Fossils
Chapter 10 Sections 1+2
- Fossils
How a Fossil Forms
• 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.
- Fossils
Changes Over Time
• The fossils record provides
evidence about the history
of life and past
environments on Earth.
The fossil record also
shows that different groups
of organisms have
changed over time. Fossils
of many different kinds of
organisms were formed in
this ancient lakeshore
environment that existed in
Wyoming about 50 million
years ago.
- Fossils
Changes Over Time
• From fossils, scientists have reconstructed the
paleomastodon. This animal had a short trunk and short tusks
on both the upper and lower jaws. The paleomastodon is an
ancestor of the modern elephant.
Using Prior Knowledge
1.
2.
3.
What You Know
Fossils come from ancient organisms.
Fossils are found in hardened rock.
Fossils show us how some present-day organisms looked
different in the past.
1.
2.
3.
What You Learned
Molds and casts are types of fossils.
Organisms are also preserved in amber, tar, and ice.
Fossils tell us about past climates, changes in Earth’s surface,
and how organisms have changed over time.
- The Relative Age of Rocks
The Position of Rock Layers
• According to the law of superposition, in horizontal
sedimentary rock layers, the oldest layer is at the bottom.
Each higher layer is younger than the layers below it.
- The Relative Age of Rocks
Determining Relative Age
• To determine relative age, geologists also study
extrusions and intrusions of igneous rock, faults, and
gaps in the geologic record.
- The Relative Age of Rocks
Determining Relative Age
• An unconformity occurs where erosion wears away layers of
sedimentary rock. Other rock layers then form on top.
- The Relative Age of Rocks
Using Fossils to Date Rocks
• Index fossils are useful because they tell the
relative ages of the rock layers in which they
occur.
- The Relative Age of Rocks
Index Fossil Activity
• Click the Active Art button to open a
browser window and access Active Art
about index fossils.
- The Relative Age of Rocks
Asking Questions
Question
Answer
What does the position of rock
layer reveal?
The oldest layers—and the oldest
fossils—are at the bottom.
How do geologists determine
the relative age of a rock?
They examine the position of rock
layer, extrusions and intrusions of
igneous rock, faults, and gaps in
the geologic record.
The age of an index fossil tells the
age of the rock layer in which it
occurs.
How are fossils used to date
rocks?
- The Relative Age of Rocks
Rock Layers
• Click the Video button to watch a movie
about rock layers.
- The Relative Age of Rocks
Index Fossils
• Click the Video button to watch a movie
about index fossils.