petrified fossils- form when minerals replace all or part of an organism. Water is full of dissolved minerals. It seeps through the layers of sediment to reach the dead organism. When the water evaporates, only the hardened minerals are left behind.
molds and casts- Organisms buried in sediment may decay or dissolve away leaving a cavity or mold. If the space is subsequently filled with sediment, an external cast can be made. Molds and casts are three dimensional and preserve the surface contours of the organism.
carbon films- Carbon film fossils are formed from the carbon residue of a soft-bodied organism that has been buried in sediment.
trace fossils- a fossil of a footprint, trail, burrow, or other trace of an animal rather than of the animal itself.
preserved remains- prehistoric organisms
