What is a fossil?
• A fossil is the preserved remains of a once-
living organism (at least 10,000 yrs. old).
• Latin: fossilis (something dug up)
What do fossils tell us?
• Fossils give clues about organisms that lived
long ago. They help to show that evolution
has occurred.
• They also provide evidence about how
Earth’s surface has changed over time.
• Fossils help scientists understand what past
environments may have been like.

The Fossil Record
All of the fossils that exist are part of the
Fossil Record
•Includes fossils that have yet to be
discovered
•Paleontologists- study life in the past
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Questions
1. What types of organisms existed in the
past?
2. What did the oceans look like over
different time periods?
3. What did the land look like over
different time periods?
4. How did organisms move about?
5. How did behaviors, such as flight,
evolve? 4

HOW IS A FOSSIL FORMED?
1. Sediment 2. Layers 3. Movement 4. Erosion
An animal is buried by More sediment layers Movement of tectonic Erosion from rain,
sediment, such as accumulate above the plates, or giant rock rivers, and wind wears
volcanic ash or silt, animal’s remains, and slabs that make up away the remaining
shortly after it dies. Its minerals, such as Earth’s surface, lifts rock layers. Eventually,
bones are protected silica (a compound of up the sediments and erosion or people
from rotting by the silicon and oxygen), pushes the fossil digging for fossils will
layer of sediment. slowly replace the closer to the surface. expose the preserved
calcium phosphate in remains.
the bones.

FIVE MAIN TYPES OF FOSSILS
Petrified Molds and Carbon
Fossils Casts Films
Trace Preserved
Fossils Remains

PETRIFIED FOSSILS
• The word “petrified” means
“turning into stone.”
• Petrified fossils form when
minerals replace all or part
of an organism.
• Water is full of dissolved
minerals. It seeps through
the layers of sediment to
reach the dead organism.
PETRIFIED FOSSIL
The Field Museum in Chicago When the water evaporates,
displays a fossil of a only the hardened minerals
Tyrannosaurus rex.
are left behind.

MOLDS AND CASTS
• A mold forms when hard parts of an
organism are buried in sediment,
such as sand, silt, or clay.
MOLD FOSSIL • The hard parts completely dissolve
This mold, or imprint, is of over time, leaving behind a hollow
an extinct mollusk called area with the organism’s shape.
an ammonite.
• A cast forms as the result of a mold.
• Water with dissolved minerals and
sediment fills the mold’s empty
spaces.
CAST FOSSIL • Minerals and sediment that are left
This ammonite cast was
in the mold make a cast.
discovered in the United
Kingdom. • A cast is the opposite of its mold.

CARBON FILMS
• All living things contain an
element called carbon.
• When an organism dies
and is buried in sediment,
the materials that make up
the organism break down.
• Eventually, only carbon
FERN FOSSIL remains.
This carbon-film fossil of a • The thin layer of carbon
fern is more than left behind can show an
300 million years old.
organism’s delicate parts,
like leaves on a plant.

TRACE FOSSILS
• Trace fossils show the
activities of organisms.
• An animal makes a footprint
when it steps in sand or mud.
• Over time the footprint is
buried in layers of sediment.
Then, the sediment becomes
FANCY FOOTWORK
This dinosaur footprint was solid rock.
found in Namibia, Africa.

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, An organism, An organism,
such as an insect, such as a such as a woolly
is trapped in a mammoth, is mammoth, dies in
tree’s sticky resin trapped in a tar pit a very cold region.
and dies. More and dies. The tar Its body is frozen
resin covers it, soaks into its in ice, which
sealing the insect bones and stops preserves the
inside. It hardens the bones from organism—even
into amber. decaying. its hair!

Earth Systems Science
Most fossils form when organisms are
buried in sediment
Natural Cycle: The Rock Cycle
•Abiotic factors such as wind, rain, ice,
and sun weather rocks
•Form sediments that are eroded by
water, landslides, or wind
•Deposited and accumulate over time to
cover the remains of organisms/traces
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Burial
• Quick burial is important!
• Abiotic (non-living)and biotic (living)
factors destroy most bodies before they
become fossils.
• Macroscopic and microscopic
scavengers, predators, decomposers,
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Which Organisms Become
Part of the Fossil Record?
• Soft Body Parts: skin, muscle
• Hard Body Parts: bone, teeth, shells,
and wood.
• Oldest body fossils?
• Phanerozoic Eon, Cambrian Period
(Trilobites)
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Review Pre-Cambrian Life
• Hadean Eon: no fossils
• Archean Eon: only trace fossils of
prokaryotic organisms (no true nucleus,
other organelles) Stromatolites
• Proterozoic Eon: first eukaryotic
organisms, Stromatolites reach greatest
abundance, still no body fossils
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Inorganic vs. Organic
• Inorganic exoskeletons very resistant to
destruction
• Calcium
• Some inorganic exoskeletons
connected with organic material.
Unlikely to find these species in one
“piece” ie. sea urchin
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Location, Location, Location…
• Underwater
• Close to run-off carrying sediment
• Still water
Nice neighborhoods:
• Lakes, ocean, rivers, swamps, deltas,
lagoons
Bad neighborhoods:
• Intertidal zone, Rainforests
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The Fossil Record
The Fossil Record is NOT complete
•Organisms live and die in different
environments
•Only some remains resist destruction
•Of those only some will fossilize
Fewer than 10% of the organisms alive
today will become fossils.
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