1. Relate Cause and Effect Why have so few organisms become fossilized
Draw Conclusions Many more fossils have been found since Darwin’s day, closing many fossil gaps. How would this information make relative dating more accurate
Review Describe three processes that have affected life on Earth

2. Ch 19 History of Life
19.1 The Fossil Record

3. Paleontologists
Researchers who study fossils to learn about ancient life.

4. Fossils vary enormously in size, type, and degree of preservation
Form only under certain conditions.

5. Types of Fossils
Fossils can be as large and perfectly preserved as an entire animal
With hair, skin, scales, or feathers
Be as tiny as bacteria, developing embryos, or pollen grains
Many are just fragments
Trace fossils
Casts of footprints, burrows, tracks, or even droppings.

6. Fossils in Sedimentary Rock
Most common
Sedimentary rock usually forms when small particles of sand, silt, or clay settles to the bottom of a body of water
Covers dead animals as it builds up.

7. Layers of sediment continue to build up over time
Remains are buried deeper and deeper
Pressure gradually compresses sediment into rock.

8. Preserved remains may later be discovered and studied.

9. Usually soft body structures decay quickly after death
Usually only hard parts like wood, shells, bones, or teeth remain
Hard structures can be preserved if they are saturated or replaced with minerals.

10. Soft tissues can be preserved from aerobic decay if buried quickly.

11. Fossil Information
Bone structure and trace fossils, like footprints, indicate how animals moved
Fossilized plant leaves and pollen suggest whether the area was a swamp, a lake, a forest, or a desert
Comparing body structures in fossils to body structures in living organisms offer clues about how body structures and species have evolved.

12. Relative Dating
Places rock layers and their fossils into a temporal sequence
Lower layers of sedimentary rock and fossils are generally older than upper layers.

13. Index Fossils
Distinctive fossils used to establish and compare the relative ages of rock layers and the fossils they contain
Easily recognized and will occur in only a few rock layers.

14. Radiometric Dating
Uses radioactive isotopes, which decay, or break down, into nonradioactive isotopes at a steady rate
Compares the amount of radioactive to nonreactive isotopes in a sample to determine its age.

15. Half Life
Time required for half of the radioactive atoms in a sample to decay
The half-life of potassium-40 is 1.26 billion years.

16. Carbon-14
Radioactive form of carbon naturally found in the atmosphere that is taken up by living organisms along with “regular” carbon
Used to date material that was once alive
Half-life of only about 5730 years
useful for dating fossils no older than about 60,000 years.

17. Fossils older than 60,000 years
Can’t use Carbon 14
Use other elements to date surrounding rock.

18. Geologic Time Scale Based on both relative and absolute dating
Major divisions of the geologic time scale are eons, eras, and periods.

21. Changing Planet
Building mountains, opening coastlines, changing climates, and geological forces have altered habitats
Actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth.

22. Physical Forces
“Heat wave” of the Mesozoic Era, average temperatures were only 6°C to 12°C higher than 20th century
Ice ages had average temperatures only about 5°C cooler than 20th century.

23. Plate Tectonics
Theory that explains how solid continental “plates” move slowly above Earth’s molten core (continental drift).

24. Biological Forces
Proterozoic Eon photosynthetic organisms produced oxygen gas and also removed large amounts of carbon dioxide from the atmosphere
Provided oxygen for aerobic processes
Removed carbon dioxide which cooled the planet.