FossilsFossils

•The earliest clues about life on earth are about

•3.5 billion years old!!!!

What are FossilsWhat are Fossils

• The remains of organisms that lived a long time ago– Shells– Bones and Teeth– Fossils of Footprints– Leaves and Seeds– