I read an interesting comment on Reddit recently that the higher oxygen content of the atmosphere is what primarily supported massive life forms (and that seeme

I read an interesting comment on Reddit recently that the higher oxygen content of the atmosphere is what primarily supported massive life forms (and that seemed to be proven at least plausible by a recent experiment where researchers were able to grow larger dragonflies in an oxygen-rich environment). Given the current descriptions of the environment at the time -- higher volcanism, an asteroid impact, global wildfires -- is it reasonable to conclude that this had a huge impact on the oxygen content of the atmosphere? And, if so, couldn't that have simply led to a tremendous selection pressure on species, where larger animals were constantly oxygen-depleted and smaller, more efficient ones were able to continue thriving? Think about it: a Tyrannosaurus would require an immense amount of oxygen to be able to maintain an active hunting pattern; if the oxygen in the atmosphere were depleted by even just a few percent, it would find things like running nearly impossible, and long-distance travel would require frequent rest breaks. I'm not sure how this half-assed notion would fit into the oceanic species' extinction, but it makes a lot of sense for why so many of the larger species died out over a period of time while the smaller ones survived.