different type of fossils Preservation

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TO BE PRESERVED A FOSSIL MUST BE:

  • rapidly buried (otherwise organism will be weathered,eroded,eaten etc.)
  • be in an anaerobic environment (low oxygen so no decay)
  • be in a low energy environment (no erosion)
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TO HELP PRESERVATION:

  • hard parts (ex: shell, skelenton because soft parts are usually eaten/decomposed before preservation)
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Rocks were it mostly happens

  • mudstone
  • shale
  • siltstone
  • limestone
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were it happens

Because the rocks form in environments where many organisms live, the fossils won't be crushed by large grains, they undergo rapid sedimentation (fast burial) and are found inanaerobic environment.

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REPLACEMENT/MINERALISATION/PETRIFICATION

Replacement of the hard parts with minerals dissolved in water present in the sediment (replaced molecule by molecule):

  • pyritisation- replaced molecule by molecule by pyrite (ex: ammonites, graptolites)
  • calcification- replaced molecule by molecule by calcite (ex: bivalves, brachiopods)
  • silicification- replaced molecule by molecule by silica (quartz) (ex: wood)
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ALTERATION

  • many shells are a mixture of aragonite and calcite
  • aragonite is unstable so changes to calcite by recrystallisation (destroys the internal structure of shell layers but not the shape of it)
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CARBONISATION

  • organims with high carbon content (ex; graptolites, plants) are buried and compressed
  • volatile organic molecules driven off but a film of carbon stays (IS NOT ADDED)
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CAST AND MOULD PRESERVATION

  • the physical characterestics of organisms are impressed onto rocks
  • when organisms are buried/trapped in mud/clay/other materials these harden around the organism (cast)
  • bodies decay and leave a mould of the organism
  • cast=solid
  • mould=empty

External mould and cast: external views

Internal mould and cast: internal views

  • empty and internal features : internal mould
  • empty and external features: external mould
  • solid and internal features: internal mould
  • solid and external features: external mould
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Destruction of fossils

Fossils are very easily destroyed. They can be destroyed by:

  • acidic groundwater which dissolves fossils
  • metamorphism
  • melting of rock
  • erosion (the rock cycle can destroy fossils)
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