Organisms can leave behind indirect fossil evidence.

• The tracks, or "footprints" of certain animals may be found, even if the organism itself is not found. The burrows of small, soft creatures are often found.

• Fossil molds form when an organism's hard parts, such as bones or shells, dissolve after they have been embedded into a rock. These hollow spaces are called molds. If the molds then get filled with sediments or minerals that later hardens, the hardened part is called a cast.

• Petrified remains form when water is continually running over wood. The minerals that are left behind from the water replace the wood leaving something that looks like wood, but is hard like stone.

In all three of these cases, the living organism itself no longer exists. However, the evidence left behind can give many clues about the organism's life.