36. What are several ways in which fossils are formed?
A: - Permineralization: occurs when minerals carried by water are deposited around a hard structure.

- Natural casts: occurs whens water removes all original fossils and minerals fill in and recreate the original shape.

- Trace fossils: records the activity of an organism (Ex. footprints)

- Amber-preserved fossils: organisms trapped in tree resin that hardens into amber.

- Preserved remains: forms when an entire organism becomes encased in material such as ice.

37. What is relative dating?
A: It estimates the time during which an organism lived by comparing placement of fossils of that organism with placement of fossils in other layers of rock.  This does not provide the actual ages.

38. How is the actual age of a fossil determined?
A: Radiometric dating can be used which uses natural decay rate of unstable isotopes found in materials in order to calculate the age of that material.  A half-life is the amount of time it takes for half of the isotope to decay into a different element.

39. What are index fossils?
A: They are fossils of organisms that existed only during specific spans of time over large geographic areas.

40. What are the three basic units that make up the geologic time scale?
A: - Eras: lasts hundreds of millions of years

- Periods: lasts tens of millions of years and is associated with particular type of rock system

- Epochs: lasts several million years

Mass extinction events often define the boundaries between many geologic periods.

41. List and describe the three main eras from oldest to youngest.
A: - Palezoic era: existing animal phyla developed through Cambrian Explosion.  Marine invertebrates evolved and earliest land plants arose.  Later, fish diversified and modern pine trees appeared.

- Mesozoic Era: dinosaurs evolved after the largest mass extinction and they diversified.  As they became extinct, birds survived.

- Cenozoic Era: mammals, flowering plants, and birds diversified while primates evolved.  This period includes all modern forms of life and still continues today.

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