A Trip Through Geologic Time

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A Trip Through Geologic Time

• Division 2 MST

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Evidence of Ancient Life

• Fossils
• Preserved remains or traces of living things.
• Provide evidence of how life has changed through
  time.
• Help scientists infer how Earths surface has
  changed.
• They are clues to what the environments of the
  past were like.

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How do fossils form?

• Most form in areas where sediments build up.
• Near swamps, lakes or shallow seas
• Only the hard parts of the animals are left after
  the soft tissues decay or are eaten.
• The remains must then be protected from decay

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How do fossils form?
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related
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Kinds of Fossils

• Petrified fossils
• Form when the remains of an organism become
  petrified (in other words "turned to stone)
• Minerals replace all or part of an organism

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Petrified Ammonite
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Petrified Wood
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Petrified Wood
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Kinds of Fossils

• Molds and Casts
• Most common
• Copy shape of organism
• Mold is a hollow area in sediment in the shape of
  the organism
• Forms when hard parts buried in sediment
• Cast is a copy of the shape of the organism due
  to dissolved minerals and sediment deposited in
  the empty space of a mold.
• Molds and Casts are Opposites!

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Which is which? Cast or Mold?
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Kinds of Fossils

• Carbon Films
• An extremely thin coating of carbon on rock
• When sediment buries an organism, the weight of
  the sediment squeezes almost all of the decaying
  organism away, until only a thin film of carbon
  remains.

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Carbon Film FossilRedwood Tree leaf
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Kinds of Fossils

• Trace Fossils
• Provide evidence of activity
• Example a footprint, burrow trails
• Trace fossils provide geologists with much useful
  information about ancient water depths,
  paleocurrents, availability of food, and sediment
  deposition rates

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Trace Fossils
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Kinds of Fossils

• Preserved Remains
• Some processes preserve the remains of organisms
  with little or no change.
• Example Trapped in Tar, amber or freezing

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Insect in Amber
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Kinds of Fossils

• Index Fossils
• A fossil preserved in a rock layer that is
  characteristic of a certain span of geologic time
  or environment.
• Must represent an organism that existed only
  briefly
• They tell the relative ages of rock layers in
  which they occur

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Index FossilTrilobite
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More Examples...
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Review Time!

• Answer the following questions IN COMPLETE
  SENTENCES and in your own words, on the sheet at
  your table to be passed in at the END of class
• Describe the process which most fossils are
  formed in rock.
• How does the fossil record support the theory of
  evolution?