Download Fossils: Evidence of Past Life

Warm Up
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 Chapter 12 Assessment #1-33 (pg. 259260)
For Section 2: Do #’s 3, 6, 16-18, 25, 27,
28, 30, & 31