Some fossils are extremely rare, and only reproductions are available for exhibit.
This is true for the giant sea turtle, Archelon, in this exhibit. Some specimens, like the Tylosaurus in this exhibit, have such heavy skulls that it would be unsafe to mount the original fossils high above the floor. This is especially true in earthquake areas like Los Angeles or San Francisco. Many of the original bones were so thin and brittle they might have broken during transportation and set-up. Most excavated skeletons are missingat least a few bones.