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.
I knew enough to see the graph here in the link and know it was bullshit because I’ve read stories and taken geography classes to know that people live at very high elevations all their lives and I remembered that those elevations have much lower oxygen concentrations (think all the sherpas that escort mt everest climbers up without needing any oxygen). So then I did just a little google fu to find a big city at a high elevation and cross referenced that with a chart showing oxygen concentrations at certain elevations.