KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form.

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KEY CONCEPT Specific environmental conditions
are necessary in order for fossils to form.
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Fossils can form in several ways.

• Permineralization occurs when minerals carried by
  water are deposited around a hard structure.

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• A natural cast forms when flowing water removes
  all of the original tissue, leaving an impression.

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• Trace fossils record the activity of an organism.

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• Amber-preserved fossils are organisms that become
  trapped in tree resin that hardens after the tree
  is buried.

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• Preserved remains form when an entire organism
  becomes encased in material such as ice.

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• Specific conditions are needed for fossilization.

• Only a tiny percentage of living things became
  fossils.

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Radiometric dating provides an accurate way to
estimate the age of fossils.

• Relative dating estimates the time during which
  an organism lived.
• It compares the placementof fossils in layers of
  rock.
• Scientists infer the order inwhich species
  existed.

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• Radiometric dating uses decay of unstable
  isotopes.

• Isotopes are atoms of an element that differ in
  their number of neutrons.

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• Radiometric dating uses decay of unstable
  isotopes.

• Isotopes are atoms of an element that differ in
  their number of neutrons.

• A half-life is the amount of time it takes for
  half of the isotope to decay.