A paper from Jul 2013 'Stable Carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic', available for free as a PDF, suggests that O2 levels were much lower than previously thought, and only about 10-15 % concentration in the atmosphere.     This seems to go against the commonly held belief that higher O2 levels allowed Dinosaurs to grow to such large sizes. Volcanic activity was much higher in that era, the seas much shallower and CO2 concent