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Fossils
What is a fossil?
A fossil is an impression, cast,
original material or track of
any animal or plant that is
preserved in rock after the
original organic material is
transformed or removed.
A fossil may be:
• an original skeleton
or shell;
• a mold or cast;
• material that has
replaced the once
living thing;
• traces such as
footprints or worm
tubes
What conditions promote
fossilization?
• Hard body parts
such as skeletal
bones or
exoskeletons
What conditions promote
fossilization?
• Rapid burial and/or
lack of oxygen
HOW IS A FOSSIL FORMED?
1. Sediment
2. Layers
3. Movement
4. Erosion
An animal is buried by
sediment, such as
volcanic ash or silt,
shortly after it dies. Its
bones are protected
from rotting by the
layer of sediment.
More sediment layers
accumulate above the
animal’s remains, and
minerals, such as silica
(a compound of silicon
and oxygen), slowly
replace the calcium
phosphate in
the bones.
Movement of tectonic
plates, or giant rock
slabs that make up
Earth’s surface, lifts
up the sediments and
pushes the fossil
closer to the surface.
Erosion from rain,
rivers, and wind wears
away the remaining
rock layers. Eventually,
erosion or people
digging for fossils will
expose the preserved
remains.
What are the fossil types?
• Body fossils – actual parts of an
organism, unaltered or altered
bones, shells, leaf imprints
• Trace fossils – evidence of life that is
not a body fossil
tracks, burrows, casts
Body fossils
Trace Fossils
What are the modes of fossil
preservation for body fossils?
Unaltered
Original Material - original,
unaltered material from the
living organism
unaltered bone or shell
Encrustations or
entombments –
material is trapped inside
coating such as amber
What are the modes of fossil
preservation for body fossils?
Unaltered
Mummification - quickly
dried material
Refrigeration –
material is trapped
inside ice and tissue is
preserved
PRESERVED REMAINS
Some organisms get preserved in or close to their
original states. Here are some ways that can happen.
Amber
Tar
Ice
An organism,
such as an insect,
is trapped in a
tree’s sticky resin
and dies. More
resin covers it,
sealing the insect
inside. It hardens
into amber.
An organism,
such as a
mammoth, is
trapped in a tar pit
and dies. The tar
soaks into its
bones and stops
the bones from
decaying.
An organism,
such as a woolly
mammoth, dies in
a very cold region.
Its body is frozen
in ice, which
preserves the
organism—even
its hair!
What are the modes of fossil
preservation?
• Altered
Permineralization –
pores in tissue are
filled by minerals
Replacement –
replacement of
tissue with minerals
What are the modes of fossil
preservation for body fossils?
• Altered
Carbonization – tissue
material is
decomposed or
reduced to a film of
carbon
More on trace fossils
• Mold – reproduction
of the inside or
outside surface of a
living thing
• Cast – duplicate of
the original
organism; usually
formed by
replacement of
inside of living thing
MOLDS AND CASTS
• A mold forms when hard parts of an
organism are buried in sediment,
such as sand, silt, or clay.
MOLD FOSSIL
This mold, or imprint, is of
an extinct mollusk called
an ammonite.
• The hard parts completely dissolve
over time, leaving behind a hollow
area with the organism’s shape.
• A cast forms as the result of a mold.
• Water with dissolved minerals and
sediment fills the mold’s empty
spaces.
CAST FOSSIL
This ammonite cast was
discovered in the United
Kingdom.
• Minerals and sediment that are left
in the mold make a cast.
• A cast is the opposite of its mold.
More on trace fossils
• Burrows or borings –
Spaces dug out by
living things and
preserved as is or
filled in
More on trace fossils
• Gastroliths – smooth
stones from
abdominal cavity of
dinosaurs
• Coprolites –
fossilized
excrement; usually
preserved by
replacement
More on trace fossils
• Tracks – impressions
of passage of living
things