Part D: Fossils
Fossils are the remains or traces of prehistoric life. Why are they important to
science today?
-Reveal information about past environments (depositional environments):
water salinity, temperature, turbidity, depth, sediment type and can be used for dating
purposes.
Fossils are grouped into two categories:
a) Body fossils: Evidence of tissue (hard or soft)
b) Trace fossils: Evidence of an organism’s activities, but not the organism itself.
Have all organisms that lived on Earth been fossilized? Why or why not?
-No, fossilization requires:
a) Hard body parts: Physically and chemically durable. Soft body parts often
decompose (only preserved with rapid burial and low O2 levels).
b) Quick burial: Preserve organic remains from scavengers/chemical breakdown.
Soft Body Part Preservation Methods
*All of the methods below involve quick burial and low O2 levels which isolate the
remains from scavengers and microbes (decomposers).
1. Unaltered Soft Body Part Preservation
-Very rare, only occurs in amber, peat bogs, tar pits, permafrost, deserts.
2. Altered Soft Body Part Preservation
a) Carbonization:
- Tissue preserved as a carbon film after the loss of hydrogen, oxygen and nitrogen and
deposition of clay/mica minerals on the tissue surfaces.
-Burgess Shale fossils.
b) Imprints: Impression of organism in sediments. Typical of Pre- Cambrian
organisms, such as the Jellyfish to the left.
Geology 12: Sedimentary Rock
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Hard Body Part Preservation Methods:
1. Unaltered Hard Body Part Preservation
- Hard body parts made up of very stable materials can survive unchanged,
such as shells and sharks’ teeth.
2. Altered Hard Body Part Preservation
a) Permineralization (Petrifaction):
-The pores in hard parts are filled by minerals that precipitate from
surrounding water (which is found in the pore space).
-Very common in porous material, such as wood or bone.
b) Replacement:
-Substitution of original material by another substance atom by atom.
-The original structure is accurately preserved.
-For example, calcium carbonate shell replaced by pyrite or silica.
Geology 12: Sedimentary Rock
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c) External Mold:
-The sediments surrounding an
organism take on its external shape.
-For example, a periwinkle shell in soft
sand.
-Negative relief
d) Internal Mold:
-The sediments take on the internal
shape of an organism.
-For example, the soft sand manages to
fill the shell’s empty interior.
e) Cast:
-The shell dissolves away and leaves an empty space, an external mold and an internal mold.
-Empty space is filled with minerals = a replica of the exterior of the original shell.
-Positive relief.
Geology 12: Sedimentary Rock
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