Preservation of Fossils

1
Preservation of Fossils

• The specification states that you need to be able
  to
• Explain the exceptional preservation of body
  fossils in
• Amber
• Tar
• The Burgess Shale
• Describe and explain the preservation of hard
  skeletal tissues
• Replacement
• Preservation by alteration of aragonite to
  calcite
• Pyritisation
• Carbonisation
• Silicification
• Cast and mould preservation

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Preservation of fossils

• Only a tiny fraction of the dead animals and
  plants become fossils. It is estimated that just
  one species out of every 5,000 will have had a
  chance of surviving in fossil form until the
  present.
• There need to be special conditions to allow
  fossilisation.

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Ideal conditions for preservation

• In order to be preserved the following conditions
  must occur
• There must be a quite rapid burial or else the
  organism will be weathered, eroded, scavenged and
  broken up.
• Ideally the environment should be anaerobic as
  soon as possible e.g. a swamp or deep in the
  ocean.
• The environment should be low energy i.e. no fast
  flowing currents.

4
Hard parts for preservation

• The following make an organism more likely to be
  preserved
• The organism has Hard parts which are often
  preserved whilst the soft parts are usually
  lost.
• The hard parts usually occur as rigid structures
  either on the outside of the animal protecting
  the soft parts (e.g. shell) , or within the body
  (e.g. skeleton), falling apart (disarticulating)
  as the soft parts decay.

5
Soft parts

• The soft parts are either eaten or decompose
  before they can become preserved
• therefore animals consisting solely of soft parts
  are rarely preserved.

6
Rock Types That Have Fossils

• Fossils are usually found in sedimentary rocks.
• Only rarely in low-grade metamorphic rocks and
  volcanic igneous rocks.
• Fossils are also mostly found in fine-grained
  sedimentary rocks mudstone, shale, siltstone and
  limestone.
• ? Why?

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Rock Types That Have Fossils 2

• These are the types of environments where many
  organisms live.
• They will not be crushed by large clasts.
• They are environments where there may be rapid
  sedimentation and therefore burying the organism
  before it decomposes.
• May be anaerobic (Deep Ocean, swamps).

8
Exceptional Preservation of Fossils

• These include
• Amber especially insects
• Tar
• The Burgess Shale
• Such exceptional preservation such as the Burgess
  Shale can be called Lagerstätten

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Lagerstätten

• One example is the Tar Pits of Rancho La Brea, in
  Los Angeles, where hundreds of sabre-toothed cats
  and many other recently extinct creatures are
  fossilized.
• Another is the Burgess Shale, in the Canadian
  Rockies.
• Europe has several, including the Solnhofen
  Limestone in which Archaeopteryx, the famous
  dinosaur-bird, was first found in all its
  feathered glory.
• And Australia has the extremely old and strange
  Precambrian fossils of the Ediacara Hills.

10
The Burgess Shale (Homework)

• You need to research the Burgess Shale and
  produce a word processed presentation that covers
  the following points
• What is the geological significance of the
  Burgess shale.
• When was it deposited.
• What kind of organisms were fossilised.
• Why were the fossils so well preserved (what were
  the conditions) /what was the palaeoenvironment.

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ALTERATION OF THE ORIGINAL SHELL/HARD PARTS

• Many shells consist originally of a mixture of
  aragonite and calcite.
• Both minerals are CaCO3 polymorphs.
• Aragonite is highly unstable and under higher T
  and P changes to calcite which is more stable.
• This slow change occurs by recrystalisation,
  destroying the internal structure of the shell
  layers but not the overall shape.
• As well as converting from one polymorph to
  another there can be a complete substitution of
  chemicals

12
Replacement/ Mineralisation

• This involves the replacement of the hard parts
  with minerals dissolved in water present in the
  sediment.
• The most commonly occurring ones are calcite,
  pyrite, silica, phosphates, etc.
• Pyritisation
• Carbonisation
• Silicification

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Pyritisation

• Very simply this is where the original material
  such as wood, shell or bone is replaced molecule
  by molecule with Iron Pyrite (FeS).
• Pyrite forms in anaerobic conditions and so
  organisms that are deposited in such environments
  may be converted to pyrite during diagenesis.
• Pyritisation is common therefore in swamps and on
  the abyssal plain.
• The most common fossils to be pyritised are wood
  and graptolites (which are C rich) and ammonites.

14
Carbonisation

• This is slightly different.
• Carbon rich organisms like wood and graptolites
  are buried.
• They are compressed and heated up.
• Volatile organic molecules are driven off and a
  film of carbon remains as a film on the bedding
  plane.

15
Silicification

• This is where any organism has its original
  mineralogy replaced bit by bit by silicon (Si).
• It will be either quartz or flint
  (chalcedony/agate).
• This often happens where a fossil is preserved in
  a Si rich sediment such as a quartz rich
  sandstone or a volcanic ash.
• Water permeating through this sediment is Si rich
  and will replace the original minerals in the
  fossil.

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Cast and Mould Preservation
   

• Casts and moulds are types of fossils where the
  physical characteristics of organisms have been
  impressed onto rocks.
• This happened when organisms became buried or
  trapped in mud, clay, or other materials which
  hardened around them. 
• The bodies decayed, leaving moulds of the
  organism.
• There are two types of moulds
• External
• Internal

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Cast and Mould Preservation 2

• The external is created when the bodies dissolve
  and leave empty cavities imprinted with the
  external details of the organisms.
• Internal moulds form from hollowed structures
  when the interior of an organism is filled with
  inorganic material.
• When the shell deteriorates, an impression of the
  interior is left.

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Cast and Mould Preservation 2

• Internal cast is where an internal mould is
  filled with sediment or cement.
• If this is weathered out the fossil will show the
  impression of the internal structures (e.g.
  muscle scars).
• External cast is where an external mould is
  filled in by sediment or cement.
• If this is weathered out the fossil will show the
  impression of the external features such as ribs
  and growth lines.

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What are fossils made of?

• Most shells are made of calcite or aragonite
  (CaCO3).
• During diagenesis when the original shell may be
  dissolved (see casts and moulds) the void can be
  filled by calcite or quartz (the same minerals
  that make most cements).
• If the mineral is replaced it could be by
• Silica (silicification)
• Pyrite (pyritisation)

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Destruction of Fossils

• Once a fossil has formed it is still unlikely
  that it will survive until it is found by a
  geologist at the surface.
• How may it be destroyed
• Acid groundwater may dissolve the fossil during
  diagenesis.
• Metamorphism.
• Melting of the rock.
• Erosion