Actually, they’d be perfectly capable of flight, as the atmosphere wasn’t much denser back then if at all, but they would quickly suffocate, because the O2 concentration is too low to permeate their bodies properly.
“Actually, they’d be perfectly capable of flight...”
I don’t think so. There’s a reason we don’t have 3 foot long creatures with an exoskeleton flying around anymore, insects or otherwise, because it’s just too heavy a thing to carry around once you size it up. Even creatures with lighter internal skeletons that fly generally have very thin hollow bones. An exoskeleton for anything but the tiniest fliers is flat out in the current atmospheric conditions.
The only way they could have flown is if the atmosphere were at a significantly higher pressure to provide more buoyancy/lift, or if the gravity was lower, and it’s a whole lot more likely that the atmospheric pressure could have varied to the necessary degree than that the mass of the planet could have. Plus, we know that the atmospheric pressure has been steadily decreasing since we have been recording reliable measurements of it, so there is corroborating evidence that you could extrapolate based on if you apply standard uniformitarianism.
“but they would quickly suffocate, because the O2 concentration is too low to permeate their bodies properly.”
Yes, that’s true, and do we know the O2 levels were higher in the past. However, a higher amount of O2 and a higher atmospheric pressure may well have gone hand in hand. We can say with certainty that the atmospheric conditions back then were quite different than today, and we have hard evidence that the composition was different, while we only have inferential evidence that the pressure was higher too.