Sorry for the late reply. When animals are big the ratio of their surface area to mass becomes small. So large animals either need bigger lungs, higher barometric pressure, more oxygen, or all three to adequately ventilate their blood. Consequently, insects like the giant centipede shown above which existed 300 mya could not possibly exist in today's thin depleted atmosphere. Also, dinosaurs appear to be large versions of reptiles and do not have exaggerated lungs. The atmosphere that fueled Earth's hay days had much more CO2, oxygen and barometric pressure. The amount of CO2 in today's atmosphere is trivial in comparison.
