Fossils are the remains, preserved in rock, of prehistoric animals or plants. Fossils usually represent only the hard portions (and not soft tissues) of organisms, such as the bones or shells of animals and the leaves, seeds, or woody parts of plants.
Fossils are formed when a dead plant or animal becomes buried in soil or clay. As the organism decomposes, its hard body parts leave an imprint in the ground. As ground water seeps past, minerals (such as silica) from the water fill in the imprint and eventually harden into stone in a process called petrification.
Molds and casts are other common fossil types. A mold is made from an imprint, such as a dinosaur footprint, in soft mud or silt. This impression may harden, then be covered with other materials. The original footprint forms the mold and the sediments filling it in form the cast.