Does Air Density Make a Difference?

This is essentially a cross-post from H2VP (with some additions)

One thing I have been asked with some regularity is whether or not a somewhat denser Mesozoic atmosphere, particularly in the Cretaceous (compared to the modern one), could explain the giant size of Late Cretaceous pterosaurs or large dinosaurs.  In short, the answer is: probably not.

There is a reasonably good body of information regarding atmospheric composition during the Mesozoic.  During the Cretaceous, both oxygen and carbon dioxide levels rose slightly, and the total atmospheric density would have been slightly greater as a result - but the difference would have been relatively mild for large vertebrates.

Here is an example of a paper published on the effects of Cretaceous oxygen concentrations on plants: http://jxb.oxfordjournals.org/content/52/357/801.full, and there is a manuscript examining the effect of paleoatmosphere conditions on insects: http://jeb.biologists.org/content/201/8/1043.full.pdf.  There is a relatively recent paper on the Late Cretaceous atmosphere and its potential relationship to mass extinction as well: http://jxb.oxfordjournals.org/content/52/357/801.full

As you can see, plants and insects probably felt the effects of slightly higher oxygen and carbon dioxide concentrations, and indeed the insects of the Cretaceous included some relatively large species, as would be expected.  A slight increase in atmospheric density would have relatively little impact on the maximum size of dinosaurs or pterosaurs, however, and there is not actually any need for an extreme explanation for their size, anyway - despite being larger than living animals with similar lifestyles, none of the giant dinosaurs exceeded the expected maximum size for a walking animal, and no pterosaurs exceeded the limits for biological flight.  Quite a few pterosaurs exceeded the estimated limit for continuous flapping flight in a vertebrate animal (limit is roughly 25-30 kg, give or take), but that only means that they could not flap continuously over long distances and would have switched to soaring flight for long trips; it does not forbid them from flying.

There are three reasons why changes in atmospheric conditions have greater impacts on insects than vertebrate flyers.  First, the tracheal system that insects use for respiration is highly sensitive to oxygen partial pressure.  Second, since insects are typically small, they are often highly reliant on unsteady aerodynamics, which are much more sensitive to air density than steady dynamics.  Finally, insects are almost purely aerobic flyers, while many vertebrates can utilize some degree of anaerobic power (in large flying vertebrates, anaerobic power dominates).  Using anaerobic flight muscle provides a very large burst of power, without using oxygen, after which the muscle quickly fatigues.  Large vertebrates can therefore flap for short bursts, followed by periods of gliding, even when oxygen levels are low.  This option is typically unavailable to insects.