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Re: Pterosaur size

Re high-altitude migratory
birds-- You cite their performance over a wide range of altitudes (.59 atm-1 atm, to use Jim’s
numbers) as evidence these large birds would be unaffected by small changes
(~15%) in average global density. Relative
to a given process, selection occurs at the point of maximum stress, and it
isn’t surprising that you don’t observe a large change in behavior or
morphology from the point of maximum stress to the point of least stress. The appropriate question is; how will they
perform in a habitat range of .5atm-.85atm, or .68-1.15atms?

That is a good question, and it should be approached both mechanically and with experimental data. At the moment, we only have mechanical models for the extinct mega-volants, but they are robust and suggest that the mega-volants would perform about the same across a wide range of atmospheric conditions, just as modern flying vertebrates do. They might or might not be optimized at various altitudes, though accommodation within individuals make finding a single "optimal" altitude problematic.

In any case, you pose interesting evolutionary questions about selection. However, if you want to answer the question of "could a large pterosaur launch in a modern atmosphere" then you are asking an aerodynamic/mechanical question. I think you're asking multiple questions; some are evolutionary and some are biomechanical. We have quantitative answers for the biomechanical ones, but the evolutionary ones are still less certain. For example, we can demonstrate quantitatively that azhdarchids would launch without difficulty in a modern atmosphere, but determining what altitude was most important in selecting for their morphology is much more difficult to determine. --MH

Another extant maximal volant,
the hummingbird, has a hovering failure density of .49, IIRC. They are very
sensitive to average density changes in the evolutionary sense because they are
the maximal animals in a size-limited locomotive process. However, hummingbirds
can also fly in the flapping style. Note that when they do, they become the
extant minimal flappers. Do you think that their failure density while flying
in flapping style is higher than that of the swan?

Interesting, I did not realize they shifted to a standard kinematic; that would seem difficult. If you have a reference, I'd be very interested in reading it. In any case, the flapping-style hummingbird would be proportionately more affected because of small size. Since hummingbirds cannot adjust planform, it would also be more constrained in that regard. --MH

That would seem to be
predicted by the statement, “small fliers are more affected by medium density
changes than large”. I take your point about weight to density ratios, but from
the ecological perspective, which bird is more likely become extinct or alter
it’s lifestyle if we dial in .85 as sealevel pressure?

I'm not sure either would become extinct, but I would predict that the large-bodied soaring form would be less affected because of the flow regime in which it works, and because of its flight kinematics. But really, to work that out, you would want (again) a combination of experimental work and quantified biomechanical work. --MH

And which bird has a
large enough functional envelope to remain flexible relative to those two
options? What benefit would accrue to all
the “feeble launchers” and “crash landers” from 1.15 atms?

Probably very little benefit; perhaps a little less running for the runway launchers. Landing would be affected less. That's a different question from the one we started with though, because pterosaurs were neither feeble launchers nor crash landers (even if you assume atmospheric conditions were identical to modern ones). --MH


--Mike H.