Lesson 3
What Do Fossils Tell Us?
Fast Fact
Big Shell Ammonites (AM•uh•nyts) were animals similar to squids with shells. They
lived millions of years ago. The largest ammonite fossil found so far has a shell
almost 2 meters (6½ ft) across. In the Investigate, you will model another type of
Sets of Animal Tracks
 poster board
 animal footprint stamps
 ink pad
 markers, crayons, or colored pencils
1. Old animal tracks, or fossil footprints, are one thing that helps scientists learn about
animals from the past. On the poster board, draw a picture of an area where you
might find animal tracks, such as a riverbank or a sandy beach.
2. Each person in your group should choose a different animal. Using an ink pad and
stamps or other materials, mark the animal's tracks on the poster board. Keep a
record of which animal made tracks first, second, third, and so on.
3. Trade finished poster boards with another group. Figure out the order in which the
other group's tracks were made. Record your conclusions in an ordered list. Give
reasons for the order you choose.
Draw Conclusions
1. Did all the animals move in the same way? How could you tell what kind of animal
made the tracks?
2. Inquiry Skill Scientists often observe an ecosystem at different times of day to see
animals that are out at different times. Predict which tracks you might see if the
picture showed tracks of night animals.
Investigate Further
Make animal tracks on a sheet of paper. Have a classmate infer from the tracks
how the animal moves. Does it slither, walk, or jump?
Reading in Science
fossil p. 216
fossil record p. 218
 what fossils are and how they form
 what the fossil record is
READING FOCUS SKILL SEQUENCE Look for the steps in the formation of
Have you ever seen a movie about dinosaurs? The movie probably showed how
dinosaurs looked, how they moved, and what they ate. Dinosaurs became extinct
millions of years ago. That was long before there were people on Earth. So, how do
people today know so much about dinosaurs?
People today know about many plants and animals of the past because of fossils. A
fossil is the remains or traces of an organism that lived long ago.
Most fossils form in sedimentary rock. First, sediment covers an organism. Then, the
sediment hardens into rock, preserving the fossil shape. The soft parts of organisms
break down quickly and decay. Because of this, most fossils
are formed from only the hard parts of living things, such as shells, bones, and teeth.
The numbered diagram shows the steps of forming a mold and cast fossil.
There are other kinds of fossils. When minerals fill the cells of once-living things, a
different kind of fossil forms. Petrified wood is an example. It is the wood of a tree
that has been replaced by rock. The Petrified Forest, in Arizona, has thousands of
stone logs that were trees millions of years ago.
Another type of fossil is a trace fossil. It doesn't show how a whole plant or animal
looked, but it tells something about it. A fossil footprint is a trace fossil that helps
tell about an animal's size or how it moved. Fossils of animal droppings show what
an animal ate.
Some fossils are the remains of whole animals. They were trapped in ice or tree sap that
hardened. Scientists have found woolly mammoths preserved in ice in Siberia.
These animals died long ago. People know about them because of fossil evidence.
SEQUENCE How do a mold and a cast form?
---see page 216
Mold and Cast Formation
1. Sediment covers a clam. The soft parts of the clam decay.
2. Its shell leaves a clam-shaped hole in the sedimentary rock that forms. This is a
fossil mold.
3. The mold fills with minerals. They form a rock cast in the shape of the clam inside
the rock.
---see picture pg. 216
Molds and casts, like those of this trilobite, are common fossil types.
---see pictures page 217
Minerals have seeped into these dinosaur eggs and turned them into stone.
Amber has encased this insect. Amber is sticky tree sap that trapped a living
insect and then hardened around it.
A fossilized dinosaur track gives clues about the animal's size.
Fossil Record
Earth is about 4.5 billion years old. People have lived on Earth for a very small part of
that time. Scientists have found clues about Earth's past by using fossils as a record
of ancient times. The fossil record is the information about Earth's history that is
contained in fossils. It's the main source of clues about Earth's past life and
Because of the fossil record, we know about animals that lived and died long, long ago.
