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atmospheric density

I have seen several mentions of increased oxygen content in the Jurassic
and Cretaceous atmosphere, approximately 35% vs our 21%.  This also
implies an increased carbon dioxide percentage, and both would
contribute to increased mass in the atmosphere and therefore increased
density near the surface, thereby lowering the density altitude more
than the increased temperature would raise it.  So in addition to
providing for enhanced oxyenation; compared to the modern atmosphere,
the denser early atmosphere would have allowed flight at reduced
airspeeds for a given weight and wing area, or at present airspeeds for
a given weight and reduced wing area.  The advantages to the development
of flight are obvious.  Can you or anyone on the list provide me with
suggested references that actually attempt to quantify the properties of
the atmosphere during the periods when pterosaur and bird flight were
                                        Jim Cunningham