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EOSC 326 Module C.docx

Earth and Ocean Sciences
Course Code
EOSC 326
Michael Wheeler

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Lesson 10: Fossil Preservation and Modes of Life 12/8/2012 9:40:00 AM
Learning Objectives:
Describe why organisms have different potential for preservation
Compare and Contrast body and tract fossils
Compare and contrast the different methods of fossil preservation
Define and identify an external mold, a cast and an internal mold
List and define the different modes of life that have been exploited
by marine organisms over time
Fossil Types.
2 main types of fossils:
o trace fossils impression made on substrate which record
past activities of organisms
trails, tracks, burrows, excrement
o body fossils preserve a record of actual organism or some
part of it
hard parts of organism are preserved bones, shell
more rarely, soft parts are preserved skin, muscles,
tendons, and organs
Preservation Methods
2 methods of preservation:
direct preservation: material is preserved without any chances,
aside from removal of less stable organic matter (eg. Soft tissues)
o common in corals and sponges
indirect preservation: original organic material is partially to fully
changed into new material
o occurs via carbonization, petrifaction, dissolution and
o carbonization: fossils preserved as thin films of carbon
material in question is buried
chemical reaction that water transforms the organic
material of plant/animal into thin film of carbon

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nitrogen, hydrogen and oxygen are driven off as gases,
leaving a carbon outline of original organism
common in fish, insects and plants
o petrifaction: new material completely fills up available pore
spaces and fossil becomes a solid rock which is still partially
composed of original materials
material is buried
groundwater percolates through pore spaces present in
biological tissue
solution that is supersaturated in either calcium
carbonate or silica precipitates minerals in the pore
common in bones, some groups of plants and
o Dissolution and Replacement
Material is buried
Groundwater seeps through sediments and original
material dissolves leaving a void, which preserves the
shape of the organism
Void is filled with sediment or another mineral which
precipitates in the void as calcite
Common in shells and trilobites
2 different situations can occur during initial burial
situation 1impression of organism on the
surrounding rock will be preserved external
cast, replica of original shell present cast
when shell buried, inside of shell originally
filled with soft tissue, does not get filled
with sediment and remains empty
when shell dissolves, entire central cavity is
this space is infilled with either additional
sediment or mineral precipitate

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situation 2 external mold and cast of shell
preserved but internal cavity of organism gets
filled with sediment upon initial burial. Impression
of inside of shell also preserved internal mold
when shell buried, inside of shell originally
filled with soft tissue does get filled with
when original shell dissolves, only the space
that was actually occupied by shell is empty
and filled with either additional sediment or
mineral precipitate
Marine Lifestyles
2 different marine lifestyles:
pelagic: organisms that live up water column, these organisms
with this lifestyle have maximum dispersal
o 2 different methods of functioning in water column:
nektonic groups: swim freely and not dependent on
groups range widely from microscopic to many
metres long
planktonic groups: move through water column by
simply floating or drifting
tend to be microscopic
benthic: organism live either on or within substrate, these
organisms have limited dispersal
o epifaunal: live on substrate
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