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G10 - Fossils

I can describe how fossils are formed and explain how they are used to determine the age of the rocks


For thousands of years, people have discovered fossils. They have wondered about the creatures that left them. In ancient times, fossils inspired myths. Stories were told about monsters and other incredible creatures. For example, dinosaur fossils discovered in China two thousand years ago were thought to be dragon bones.

Do you know what fossils are? Do you know how they form? And do you know what they can tell us about the past?

What Are Fossils?

Fossils are preserved remains or traces of organisms that lived in the past. Most preserved remains are hard parts, such as teeth, bones, or shells. Examples of these kinds of fossils are pictured in Figure below. Preserved traces can include footprints, burrows, or even wastes. Examples of trace fossils are also shown in Figure below. You can learn more about both types of fossils at this link:

A variety of fossil types are pictured here. Preserved Remains: (A) teeth of a 15 million-year-old horse, (B) nearly complete dinosaur skeleton embedded in rock, (C) sea shell preserved in limestone. Preserved Traces: (D) dinosaur tracks in mud, (E) fossil animal burrow in rock, (F) fossil feces from a meat-eating dinosaur in Canada.

How Fossils Form

The process by which remains or traces of living things become fossils is called fossilization. Most fossils are preserved in sedimentary rocks.

Fossils in Sedimentary Rock

Most fossils form when a dead organism is buried in sediment. Layers of sediment slowly build up. The sediment is buried and turns into sedimentary rock. The remains inside the rock also turn to rock. The remains are replaced by minerals. The remains literally turn to stone. Fossilization is illustrated in Figure below.

Fossilization. This flowchart shows how most fossils form.

Other Ways Fossils Form

Fossils may form in other ways. With complete preservation, the organism doesn't change much. As seen below, tree sap may cover an organism and then turn into amber. The original organism is preserved so that scientists might be able to study its DNA. Organisms can also be completely preserved in tar or ice. Molds and casts are another way organisms can be fossilized. A mold is an imprint of an organism left in rock. The organism's remains break down completely. Rock that fills in the mold resembles the original remains. The fossil that forms in the mold is called a cast. Molds and casts usually form in sedimentary rock. With compression, an organism's remains are put under great pressure inside rock layers. This leaves behind a dark stain in the rock.

You can read about them in Figure below. You can also learn more about them at this link:





Ways Fossils Form. (A) Complete Preservation. This frog looks the same as it did the day it died millions of years ago! (B) Molds and Casts. A mold is a hole left in rock after an organim's remains break. A cast forms from the minerals that fill that hole and solidify. (C) Compression. A dark stain is left on a rock that was compressed. These ferns were fossilized by compression.

Why Fossilization is Rare

It’s very unlikely that any given organism will become a fossil. The remains of many organisms are consumed. Remains also may be broken down by other living things or by the elements. Hard parts, such as bones, are much more likely to become fossils. But even they rarely last long enough to become fossils. Organisms without hard parts are the least likely to be fossilized. Fossils of soft organisms, from bacteria to jellyfish, are very rare.

Learning from Fossils

Of all the organisms that ever lived, only a tiny number became fossils. Still, scientists learn a lot from fossils. Fossils are our best clues about the history of life on Earth.

Fossil Clues

Fossils give clues about major geological events. Fossils can also give clues about past climates. Here are just two examples. Both are illustrated in Figure don't purge me.

  • Fossils of ocean animals are found at the top of Mt. Everest. Mt. Everest is the highest mountain on Earth. These fossils show that the area was once at the bottom of a sea. The seabed was later uplifted to form the Himalaya mountain range.
  • Fossils of plants are found in Antarctica. Currently, Antarctica is almost completely covered with ice. The fossil plants show that Antarctica once had a much warmer climate.

What can we learn from fossil clues like these? (A) A fish fossil found in the Wyoming desert. (B) Plant fossils like this one in Antarctica.

Index Fossils

Fossils are used to determine the ages of rock layers. Index fossils are the most useful for this. Index fossils are of organisms that lived over a wide area. They lived for a fairly short period of time. An index fossil allows a scientist to determine the age of the rock it is in.

Trilobite fossils, as shown in Figure below, are common index fossils. Trilobites were widespread marine animals. They lived between 500 and 600 million years ago. Rock layers containing trilobite fossils must be that age. Different species of trilobite fossils can be used to narrow the age even more.

Trilobites are good index fossils. Why are trilobite fossils useful as index fossils?

Lesson Summary

  • Fossils are preserved remains or traces of organisms that lived in the past. Most fossils form in sedimentary rock. Fossils can also be preserved in other ways. Fossilization is rare. It’s very unlikely for any given organism to become a fossil.
  • Fossils are the best form of evidence about the history of life on Earth. Fossils also give us clues about major geological events and past climates. Index fossils are useful for determining the ages of rock layers.

Lesson Review Questions


1. What are fossils?

2. Give examples of trace fossils.

3. Why are most preserved remains teeth, bones, or shells?

4. Describe how fossils form in sedimentary rock.

5. Why is fossilization rare?

Apply Concepts

6. Create an original diagram to explain the concept of index fossil. Your diagram should include sedimentary rock layers and fossils.

Think Critically

7. Compare and contrast the frog fossil in Figure above and the fossil dinosaur tracks in Figure above. Infer what you might learn from each type of fossil.

8. Earth’s climate became much cooler at different times in the past. Predict what fossil evidence you might find for this type of climate change.

Points to Consider

Fossils can help scientists estimate the ages of rocks. Some types of evidence show only that one rock is older or younger than another. Other types of evidence reveal a rock’s actual age in years.

  • What evidence might show that one rock is older or younger than another?
  • What evidence might reveal how long ago rocks formed?