One hypothesis noted on the History channels show “the creation of earth” postulated that the atmoshere was much richer in CO2 causing plants to more than flourish. Consequently, they went further by saying since the jungles could support it, the dinosaurs evolved.
Perhaps this difference in atmospheric density played a role in the flight of the pteradactyls(sp) and other flying dinosaurs. Just a guess on my part.
What about the dragonflies with those 3 to 4 feet wingspans? Those membranous wings would probably break even if the air was replaced by vacuum. If the medium were thicker than present-day air, even then they have too long a span owing to which they most likely wouldn’t be able to handle the inertial stresses developed during flapping.
CO2 enrichment wouldn’t make plants flourish. Plants have to have CO2, obviously, but they flourish just fine with our current 150 to 300 ppm. Plants flourish from adequate sunlight and water, provided the temps are suitable (the higher up in latitude or altitude, the fewer the plants).
Also, if you’ve seen the movie “Apollo 13”, you know that too much CO2 causes brain asphyxia.