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:
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