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FOSSIL FORMATION AND STRATIGRAPHY What is the role of fossils in expanding…
FOSSIL FORMATION AND STRATIGRAPHY What is the role of fossils in expanding what is known of geological time and past life on earth?
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FOSSILS & HOW THEY FORM
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fossils are rare because when most organisms die they are eaten by scavengers or broken down by decomposers
conditions right for fossilisation: rapid burial in sediments, preservation and then later compression of those sediments to form sedimentary rock.
Moulds
A mould fossil forms when the actual bone or shell (etc.) rots completely away but leaves a hollow cavity in the sediments which become rock.
Casts
If the cavity/mould is filled with sediments which become
rock, it forms a “cast” of the original bone or shell.
Carbonisation
This is when the body of the plant or animal is reduced to a flat imprint made of a thin, coal-like carbon impression, often showing fine details.
Mineralisation
this occurs when minerals precipitate into the hollow parts of the fossil, hardening the fossil. e.g petrified wood
Replacement
this occurs when the chemical structure of the hard parts of an organism is gradually dissolved by chemicals in the water seeping through the rock and is completely replaced by minerals
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Trace fossils
record activities of organisms: footprints, burrows, tracks
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INDEX FOSSILS
These are fossils of species which were very common and widespread at a particular time in Earth history.
The presence of an index fossil identifies the period to which rock strata belong and helps a palaeontologist analyse fossil sequences.
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MASS EXTINCTIONS
A mass extinction has occurred when 30% or more of species all around the world become extinct within 2 million years
The eras/periods in the Phanerozoic eon are determined by mass extinctions revealed in the fossil record
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