FossilisationFossilisation The specification states that you need to be able to: Explain the exceptional preservation of body fossils in:

AmberTarThe Burgess Shale

Describe and explain the preservation of hard skeletal tissues:

ReplacementPreservation by alteration of aragonite to calcitePyritisationCarbonisationSilicificationCast and mould preservation

Only a small number of dead organisms become fossilised, and 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.

Usually the hardest part of the organism (bone, husk, shell, etc) is fossilised, though these may be subject to physical alteration, impact fracture, disintegration or dispersal caused by atmospheric phenomena.

The hard parts may survive even if subject to such traumas, but the softer parts of an organism rarely survive because they are vulnerable to predation and decomposition.

The rapidity with which an organism becomes covered in sediment is important in fossilisation.

In view of this consideration, it is easy to understand why most ancient organisms known to us are either marine or lacustrine; terrestrial organisms can only be preserved if

they die in a marine or fluvial environment.

Only a tiny fraction of the dead animals and plants become fossils.

There need to be special conditions to allow fossilisation.

In order to be preserved the following possibilities must occur:

1. There must be a quite rapid burial or else the organism will be weathered, eroded, scavenged and broken up.

2. Ideally the environment should be anaerobic as soon as possible e.g. a swamp or deep in the ocean.

3. The environment should be low energy i.e. no fast flowing currents.

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.

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

Rock Types That Have Fossils

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?

Rock Types That Have Fossils 2

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).

Exceptional Preservation of Body Fossils

Exceptional Preservation of Body Fossils

These include:Amber

Tar

The Burgess Shale Such exceptional preservation

can be called Lagerstätten Lagerstätten are deposits that

exhibit extraordinary fossil richness and completeness.

LagerstättenLagerstätten 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.

The Burgess Shale (Homework)

The Burgess Shale (Homework)

To be handed in by Friday’s lesson next week. You need to research the Burgess Shale and

produce a word processed or DTP hand-out or PowerPoint 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.

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:

Replacement/ Mineralisation: 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

PyritisationPyritisation Very simply this is where the

original material such as wood, shell or bone is replaced molecule by molecule with Iron Pyrite (FeS).

Pyrite tends to form 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.

CarbonisationCarbonisation This is slightly different. C 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.

SilicificationSilicification 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.

Cast and Mould Preservation

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

   

Cast and Mould Preservation 2Cast 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.

Cast and Mould Preservation 2Cast 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.

What are fossils made of?What are fossils made of? As mentioned previously 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:Silicon (silicification)Pyrite (pyritisation)

Destruction of FossilsDestruction 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