1. How do scientists know that different kinds of plants & animals lived in the past?

2. Fossils Preserved remains/traces of living things Clues to the past
Usually found in sedimentary rocks
Bones, shells, teeth, seeds, woody stems become fossilized
It is rare for the soft part of an organism to become a fossil

3. Fossils provide information for:
What the environment looked like
Climate
Shape of Earth’s surface

4. How do fossils form? Living things are buried by sediments
Slowly harden into rock
The shape of the organism is preserved

5. Who studies fossils?
Paleontologists!

6. Kinds of Fossils: Petrified fossils Molds Casts Carbon Films
Trace fossils
Other

7. Petrified Fossils Petrified means “turning into stone”
Minerals replace all/part of an organism

8. Molds & Casts
Mold is a hollow area in sediment in the shape of an organism/part of an organism
Cast is a copy of the shape of an organism when a mold is filled with mud, ash, lava
Most common types of fossils

9. Carbon Films Extremely thin coating of carbon on rock
Carbon comes from living things buried underground
Carbon can preserve delicate parts of plants & insects

10. Trace Fossils Provide evidence of the activities of ancient organisms
Ex: footprint, burrows, trails

11. Other
Some processes preserve the remains of organisms with little/no change
Ex: tar, amber resin, freezing

12. How are fossils classified?
Paleontologists classify fossils in order in which they lived from earliest to latest into a fossil record
Fossil records show how organisms have changed over time

13. How do geologists use fossils to decide how old the rocks are?
Relative age
Absolute age
Law of Superposition
Index fossils

14. Relative Age
The rock’s age compared to the ages of other rocks

15. Absolute Age The number of years since the rock formed
Geologists use radioactive dating to determine this
Carbon-14 dating
Potassium-Argon dating

16. Law of Superposition
In horizontal sedimentary rock layers, the oldest layer is at the bottom and each layer above it is younger

17. Index fossils A fossil found in many different areas
Represents a type of organism that existed only briefly
Geologists look for index fossils in rock layers
Index fossils are useful because they tell the relative age of the rock layers they are in

18. Index Fossil Example
Trilobite: hard-shelled animal with 3 distinct body sections
Many types of trilobites existed over 500 million years ago
Geologists must find one trilobite that is different in some way than the others to date the rock layer in which it was found
Ex: Large eyes

19. What if a small piece of time is missing?
Unconformity is a gap in the geologic record
Some rock records are lost due to erosion

20. What is the geologic time scale?
A record of the life forms and geologic events in Earth’s history
First developed by studying rock layers & index fossils
Later radioactive dating helped to determine the absolute age of each divisions of the time scale

21. Different units of the time scale:
Eras
Periods
Epochs

22. Eras
Long units of time
Precambrian
Paleozoic
Mesozoic
Cenozoic

23. Periods Sub-divided units of an era
Range in length from 2-10million years
Names came from where geologists first discovered the rocks/fossils of that period
Ex: Jurassic

24. Epochs
Sub-divided units of a period

25. Precambrian Era 4.6billion-544million years
Earliest life forms similar to bacteria of today called cyanobacteria
Made food through the use of photosynthesis

26. Paleozoic Era 544million-245million years Explosion of life forms
Animals began coming onto land
Evolving:
Hard-shelled organisms
Invertebrates (jellyfish)
Vertebrates (jawless fish)
Amphibians
Small reptiles
Mass extinction ended this era

27. Mesozoic Era 245million-65million years ago
Called the “Age of Reptiles”
Evolving:
Dinosaurs
Mammals
Birds
Flowering plants
Jurassic Period-dinosaurs ruled Earth for about 150million years
Mass extinction occurred when an object from space hit Earth’s surface causing a dust cloud which blocked the Sun
No dinosaurs survived!

28. Cenozoic Era 65million-Present Evolving: Marine mammals (whales) Grass
Mammals, birds, & insects continue to flourish
Fossil records show that humans appeared about 3.5million years ago