Transcript Fossils: Evidence of Past Life

Warm Up 1/7/09
An example of a geologic event that could be recorded in rocks
is ____.
a. a lunar eclipse
c. a civil war
b. a volcanic eruption d. the occurrence of sun spots
2. Nicolaus Steno proposed the most basic principle of relative
dating, the law of ____.
a. gravity
c. superposition
b. fossil succession
d. correlation
3. If a layer of sandstone is in contact with a mass of granite that
contains small pieces of the sandstone, which rock is older?
a. Both are the same age.
b. the granite
c. the sandstone
d. It is not possible to determine which rock is older from this
information.
Answers: 1) b. 2) c. 3) c.
1.
Fossils: Evidence of
Past Life
Chapter 12, Section 2
Fossil Formation
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Fossil – the remains or traces of an organism preserved
from the geologic past
The type of fossil that is formed is determined by the
conditions under which an organism died and how it
was buried
Some remains of animals—such as teeth, bones, and
shells—may not have been altered hardly at all over
time (unaltered remains)
The remains of an organism are likely to be changed
over time (altered remains)
Fossils often become petrified, mineral-rich water soaks
into small cavities and pores of the original organism,
and replace the original material
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Molds and casts are another common type of
fossil; it is created when a shell or other
structure is buried under sediment and then
dissolved by groundwater
Carbonization is effective in preserving leaves
and delicate mineral forms, occurs when an
organism is buried under fine sediment and
subjected to pressures which squeeze out its
liquid and gases
Trace fossils are indirect evidence of prehistoric
life (tracks, burrows, dung, etc.)
Two conditions are important for preservation:
rapid burial and the possession of hard parts
Fossil Formation
Fossil Types
Concept Check
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What are three types of fossils?
Petrified remains, molds, casts, impressions, and
trace fossils
Fossils and Correlation
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In the late 18th Century, William Smith found that
fossils were not randomly distributed throughout the
rock layers, but each layer contained a distinct
assortment of fossils that did not occur in other layers
The principle of fossil succession states that fossil
organisms succeed one another in a definite and
determinable order; any time period can be recognized
by its fossil content
Based on the rock record from around the world,
geologists have identified an order of fossils: an Age of
Trilobites, an Age of Fishes, an Age of Coal Swamps,
an Age of Reptiles, and an Age of Mammals
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Index Fossil – a fossil that is associated with a
particular span of geologic time
Index fossils are widespread geographically, are
limited to a short span of geologic time, and
occur in large numbers
Their presence provides an important method of
matching rocks of the same age
Fossils can also be used to interpret and describe
ancient environments
Fossils and Correlation
Index Fossils
Assignment
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Read Chapter 12, Section 2 (pg. 343-346)
Do Section 12.2 Assessment #1-5 (pg. 346)