They mold the original bones and then make casts of them. That way more places can have them on display. Then the real bones are used for studying or just kept safe. Shoot, some dinosaur skeletons had bones broken to make them fit the way they thought they should be. It's a very interesting field. I got to do some fossil hunting out on Calvert Cliffs, but danged if I could find one of those awesome Megaladon teeth. That woulda been so cool. *sigh* I did find a crocodile tooth, so that was pretty neat.