Molds and Casts
The cavity in which a fossil rests is also considered a fossil. It preserves the shape of the outer material of the organisms. Molds are often forms as the organic material dissolves away. A cavity is left and sometimes filled with another dissolved substance that replaces the void in the cavity. Molds include imprints, like this leaf below, as well as the compression fossil of the soft-bodied Tullymonster of northeastern Illinois.