1. Geologic Time Scale, Fossils, and Dating

2. Geologic Time Scale Represents the history of Earth.
Earth is thought to be about 4.5 billion years old.
Timeline is broken down
as follows:
Eras
Periods

3. Major Events of each Era
Precambrian
Earth forms
First life (unicellular)
Oxygen produced by life forms
Paleozoic
Oxygen levels increase
First insects, plants, amphibians, marine animals
Pangea forms

4. Major Events of each Era
Mesozoic
Dinosaurs, large animals
1st birds, mammals, flowering plants
Pangea breaks up
Cenozoic
Mass extinction of large mammals
Mammals dominate
Humans appear

5. Extinction About 99% of organisms have become extinct.
Often occurs due to natural
selection, may open opportunity for new species to develop.
Mass extinctions destroy many species at a global level Caused by catastrophic events (meteors, volcanic eruptions).

6. How is Age Determined? Relative dating-
Estimates when an organism lived by comparing the location of fossils in layers of rock.
Older organisms are in lower layers, younger organisms are in top layers.

7. Index Fossils Fossil from a “known” time period.
Used to date fossils in corresponding layers.

8. How is Age Determined? Radioactive dating:
Find age by comparing the amount of decayed material to material that is not decayed.
Half-life = the amount of time it takes for half of the material to decay.
Carbon-14 decays to
nitrogen (5730 yrs)
Uranium decays
to lead (700
million years)

9. Fossils Preserved bones or other traces of ancient organisms.
Found in layers of sedimentary rock.
Shows how species may have changed through time.

10. How Fossils Form Petrification
Occurs when rock-like minerals replace organic tissue.
Cast
Dead organism is covered with sediment, all original tissue is removed leaving a mold.

11. How Fossils Form
Amber: organisms become trapped in tree sap that hardens.
Ice: organisms are preserved when encased in ice.
Trace fossil: records activity of organisms. Ex: footprints, nests.

12. Transitional Fossils
Link modern organisms to extinct organisms.

13. Why is the fossil record incomplete?
Not all organisms that have ever lived
became fossilized because specific conditions are needed for fossils to form.