https://en.wikipedia.org/wiki/Hyperoxia
This was a screenshot I took months ago while watching a Geology Hub upload on YT. It was a lightbulb moment for my understanding of mass extinction events, (the largest was 250ma). I’ve referenced this multiple times, so thought I might share. Perhaps you find it as interesting as I do.
Oh yeah, there was a whole era where the earth was extra oxygenated so you had XXL insects flying around. They have even done experiments raising modern insects in oxygen rich tanks and they grow (proportionally) quite large!