Dinosaurs and trilobites are examples of such animals. No one has ever seen a
living one. We know about them because people have found and studied their
The fossil record also shows how some species changed over time. Mammoths lived
during the last Ice Age. At that time, ice sheets covered much of Earth. The Ice Age
ended, and the mammoths died out. Other animals much like them continued to
live. It is likely that the elephants of today are related to some of these animals.
---see pictures
Sediment covered the reef, and it became fossilized over millions of years.
Millions of years ago, this reef was home to corals and many other sea animals.
Scientists study reef fossils to find out about animals that lived in oceans and on
reefs long ago.
---see pictures
A dinosaur laid these eggs millions of years ago.
The eggs have become part of the fossil record. The size and number of eggs tell
scientists about the dinosaur that laid them.
The fossil record helps scientists learn how Earth's environment has changed over time.
Today, palm trees live in warm areas. Scientists have found fossils of palm trees in
Wyoming, where it's too cold for palms to grow today. From this evidence,
scientists infer that the climate there must have been much warmer in the past.
Scientists have also found fossils of sea animals in Kansas. Today, Kansas is far from
any ocean. Scientists have inferred that a shallow sea covered parts of Kansas long
SEQUENCE What does the fossil record tell us about climate change in
Fossil Hunt
Get a cupful of soil from outside. Examine it closely with a hand lens. Describe what
you see. Can you see any evidence of fossils? Why or why not?
Geologic Time Scale
Many living things have lived and died out during Earth's long history. Scientists use
the geologic time scale to understand better what was living during each part of this
The scale has several divisions. The table here shows the four eras of the time scale.
Each era is millions of years long. In the middle of the Paleozoic
(pay•lee•uh•ZOH•ik) Era, there were more fish than any other life form. In the next
era, the dinosaurs became the most common vertebrate.
Why is the geologic time scale divided the way it is? The scale shows the waylife has
changed over time. The fossil record shows that animals died out at certain times
during Earth's history. Scientists use these times to mark when eras start and end.
For example, trilobites were common at the start of the Paleozoic Era. They died
out about 248 million years ago. That marks the end of the Paleozoic Era and the
start of the next era. Dinosaurs became extinct about 65 million years ago. That
time marks the end of the Mesozoic Era and the start of the present era.
SEQUENCE What are the four main eras of the geologic time scale, from the
earliest to the present?
Geologic Time Scale
---see chart
Cenozoic Era
The Cenozoic Era includes the present. Humans first appeared during this era.
Mesozoic Era
Dinosaurs were dominant during the Mesozoic Era. An unknown event at the end
of this era caused them to die out.
Paleozoic Era
Trilobites were dominant during part of the Paleozoic Era, but they died out at the
end of it.
Precambrian Time
One-celled organisms were dominant during the Precambrian Era, which covers
Earth's early history.
Reading Review
1. SEQUENCE Copy and fill in the graphic organizer below.
---see chart
Small sea animal with shell _____
Soft parts of the animal decay.
A space forms in the rock where the soft parts used to be.
Minerals fill the space.
_____ forms in the rock.
2. SUMMARIZE Write a brief summary of this lesson. Begin with the sentence
Fossils hold clues to Earth's past.
3. DRAW CONCLUSIONS Are fossils being formed today? Explain.
4. VOCABULARY Use each of the lesson's vocabulary terms in a sentence.
Test Prep
5. Critical Thinking In which type of rock are you most likely to find a fossil? Why?
6. Which kind of fossil is a dinosaur footprint?
A. tar pit fossil
B. cast fossil
C. petrified fossil
D. trace fossil
Narrative Writing
Suppose you are hiking near a cliff. You see a large bone trapped in rock. Write a story
that describes the animal whose fossil you found. Tell how you think it lived and
how the fossil formed.
Compare Two Whole Numbers
Ammonites were like squids with shells. Use an encyclopedia to find the size of squids
today. Compare their size with the size of the largest ammonite.
Social Studies
Make a Brochure
Research a place in the United States where people can see fossils. It could be a national
park, a museum, or another type of area. Design a brochure that encourages people
to visit it. Share it with the class.
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