Fossils provide clues about prehistoric life by preserving remains of organisms from over 10,000 years ago. They reveal how Earth's surface and environments have changed over time. The fossil record includes all fossils discovered and yet to be discovered, studied by paleontologists, to answer questions about past life like what organisms existed, and what the oceans, land, and behaviors of organisms looked like in different eras. Fossils form when organisms are quickly buried after death in sediment layers, and minerals slowly replace the organism's bones and tissues over millions of years.

2. 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.

3. The Fossil Record
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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

4. Questions
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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?

5. HOW IS A FOSSIL FORMED?
1. Sediment
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.
2. Layers
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.
3. Movement 4. Erosion
Movement of tectonic Erosion from rain,
plates, or giant rock rivers, and wind wears
slabs that make up away the remaining
Earth’s surface, lifts rock layers. Eventually,
up the sediments and erosion or people
pushes the fossil digging for fossils will
closer to the surface. expose the preserved
remains.

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

7. •
•
• 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.
When the water evaporates,
only the hardened minerals
are left behind.
PETRIFIED FOSSILS
PETRIFIED FOSSIL
The Field Museum in Chicago
displays a fossil of a
Tyrannosaurus rex.

8. MOLDS AND CASTS
•
MOLD FOSSIL
This mold, or imprint, is of
an extinct mollusk called
an ammonite.
•
•
•
•
• A mold forms when hard parts of an
organism are buried in sediment,
such as sand, silt, or clay.
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.
Minerals and sediment that are left
in the mold make a cast.
A cast is the opposite of its mold.
CAST FOSSIL
This ammonite cast was
discovered in the United
Kingdom.

9. • 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
remains.
• The thin layer of carbon
left behind can show an
organism’s delicate parts,
like leaves on a plant.
CARBON FILMS
FERN FOSSIL
This carbon-film fossil of a
fern is more than
300 million years old.

10. 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
solid rock.
FANCY FOOTWORK
This dinosaur footprint was
found in Namibia, Africa.

11. PRESERVED REMAINS
Some organisms get preserved in or close to their
original states. Here are some ways that can happen.
Amber
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.
Tar
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.
Ice
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!

12. Earth Systems Science
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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

13. Burial
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• Quick burial is important!
• Abiotic (non-living)and biotic (living)
factors destroy most bodies before they
become fossils.
• Macroscopic and microscopic
scavengers, predators, decomposers,

14. Which Organisms Become
Part of the Fossil Record?
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• Soft Body Parts: skin, muscle
• Hard Body Parts: bone, teeth, shells,
and wood.
• Oldest body fossils?
• Phanerozoic Eon, Cambrian Period
(Trilobites)

15. Review Pre-Cambrian Life
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• 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

16. Inorganic vs. Organic
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• 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

17. Location, Location, Location…
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• Underwater
• Close to run-off carrying sediment
• Still water
Nice neighborhoods:
• Lakes, ocean, rivers, swamps, deltas,
lagoons
Bad neighborhoods:
• Intertidal zone, Rainforests

18. The Fossil Record
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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.