paleontologist Gareth Dyke investigated how the two competing arrangements would affect the aerial abilities of the Coloborhynchus – a pterosaur with a 19-foot-wide wingspan that flew over Cretaceous England about 98 million years ago.

The forward-pointing pteroid didn’t work so well. In their calculations, Palmer and Dyke found that the straight-ahead position increased strain on the little bone. Not only that, but this pteroid position would have required a very flexible membrane that could stretch 40% between furled and extended positions. This would have made the leading edge of the wing relatively weak and made it more difficult for Coloborhynchus to fly.

Coloborhynchus didn’t seem to have these problems with the inward-pointing pteroid, though. The bone would have kept the membrane as a relatively rigid, stable edge during flight. It’s indirect evidence, true, but we can assume that these animals evolved for optimal flight ability within the constraints of their anatomy, and other pterosaurs likely shared what Palmer and Dyke reconstructed for Coloborhynchus. So if you’re going to build a pterosaur suit to bring your Pteranodon impersonation to the next level, it’s wise to keep that pteroid medial.

Reference:

Palmer, C., Dyke, G. 2009. Biomechanics of the unique pterosaur pteroid. Proceedings of the Royal Society B. 277, 1684: 1121-1127. doi:

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