Soft-bodied animals such as jellyfish or worms may also be preserved as carbonaceous films in black shales. C. Imprints of Hard Parts in Sediment Many fossils are simply imprints with no shell material present at all. Hard parts are commonly destroyed by decay or dissolution after burial, but may leave a record of their former presence in the surrounding sediment. A shell buried in sandstone may be leached or dissolved by groundwater, leaving a mold of the shell in the surrounding sandstone. If the original shell was convex , the external mold will be concave .
Internal molds are imprints of the inside of the shell in the rock.
Look for such features as muscle scars , which are present on the inside of bivalve shells. Internal molds are produced when a shell is filled with sediment that becomes cemented, and then the shell is dissolved away. Internal molds are sometimes called steinkerns . A cast may be produced if a mold is filled with sediment or mineral matter. A cast is a replica of the original.