Introductory Palaeontology
(Fossils)
Preservation of Organisms
The Chances of Preservation for an Organism in Different Environments
Level of Chance of
Environment Anoxic Why?
Energy Preservation
High No Poor Organisms could be eroded and could
Desert
move before being covered
High No Moderate to Low Organism will be moved before it can be
Floodplain
covered so eroded
River High No Poor High energy so organism is broken up
Swamp on Low Yes Good Low energy so organism doesn’t
Delta move/is covered
High No Low to Medium Organism gets broken before it can be
Beach
covered by sediment
Shallow High No Moderate to High More chance of being covered before it
Marine erodes
Low In some Good to High Organism is quickly covered as it is low
Deep Marine
places energy – doesn’t move
Fossilisation
Actual Remains
The actual organism can be preserved – a very rare phenomenon and only occurs under exceptional
conditions:
• Refrigeration
• Dehydration (Mummification)
• Peat Bog (Tar Pits)
Hard Parts
An organisms hard parts are more likely to be preserved than the Actual Remains of an organism, for example:
• Hair • Horn
• Teeth • Shells
• Bone • Branches
• Scales
For the hard part to be preserved, the minerals that make it up have to be changed into a more stable one.
There are three common ways of doing this:
Replacement Method Replacement by: How?
Silicification Quartz The remains of the organism are dissolved away by
Calcification Calcite percolating waters, then the space is filled in by minerals
precipitated from solution by subsequent percolating
waters
Petrification
OR
Pyritisation Pyrite
Spaces within the organic structure are impregnated with
minerals precipitated from solution by percolating
waters, molecule by molecule
Common method of preservation of plants. Compaction
Carbonisation Carbon
squeezes out hydrogen etc. to leave carbon
, Moulds and Casts
Mould – the cavity left after the remains of an organism are dissolved
Cast – the result of the mould being filled in with mineral matter or sediment. Can
be internal or external
Shell (calcite) is dissolved by acidic water percolating through sediments to leave a
mould
Infilling a mould by percolating waters with minerals in solution e.g. calcite, quartz
and iron pyrite are most common
Trace Fossils
If a shell or hard part is not preserved, it is still possible to see evidence of life
There are many types of trace fossils, including:
• Tracks and trails
• Burrows
• Coprolites
Burrows and tracks can easily be identified so that we know what type of organism made them
Transported and Derived Fossils
Transported Fossils – organisms may have been transported from the environment where they were living
into a different environment for deposition, giving a false impression. Transported then fossilised
Derived Fossils – a fossil may have become weathered and eroded out of the rock that it was fossilised in and
transported to a younger depositional area. Fossilised then transported
Life and Death Assemblages
Life Assemblage – some organisms are preserved in their life positions e.g. a community of organisms may
have been preserved in their burrows
Death Assemblage – the remains have been moved after death but before they have been deposited
Differences Between Life and Death Assemblage:
• Death assemblage has fragmented shells, whereas life assemblage are whole
• Death assemblage has been moved whereas life assemblage hasn’t
• Death assemblage has been sorted by size whereas life assemblage has a range of sizes
Fossil Record
Fossils are the most direct piece of evidence for past life, and they have many purposes:
• To determine the relative age of the rock
• To determine the palaeoenvironment conditions at the time of deposition
• To study the process of evolution
(Fossils)
Preservation of Organisms
The Chances of Preservation for an Organism in Different Environments
Level of Chance of
Environment Anoxic Why?
Energy Preservation
High No Poor Organisms could be eroded and could
Desert
move before being covered
High No Moderate to Low Organism will be moved before it can be
Floodplain
covered so eroded
River High No Poor High energy so organism is broken up
Swamp on Low Yes Good Low energy so organism doesn’t
Delta move/is covered
High No Low to Medium Organism gets broken before it can be
Beach
covered by sediment
Shallow High No Moderate to High More chance of being covered before it
Marine erodes
Low In some Good to High Organism is quickly covered as it is low
Deep Marine
places energy – doesn’t move
Fossilisation
Actual Remains
The actual organism can be preserved – a very rare phenomenon and only occurs under exceptional
conditions:
• Refrigeration
• Dehydration (Mummification)
• Peat Bog (Tar Pits)
Hard Parts
An organisms hard parts are more likely to be preserved than the Actual Remains of an organism, for example:
• Hair • Horn
• Teeth • Shells
• Bone • Branches
• Scales
For the hard part to be preserved, the minerals that make it up have to be changed into a more stable one.
There are three common ways of doing this:
Replacement Method Replacement by: How?
Silicification Quartz The remains of the organism are dissolved away by
Calcification Calcite percolating waters, then the space is filled in by minerals
precipitated from solution by subsequent percolating
waters
Petrification
OR
Pyritisation Pyrite
Spaces within the organic structure are impregnated with
minerals precipitated from solution by percolating
waters, molecule by molecule
Common method of preservation of plants. Compaction
Carbonisation Carbon
squeezes out hydrogen etc. to leave carbon
, Moulds and Casts
Mould – the cavity left after the remains of an organism are dissolved
Cast – the result of the mould being filled in with mineral matter or sediment. Can
be internal or external
Shell (calcite) is dissolved by acidic water percolating through sediments to leave a
mould
Infilling a mould by percolating waters with minerals in solution e.g. calcite, quartz
and iron pyrite are most common
Trace Fossils
If a shell or hard part is not preserved, it is still possible to see evidence of life
There are many types of trace fossils, including:
• Tracks and trails
• Burrows
• Coprolites
Burrows and tracks can easily be identified so that we know what type of organism made them
Transported and Derived Fossils
Transported Fossils – organisms may have been transported from the environment where they were living
into a different environment for deposition, giving a false impression. Transported then fossilised
Derived Fossils – a fossil may have become weathered and eroded out of the rock that it was fossilised in and
transported to a younger depositional area. Fossilised then transported
Life and Death Assemblages
Life Assemblage – some organisms are preserved in their life positions e.g. a community of organisms may
have been preserved in their burrows
Death Assemblage – the remains have been moved after death but before they have been deposited
Differences Between Life and Death Assemblage:
• Death assemblage has fragmented shells, whereas life assemblage are whole
• Death assemblage has been moved whereas life assemblage hasn’t
• Death assemblage has been sorted by size whereas life assemblage has a range of sizes
Fossil Record
Fossils are the most direct piece of evidence for past life, and they have many purposes:
• To determine the relative age of the rock
• To determine the palaeoenvironment conditions at the time of deposition
• To study the process of evolution
