Molds & Casts
Organisms get buried in sediment and decay away, leaving a cavity (mold). If that cavity gets filled in with sediment, a cast is made. These leave 3-dimensional impressions of an organism.
Sometimes a shell gets filled in with sediment first, then the shell is dissolved away. This leaves an internal cast of the creature.
Most molds and casts do not contain the organism’s remains.
Molds: A fossilized impression made in sediment that is turned to stone. The original body parts have been decayed.
Casts: The process of minerals entering the cavity of a mold fossil, creating an image of the organism.