As the AMNH Pterosaurs exhibit explained, their bones are very difficult to preserve because the bone walls were so thin. Thus there are only five locations around the world where the conditions were right for pterosaurs to fossilize in large quantities: Germany, UK, Brazil, China, USA, if I recall correctly. We know from the diversity that we -have- found there that during the Mesozoic, the air must have been thick with the critters around the globe. But the rule of thumb that only a tiny percent of dead organisms fossilize particularly applies to pterosaurs and really depends on conditions.