Earth’s History – Course 2
4-1 Fossils
My Planet Diary pg. 140
A Dinosaur Named Sue
On a hot day in August 1990, Sue Hendrickson was hunting for
fossils near the town of Faith, South Dakota. She found some little
pieces of bone below a cliff. When she looked up at the cliff, she
saw more bones. These bones weren’t little. They were enormous!
She and other scientists determined that they were the bones of a
Tyrannosaurus rex. In fact, she’d found the largest and most
complete skeleton of a Tyrannosaurus ever discovered. Today, the
skeleton, nicknamed “Sue,” is on display at the Field Museum in
Chicago.
1. What science skills did Sue Hendrickson use when she discovered
Sue ? __________________________________________________
2. What do you think scientists can learn by studying dinosaur
skeletons?
__________________________________________________________
What are Fossils? Pg. 141
Fossils are preserved remains or traces of living things.
Most fossils form when living things die and are buried
by sediment. The sediment slowly hardens into rock and
preserves the shapes of the organisms.
Sediment is made up of rock particles or the remains of living things.
How a Fossil Forms: A fossil may form when sediment quickly
covers an organism’s body.
Assess Your Understanding pg. 141
I get it! Now I know that fossils are _______________________
___________________________________________________
What are the Kinds of Fossils? Pg. 142
Fossils found in rock include:
molds and casts: A mold is a hollow area in sediment in the
shape of an organism or part of an organism. A cast is a solid copy
of the shape of an organism.
petrified fossils: minerals have replaced part or all of an organism.
carbon films: an extremely thin coating of carbon on a fossil that
preserves the delicate parts of plant leaves and insects.
trace fossils: such as footprints provide evidence of the activities
of ancient organisms.
Preserved Remains: when the remains of organisms are
preserved in substances such as tar, amber, or ice.
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
3. Movement
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.
4. Erosion
Erosion from rain,
rivers, and wind
wears away the
remaining rock
layers. Eventually,
erosion or people
digging for fossils
will expose the
preserved remains.
FIVE MAIN TYPES OF FOSSILS
Petrified
Fossils
Molds and
Casts
Trace
Fossils
Carbon
Films
Preserved
Remains
PETRIFIED FOSSILS
• The word “petrified” means
“turning into stone.”
• Petrified fossils form when
minerals replace all or part
of an organism.
PETRIFIED FOSSIL
The Field Museum in Chicago
displays a fossil of a
Tyrannosaurus rex.
• 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.
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.
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.
FERN FOSSIL
This carbon-film fossil of a
fern is more than
300 million years old.
• Eventually, only carbon
remains.
• The thin layer of carbon
left behind can show an
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.
FANCY FOOTWORK
This dinosaur footprint was
found in Namibia, Africa.
• Over time the footprint is
buried in layers of sediment.
Then, the sediment becomes
solid rock.
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!
Video: Becoming a Fossil
For more on fossils visit:
http://www.pbs.org/wgbh/evolution/library/04/3/l_043_0
1.html
PBS
Article: Major Fossil Find
http://www2.scholastic.com/browse/article.jsp?id=3751
945
Scholastic News Online
Online Exhibit: Fossil Halls
http://www.amnh.org/exhibitions/permanent/fossilhalls
American Museum of Natural History
Interactive Game: Fossil Hunt
http://channel.nationalgeographic.com/episode/dinodeath-trap-2998#tab-fossil-hunt
Scholastic Classroom Magazines. www.scholastic.com Photo Credits:
PAGE 1: UTAH MUSEUM OF NATURAL HISTORY (DINOSAUR); PHOTOTAKE INC./ALAMY (ANT). PAGE
3: 5W INFORGRAPHIC (GRAPHIC). PAGE 4: Jason Lindsey/Alamy (DINOSAUR); Peter Bowater/Alamy (MOLD); David Lyons/Alamy (FERN); Hoberman Collection UK/Alamy (FOOTPRINT); John
Cancalosi/Alamy (MANTIS). PAGE 5: Gary Crabbe/Alamy (DINOSAUR). PAGE 6: Peter Bowater/Alamy (MOLD); Detail Heritage/Alamy (CAST). PAGE 7: David Lyons/Alamy (FERN). PAGE 8: Hoberman
Collection UK/Alamy (FOOTPRINT). PAGE 9: John Cancalosi/Alamy (MANTIS); R1/Alamy (TAR PITS); Gianni Dagli Orti/Corbis (MAMMOTH).
National Geographic
Assess Your Understanding pg. 143
1a. Identify A (mold / trace fossil) can form when sediment
buries the hard part of an organism.
b. Explain A petrified fossil forms when _________________
replace parts of a(n) _________________________.
c. Make Generalizations What might you learn from a carbon film
that you could not learn from a cast?
___________________________________________________
I get it! Now I know that the kinds of fossils are ______________
___________________________________________________
What Do Fossils Show? Pg. 144
A paleontologist is a scientist who studies fossils.
The fossil record is the combined information about fossils
collected by scientists throughout history.
•It provides evidence about the history of life and past environments
on Earth.
• It also shows how different groups of organisms have changed over
time.
The term evolution is used to identify the process of gradual change
in living things over long periods.
A type of organism is extinct if it no longer exists and will never
again live on Earth.
Fossils – Figure 3 pg. 144
Wyoming, 50 Million Years Ago
Today, Wyoming has areas of dry plateaus, but 50 million years ago it was very
different.
Infer- Identify the organism or kind of organism shown by fossils a, b, and c.
Challenge What features of Hyracotherium show that it is related to horses?
Assess Your Understanding pg. 145
2a. Explain What does the fossil record show about how life has
changed over time? ___________________________________
___________________________________________________
b. Apply Concepts Give an example of a question you could ask
about a fossil of an extinct organism. _____________________
___________________________________________________
___________________________________________________
I get it! Now I know that the fossil record shows _____________
___________________________________________________