Because it is a rock/flying type Pokemon, Aerodactyl has natural weaknesses to rock, steel, water, ice and electric attacks. These elements deal twice the normal damage they cause to a Pokemon that had zero resistance and zero weakness to them.
Aerodactyl has a high natural defense because it is a rock type. Additionally, being a flying type gives it total immunity to ground attacks. This combination makes it one of the few flying types with very good general defense and a good amount of resistances, but the majority of types, such as fairy, darkness and psychic, that do full damage are just as numerous and make Aerodactyl susceptible to status ailments.