>I’m sure when dinosaurs were dealing with 12% oxygen they had developed adaptations to handle it. Dinosaurs never had to deal with 12% oxygen. Also, mammals'

>I’m sure when dinosaurs were dealing with 12% oxygen they had developed adaptations to handle it. Dinosaurs never had to deal with 12% oxygen. Also, mammals' ancestors roamed the Earth together with dinosaurs, so they would have developed a similar adaptation if your hypothesis was true.