A six foot Pteranodon with a 25 foot wingspan?
Once you get over a certain weight, it takes more than a pound of living tissue (bone, meat, etc.) to get a pound of lift. That is why the maximum.
So the question is begged about ancient "flying" creatures. Either they didn't "fly" (glider, etc) or the environment in which they flew operated under different circumstances than ours (less gravity, heavyer "air", etc>). Each answer brings with it it's own inconsistencies.
We don't know what we don't know.