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 missing at least a few bones. To place them on display, we must recreate the missing parts from other skeletons of the same species.

In some cases, mounting the original fossils for display decreases their scientific value, since we must often drill holes or attach other hardware to recreate the skeleton.

Making a Cast

1. The original fossil is covered in silicone or rubber to create a mold. A plaster case is often made to surround and support the mold.

2. The fossil is carefully separated from the mold.

3. Casting material (plaster or plastic resin) is poured into the mold. When hard, the cast is removed. The mold can usually be reused.

4. The cast fossil may be painted to look like the original.