Studying The Past
I.
What is a Fossil?
A. Definition: The evidence or remains of once-living plants or
animals
II. Why Do We Study Fossils Found in Rocks?
A. To provide evidence of the past existence of life forms
B. To provide information about past environmental conditions
C. To provide evidence that populations have undergone change
over time due to environmental changes (evolution)
III. Types of Fossils
A. Original Preservation
1. Description: plant or animal remains that have not
undergone elemental change since death.
a. Uncommon because frozen, extremely dry, or oxygenfree environments are required to form these fossils
b. Examples:
• Mummified humans
• Frozen organisms (Ice Man)
• Mammoths & cats in La Brea Tar Pits
• Fossilized insects in tree sap (amber)
B. Altered Hard Parts
1. Description: all organic material has been removed and the
hard parts of the organism have been changed
a. Minerals seep in slowly and replace the original
organic tissue, forming a rock-like fossil
b. The fossil has the same shape as the original object,
but is chemically more like a rock!
c. Examples:
• Petrified wood
• Recrystallized shells
C. Index Fossils
1. Description: Remains of unique species that can be used to
correlate rock layers or to date a particular rock layer
a. Must be easily recognized, abundant, and widely
distributed geographically
b. Must have lived during a relatively short time period
c. Examples:
• The mollusk Ecphora has a distinctive shape
D. Molds and Casts
1. Description: Fossils that do not contain any shell or bone
a. A mold is formed when original shell parts are
weathered and eroded, leaving an impression of the
shell.
b. This cavity might later become filled with minerals or
sediment to create a cast.
c. Examples:
• Common with shellfish
E. Trace Fossils
1. Description: Indirect evidence of plant and animal life
a. Provide information about how an organism lived,
moved or obtained food
b. Examples:
• worm trails
• burrows
• Footprints
IV. Dating Fossils
A. Relative-Age Dating:
1. Definition: Dating rocks and fossils by placing them in
chronological order without exact dates.
2. Geologic Principles (used in this dating process):
a. Original Horizontality
• Sedimentary rocks are deposited in horizontal layers
b. The Law of Superposition
• in an undisturbed sequence the oldest rocks are at the
bottom and each successive layer is younger
c. Principle of Cross-Cutting Relationships:
• an intrusion or a fault is younger than the rock it cuts
across
3. Other Means of Determining Relative Age
a. Correlation
• Used to date rock layers that are far apart from each
other
• Geologists examine rocks for distinctive fossils (index)
and features to help identify and date them
Relative Dating - Applying the Concepts
1. Which is the oldest rock unit in
the outcrop?
2. Explain why the rock layers on
the west side of the outcrop do not
match the east side.
3. Which is the younger layer, layer A
or layer C?
B. Absolute-Age Dating:
1. Definition: Dating rocks and fossils by using techniques to
determine their actual age.
2. Methods:
a. Tree Rings and Seasonal Climatic Changes
• Each tree ring represents 1 year of growth
• Varves are bands of sediment that show a yearly
cycle from climate change
• Although accurate, neither method can be used to date
very far back in time.
Methods cont.:
b. Radioactive Dating: Dating fossils based on the amount of
radioactive material remaining in a substance over time
• Radioactive substances (unstable atoms) emit protons
and neutrons at a constant rate
• The original element (parent) is converted to a different
element (daughter)
• Since the rate of decay is constant, you can measure the
parent to daughter ratio to determine the age of the rock
• The length of time it takes for one-half of the
original amount to decay is called the elements halflife.
Absolute Dating - Applying the Concepts
Ex. Uranium-238 will decay into an isotope of lead, Pb - 206
Example: Uranium-238Lead 206
1 half-life
2 half-lives
0 years
4.5 billion years
9 billion years
100 % U
50 % U
25 % U
50 % lead
75 % lead
Graph
%U left
time
•Do you want to see a simulation?