Friday, 4 January 2013

Fractured Wing Bones

A number of wing bone fractures are known to have occurred in pterosaur specimens around the wrist area (Wing metacarpal and first phalange joint).  I have puzzled over why this should be the case and have not found a satisfactory explanation until now.
This Pterodactylus from the Solenhofen Limestone in Bavaria shows a fracture of the first wing phalange near to the joint with the wing metacarpal.  This appears to be a stress fracture at a weak point in the bone.  It is not typical of a failure that would occur during flight or in  predatory conflict.
The same type of fracture can be seen in the X-ray of this Tapejarid fossil, where the wing metacarpal has been broken near to the joint.  Such fractures would have collapsed one wing, making it impossible to fly.  If this type of fracture occurred causing the pterosaur to fall into water, the injury would probably have been fatal.  In both of these cases, the fracture resulted in the death of these animals.

Similar fractures can be seen on isolated bone ends of medium sized pterosaurs.  These fracture patterns are not uncommon across the order as a whole, but are seldom seen in smaller individuals.

Mike Habib has been postulating the use of quadrupedal launching for large pterosaurs for some time now and his ideas are received with some caution.  However, the biology and anatomical structure of the wing bones in such a launch would impose stress on the bone structures which, in some cases, would explain their occasional fractures.  I consider it unlikely that pterosaurs would use this launch method across all genera as a prime method of launch, but the functionality and energy conservation of such a launch method seems to me to suggest that many species of pterosaur could take advantage of this type of take off strategy.

From my own experiments with pterosaur wing efficiency in gliding, it is clear that this method of launch is very practical, especially in species with larger wing areas and larger wing spans.  It also seems that apparently older animals have relatively thinner bone cortex thickness to bone diameter ratios, making the bones more likely to be affected by take-off stress in quadrupedal launch.

I do not have conclusive data on this matter, but there is a potential thesis here for someone intent enough to take on the task.

Habib M. B., 2008, Comparative evidence for quadrupedal launch in pterosaurs. P161-168, In: Flugsaurier: Pterosaur papers in honour of Peter Wellnhofer. 2008. Hone D. W. E. and Buffetaut E. (eds). Zitteliana B, 28. 264pp

Habib M., 2011, Solving the pterosaur size problem: Quadrupedal launch in the Pterosauria, Los Angeles County Museum

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