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 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. 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
D. 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 100 % U 4.5 billion years 50 % U 50 % lead Graph 9 billion years 25 % U 75 % lead %U left time
C. A Special Case of Relative/Absolute Dating - 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. If we can date a rock layer absolutely (exact age), then we can use the index fossil to date similar rock layers absolutely d. If we can only date the rock layer relatively (approximate age), then we can use the index fossil date similar rock layers relatively. e. Examples: The mollusk Ecphora has a distinctive shape