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Re: Great in the air, not so good underwater

"...suggest that the rules were different for Cretaceous
birds (for example, we don't see higher wing loadings in arboreal
enantiornithine analogs of modern passerines)."

Hmmm. Does your measurement of wingloading in paleo-fliers have an error below +/- 10%? What is the of the error in measuring wingloading in extants?

Not certain; for species with good wing impressions, the planform measurement errors should be below 10%. The mass estimates are less constrained. However, given that the more derived enantiornithines (and Cretaceous ornithurans) were very structurally similar to modern neornithines, I'd find it odd if they didn't scale somewhat similarly. To put it another way, if we invoke atmospheric differences to account for pterosaur size, then it implicitly suggests that the Re changes were sufficient for a more than two-fold increase in mass. That's an awfully big change, and we should see more than subtle effects in other flying taxa. It is more parsimonious, and in line with known anatomy and terrestrial posture in pterosaurs, to invoke the importance of launch effects. The good news is that the launch mode mechanisms are at least partially testable, and that will somewhat implicitly answer the question regarding the importance of hyperoxic environments (since invoking hyperoxia effects is not required if quadrapedal launch was indeed used).

The little flies operate real well at O2 levels below standard pressure; I believe the effects I observed (if real) are _not_ the results of O2-enhanced performance. Sure would be nice if someone with the appropriate tools and an open mind would investigate, though.

Yes, I hope someone does so. I'm not terribly surprised that the flies do alright below standard pressure. I would still expect an improvement in performance in O2-enhanced environments (to a point), but only more data will tell. It is also expected that such environments, if sustained, would raise the body size limit for insects and possibly result in the evolution of larger-bodied forms (again, this is emphasized by R. Dudley and several other insect workers).

"The difference in maximum observed size
between pterosaurs and birds is well over 2x however, and that seems
like a larger difference than can be accounted for with a 12% O2
partial pressure jump."

As far as I am concerned, the difference to be accounted for (empirically, please) is the difference between the largest extant fliers and the largest extinct fliers, from the perspective of the performance of the extant animals. More on that later.

I think I know what you mean, but I'm not certain because there are multiple comparisons implied in your statement. If you really mean it as stated, then the atmospheric effects are a different topic altogether, because the largest flying birds (Argentavis, Miocene pseudodontorns) are extinct, and lived during periods of essentially modern atmospheric composition. I think you mean (and correct me if I'm wrong) the difference between extinct flying clades and those groups with extant members: that is, pterosaurs vs. birds and bats for vertebrates. Also, if you actually mean just comparing the extinct flying species to extant ones, the difference in mass to be accounted for is much more than 2 fold. I was comparing the largest flying birds (fossil forms) to the largest pterosaurs (the largest pseudodontorns were probably about 60kg in mass. I've seen estimates for Argentavis around 80kg, but I have not calculated it for myself. Quetzalcoatlus northropi, by comparison, could easily have exceeded 200 kg, and likely more than that).

In any case, the difference in question seems to me to be a clade effect, more than anything else, at least for vertebrates (the difference in modern insect maximum mass and late Paleozoic forms is likely to be atmospheric, more on that some other time). This can be approached empirically by quantitative assessment of the launch system in pterosaurs (through reconstruction of stance, and analysis of structural mechanics). It can also be approached by looking further into the effects of air density effects, as you suggest. However, as a quick semi-empirical point, I note that the 12-14% O2 increase in the Cretaceous only translates to a 2.5-3% increase in overall air density (given that O2 is about 21% of the atmospheric contents by mass). Since Re (and load carrying capacity) will increase in direct proportion to density, that is only about a 3% load jump, and insufficient to explain the 2.5 fold difference in mass between the largest birds and the largest pterosaurs.