Fossils

Fossil: The remains, imprints or traces of prehistoric organisms that are preserved in rock.

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Fossils are evidence of not only when and where organisms once lived, but also how they lived.

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What is a fossil?

• Fossils give clues about organisms that lived long ago.
• They also provide evidence about how Earth’s surface has changed over time.
• Fossils help scientists understand what past environments may have been like.

• A fossil is the preserved remains of a once-living organism.

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What do fossils tell us?

Fossils are rare because very “lucky” conditions are needed to create them:

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The dead organism must be protected from scavengers, microorganisms and the processes of weathering (usually this means the body is quickly covered by sediments)

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HOW IS A FOSSIL FORMED?

A fossil is more likely to form if the

organism had hard parts like bones, shells or teeth.

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Fossils are generally only found in sedimentary rocks because the heat/pressure involved in forming igneous and metamorphic rocks would destroy the organism.

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FIVE MAIN TYPES OF FOSSILS

1. Permineralized (Petrified) Remains: are hard and rock-like because the original material has been replaced by minerals

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• The word “petrified” means “turning into stone.”
• 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.

PETRIFIED FOSSILS

2. Molds/Casts: An object gets buried, then decays leaving behind a space called a mold. The mold gets filled in by other sediments which harden into rock and forms a cast of the original object

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MOLDS AND CASTS

• A mold forms when hard parts of an organism are buried in sediment, such as sand, silt, or clay.
• The hard parts completely dissolve over time, leaving behind a hollow area with the organism’s shape.

• A cast forms as the result of a mold.
• Water with dissolved minerals and sediment fills the mold’s empty spaces.
• Minerals and sediment that are left in the mold make a cast.
• A cast is the opposite of its mold.

3. Carbon Films: are when the organism gets pressed against a hard object and leaves behind a thin film of carbon residue (similar to a fingerprint)

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• All living things contain an element called carbon.
• When an organism dies and is buried in sediment, the materials that make up the organism break down.
• Eventually, only carbon remains.
• The thin layer of carbon left behind can show an organism’s delicate parts, like leaves on a plant.

CARBON FILMS

4. Trace Fossils: Tracks that have been preserved in mud/clay (similar to hand prints in cement)

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TRACE FOSSILS

• Trace fossils show the activities of organisms.
• An animal makes a footprint when it steps in sand or mud.
• Over time the footprint is buried in layers of sediment. Then, the sediment becomes solid rock.

FANCY FOOTWORK�This dinosaur footprint was found in Namibia, Africa.

5. Original or Preserved Remains: The actual organism or part of it has been preserved in amber, ice or tar (usually very rare and valuable)

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Some organisms get preserved in or close to their original states. Here are some ways that can happen.

PRESERVED REMAINS

Amber An organism, such as an insect, is trapped in a tree’s sticky resin and dies. More resin covers it, sealing the insect inside. It hardens into amber.