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 which 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.
Casts are relatively uncommon.
In rare circumstances, the soft parts of an animal may be preserved. They result from the movement of organisms across the sediment surface, or the tunneling of organisms into the sediment, or the ingestion and excretion of sedimentary materials. The study of trace fossils is called ichnology . Trace fossils provide geologists with much useful information about ancient water depths, paleocurrents, availability of food, and sediment deposition rates. In many cases, tracks of animals are the only record of their existence.