Index Fossils

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Index Fossils

• How to date rocks using fossils

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Fossils

• When living organisms die, most settle on the
  ground and decay
• Some are buried by sediment and preserved as
  fossils
• When the sediment hardens into sedimentary rock,
  the fossils become encased in it

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How fossils are created

• 1. The animal dies

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How fossils are created

• 2. The animal is covered by sediment

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How fossils are created

• 3. Minerals replace the original shell

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How fossils are created

• 4. The sediment becomes rock

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How fossils are created

• 5. Rock is eroded and fossil exposed

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How fossils are created

• 6. The fossil is collected!

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A fossil bat
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A dinosaur (Albertosaurus)
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Using fossils to date rocks

• Index fossils organisms whose range in the rock
  record has been used to represent named divisions
  of the geologic time scale
• The best index fossils are
• found in only a particular age of rock
• are common enough to be collected frequently
• easily identified

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Fossil ranges

• Some fossils have long ranges
• Some have short ranges
• Which would make the best index fossil?

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Fossil range zones

• Sometimes the overlap of index fossil ranges can
  be used to identify a rocks age

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Dating Technique
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Outcrops of rock might not preserve all the
layers, but can be combined using index fossils
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Geological Time

• Scientists have divided time since the formation
  of the Earth into a geological time scale.
• Each period has characteristics or events that
  are unique to them.

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• These logs of petrified wood are in the Petrified
  Forest National Park, Arizona.

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• This dinosaur footprint is in shale near Tuba
  City, Arizona. It tells you something about the
  relative age of the shale, since it must have
  been soft mud when the dinosaur stepped here.

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The Burgess Shale

• The Burgess Shale is is an exceptional Middle
  Cambrian age (540 Ma) fossil locality located in
  Yoho National Park in the Rocky Mountains near
  Field, British Columbia, Canada.
• It is special because of the soft-bodied
  preservation of a wide variety of fossil
  invertebrate animals.

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Opabinia
Five eyes and a clawed proboscis!
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Hallucigenia
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Anomalocaris