Although the atmospheric pressure may have been a little higher than the present day, the partial pressure of oxygen was lower at the time pterosaurs first appeared (the Triassic), and rose to match the present day only around the time they became extinct (the end of the Cretaceous).
Peter Ward, in his book Out of Thin Air, claims that the low oxygen pressures may have helped drive the evolution of the dinosaurs (in particular the efficient counterflow lung that birds have inherited). But those low oxygen levels would certainly add to the problems of a heavy flyer.

Grant Hutchison