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Re: Really, *really* big pterosaurs?

On Friday, September 9, 2005, at 07:03 PM, Patrick Norton wrote:

Impressive. I wonder what the size limits might be for vertebrate
soarers? The Reuters article suggests a very thin airfoil and very light
skeleton in this animal, which makes sense, but how far can the
vertebrate soaring bauplan be extended in this regard? I think it's
fairly well accepted that powered, flapping flyers are limited to
wingspans less than half this width (Argentavis sp), but what are the
size/mass limits for vertebrate soarers?

Good question, and I am sure you would get various answers depending on who you asked. Incidentally, the limitations on powered flight probably have more to do with mass-specific power limitations (as animals get larger, they get less bang per unit muscle weight) than it does with wingspan. Especially large fliers tend to have disproportionately long wings because large fliers tend to utilize soaring a great deal (note that oceanic soarers, like albatrosses, are built very differently from terrestrial soarers like condors). This is more complicated than it seems at first, because while part of the story is that large fliers are 'limited' somewhat to soaring by virtue of size, it is also true that large fliers often make excellent soarers. That is, there is some advantage to being big if an animal is adapted to soaring (example: wind penetration for oceanic, dynamic soarers like albatrosses). Since prolonged soaring is very efficient (and, in some cases, quite a derived form of flight), the adaptation of vertebrate fliers to soaring-oriented flight may not always be simply do to limitations by weight, and similarly, some species may be larger because they utilize soaring. To put it another way, just because a flying animal is large and soars, does not mean it is above the weight limit for powered flight, nor does it mean that soaring evolved in that lineage because of size constraints.

For very large, soaring pterosaurs, one constraint might actually be their bone properties. In order to be both light and strong, the bones of pterosaur arms and fingers (specifically the first digit) are relatively wide, but have very thin cortices. At large sizes, pterosaur bones become extremely thinly walled (presumably for weight reduction, which, due to allometry, gets more 'difficult' at larger sizes), such that they are much more likely to fail by having low stiffness than because of low strength (Currey, 2002), which means that stiffness would be the limiting factor for bone design in this case. (Stiffness=ability to deform to load and return to original shape). In other words, a gigantic pterosaur would need to have a very low mass relative to volume, which would require bones that had very low stiffness, and were likely to fail by flexing past their ability to recover. With a large number of good body mass estimates and bone cross-sections, this relationship could be worked out, but it would only be useful for calculating a maximum size if combined with some knowledge of the forces acting on a pterosaur wing.