II. Why Do We Study Fossils Found in Rocks? Studying The Past 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? To provide evidence of the past existence of life forms To provide information about past environmental conditions To provide evidence that populations have undergone change over time due to environmental changes (evolution)

III. Types of Fossils Unaltered Remains Description: plant or animal remains that have not undergone change since death. Uncommon because frozen, extremely dry, or oxygen-free environments are required to form these fossils Examples: Mummified humans Frozen organisms (Ice Man) Mammoths & cats in La Brea Tar Pits Fossilized insects in tree sap (amber)

B. Altered Remains Description: all organic material has been removed and the hard parts of the organism have been changed Minerals seep in slowly and replace the original organic tissue, forming a rock-like fossil The fossil has the same shape as the original object, but is chemically more like a rock! Examples: Petrified wood Recrystallized shells

B. Altered Remains - Continued Types of Altered Remains - Molds and Casts An organism dies and gets trapped/buried in sediment (sand, ash, etc) The original organism decomposes or dissolves away leaving a hollow impression of the organism (mold) This cavity might later become filled with minerals or sediment to create a cast. Examples: Plant fossils Trilobites

C. Indirect Evidence Description: Trace fossils of plant and animal life Provide information about how an organism lived, moved or obtained food Examples: Coprolites (animal waste) Burrows/trails footprints

IV. Dating Fossils Relative-Age Dating: Definition: Dating rocks and fossils by placing them in chronological order without exact dates. Geologic Principles (used in this dating process, also called Steno’s laws): Original Horizontality Sedimentary rocks are deposited in horizontal layers The Law of Superposition in an undisturbed sequence the oldest rocks are at the bottom and each successive layer is younger 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 Correlation Used to date rock layers that are far apart from each other Geologists examine rocks for distinctive fossils and features to help identify and date them Inclusions If a rock contains fragments of another rock, then the rock that is containing the fragments must be younger than the fragments themselves Unconformities Gaps in the geologic record May indicate erosion or deformation of the rock layers

Relative Dating - Applying the Concepts Which is the oldest rock unit in the outcrop? Which is the youngest rock unit in the outcrop? What does “H” represent? Why don’t the layers on the left side of the diagram match with the layers on the right side of the diagram? What do we call what is happening between layer “F” and “M”? I R An intrusion Because of the fault Faulting, unconformity

B. Absolute-Age Dating: Definition: Dating rocks and fossils by using techniques to determine their actual age. Radioactive Dating: Dating fossils based on the amount of radioactive material remaining in a substance over time When nuclei are unstable they spontaneously break apart (decay) in a process called radioactivity The original unstable element (parent) is converted to a different stable 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 radioactive amount to decay is called the elements half-life

Absolute Age Dating – Continued 3. Uranium 238 When rock forms from magma, it contains U-238 and there is no lead present (U-238 decays into Lead-206) U-238 can be used to date rocks formed in the early Earth 4. Carbon 14 When an organism is alive, its C-14 is continuously replaced When that organism dies, the C-14 decays to Nitrogen-14 and it does not get replaced C-14 can be used to date geologic events involving organisms within the past 30,000 years

1 half-life 2 half-lives 9 billion years 4.5 billion years 0 years Example: Uranium-238Lead-206 1 half-life 2 half-lives 9 billion years 25 % U 75 % lead 4.5 billion years 50 % U 50 % lead 0 years 100 % U Graph %U left time

C. A Special Case of Relative/Absolute Dating - Index Fossils Description: Remains of unique species that can be used to correlate rock layers or to date a particular rock layer Must be easily recognized, abundant, and widely distributed geographically Must have lived during a relatively short time period If we can date a rock layer absolutely (exact age), then we can use the index fossil to date similar rock layers absolutely If we can only date the rock layer relatively (approximate age), then we can use the index fossil date similar rock layers relatively. Examples: Ammonites were common 245 to 65 million years ago, when they went extinct