I've heard that oxygen levels are lower now than they have been in the past (this seems to change a lot on geological timescales), and that high oxygen levels m

I've heard that oxygen levels are lower now than they have been in the past (this seems to change a lot on geological timescales), and that high oxygen levels may help allow megafauna's existence. I wouldn't think it's so much the buoyancy as it is about all the other biological machinery that can operate more efficiently at higher oxygen levels. Oxygen levels are largely driven by algae and plant levels, which of course could have been dramatically affected by the impact. It's also possible you'd be better received espousing an alternative to conventional science if your handle didn't sound like a reference to Flat Earth Theory.