reading:
Manning ert al 2009. Biomechanics of Dromaeosaurid Dinosaur Claws: Application of X-Ray Microtomography, Nanoindentation, and Finite Element Analysis. The Anatomical Record 292:1397-1405.
The authors used X-Ray Microtomography to generate 3D images of the internal structure of a hand claw specimin belonging to Velociraptor mongoliensis. They then determined the strengths of trabecular and cancellous bone found in the claws of an Owl (Bubo bubo) and used those values for the same parts of the V. mongoliensis claw. They then translated all of this into a 'mesh' model of the mongoliensis claw, excluding the keratin sheath, the outside part of the claw, and applied a force to it equivalent to mongoliensis' bodymass (around 9 lbs here). The highest pressure the claw experienced (the greatest stress) was 60 Kpa. Since extant theropods (birds of course) have claws that can withstand much higher pressure 150-200 Mpa (notice thats ~10,000 times higher) without 'failure', they feel its reasonable to assume mongoliensis claws could support body weight and be used in climbing.
A few questions I have are, and that I have no idea about the answers to, are:
- Is it reasonable to exclude the keratin covering of the claw? On the one hand, if they fail, its like a broken nail, and not necessarily a catastrophe, but on the other, why no estimates anyway?
- The claw specimen was already split in two (and had been glued back together previously), so they had to use some tricks to digitally put it back together. Is it possible that this makes the claw stronger than in reality? But given the very low stress involved, seems irrelevant anyway.
- For trabecular bone, they couldn't use the indentation method to get good values, so they used one from previous literature. The journal source was listed as "J. Dent. Res." which I assume is Journal of Dental Research, which makes me wonder, are they using values for tooth material, and is that realistic?
- In the discussion, they seemed to be very careful to speak of both holding prey and holding onto a tree trunk/climbing. I got the impression that its actually diffcult to distinguish between these two activities in terms of forces.
- The big experimental conclusion was that the claw (minus the keratin at least) can 'easily' (my word) support body weight. Am I right in understanding that this means that these creastures could hand by a single claw?! Thats pretty impressive to me, imagine having to hang over a cliff by a single nail, yikes!
The authors examined a hand claw, but were also able to make some analogies to foot claws. They also had some suggestions that, because the claws could be used in tree-climbing, some of the features of the hindlimb, like a ridge running along metatarsal II, may be analagous to something called the medial plantar crest in birds. This potentially means that the tendon running alongside the ridge in mongoliensis was similar to the tendon along the medial plantar crest in birds, which has weird 'ratcheting' structures used in perching. They don't say that this means mongoliensis was a percher, but rather that retraction of the 'killing claw' was linked to lifting of the foot. Rather nicely, they suggest that in the famous 'fighting pair' of fossils from mongolia (a Velociraptor and a Protoceratops fossilized in mutual death grips), the Velociraptor couldn't let go because it couldn't lift its foot.
