4-8 Fossils

Fossils help tell the story of life on earth because they are the traces of animals or plants that lived millions of years ago, when ancient rock formed.

Most fossils are found in sedimentary rock.  Since sedimentary rock forms in layers, paleontologists (a special kind of geologist who studies the earth's past) believe the fossils found in the same layer of rock probably lived during that same time.  Fossils are formed within rock in several ways.

• Mold Fossils - A mold fossil is created when sediments build up over plants, parts of plants, bones, teeth, and even animals. These were covered by more layers of sediment.  These layers hardened and great pressure formed them into rock that is the same shape as the plant or animal that once lived.

• Cast Fossils - A cast fossil is created if a mold fills with minerals from water and water build up, and form the shape of the plant or animal.

• Trace Fossils - A trace fossil is created when trails of animals or footprints are preserved in rock.

• True-Form Fossils - True-form fossils are the hard remains of animals, like bones, shells and teeth that have been preserved.  True-form fossils of soft-bodies insects are created when the insect became covered with sticky tree sap, which became rock hard.

• Petrified Wood - Petrified wood is created when minerals gradually replace the cells of a dead plant or an animal.  It becomes a rocklike fossil that looks much like the animal or plant.

 Directions: Answer the questions about fossils and their formation.

Most fossils are found in rock.

A paleontologist is

a special kind of geologist who studies the present earth
a special kind of geologist who studies the earth
a special kind of geologist who studies the earth's past

Which of the following statements is true?

  Since sedimentary rock forms in layers, paleontologists believe the fossils found in the same layer of rock probably lived during that same time.
Since sedimentary rock forms in layers, paleontologists believe the fossils found in the same layer of rock probably lived during during an earlier time.

Identify each kind of fossil.  Use the following labels: PW-petrified wood, M-mold, C-cast, T-trace, and TF-true form

Created if a mold fills with minerals from water and water build up, and form the shape of the plant or animal.

  A rocklike fossil that looks much like the animal or plant.  Created when minerals gradually replace the cells of a dead plant or an animal. 

Created when trails of animals or footprints are preserved in rock.

The hard remains of animals, like bones, shells and teeth that have been preserved.

Created when sediments build up over plants, parts of plants, bones, teeth, and even animals. These were covered by more layers of sediment.  These layers hardened and great pressure formed them into rock that is the same shape as the plant or animal that once lived.