How Do Organisms Become Fossils?

Fossilization

• This process usually involves the replacement of organic materials with minerals at the molecular level, over long periods of time. No organic material from the animal or organism remains. All of it is replaced and eventually turned to stone. The type of materials it is replaced with depends on the location. All of the soft tissue is lost and only the hard parts, such as bones and teeth, are preserved. Sometimes a mold fossil reveals the texture of soft tissue if the imprint is preserved.

Where to Find Fossils

• Fossils can only exist in sedimentary rock such as shale or sandstone. They would be destroyed in the environments in which other types of rocks are formed. The heat and pressure to form conglomerate rock, for example, would erase any trace of fossilized remains. There are other substances that also preserve fossils such as tar, amber and ice. These sometimes preserve soft tissue, including the internal organs of an animal, in some cases. Freezing only works as far back as the last ice age. As the world's climate warms up and the ice melts, any preserved frozen fossils are destroyed.

Rock Fossils

• The way the organism is buried is critical, if it is going to be fossilized. Ideally, the organism is covered soon after death with sand or mud through some natural event. This allows it to be preserved and protected from scavengers. It then undergoes chemical changes which turn its skeleton to stone. As millions of years go by, sediments build up over the remains creating intense pressure. This may shift, bend or warp the fossil in almost any direction and turns the entire region into stone, encasing the bones in rock.

Trace Fossils

• Fossilized tracks, footprints and burrows constitute trace fossils and are all created by the activities of organisms or animals. They offer clues to what animals existed at that time, their behavioral patterns and details about their physical make-up. Worm trails are less spectacular, but dinosaur tracks, and certainly the first human footprints, are the most famous trace fossils ever discovered.