Plaster Fossil Cast

Some fossils form when a bone, shell, leaf, or other organic matter is buried in mud and this mud hardens into (sedimentary) rock. Water seeping into the rock can dissolve the organic matter, leaving only the hollow space. This bone, shell, or leaf shaped hollow space can become a fossil mold. If the mold fills with minerals or mud, the filling can later harden. The resulting fossil is called a cast. Paleontologists can also make fossil casts which are more easily moved, replicated, & shared.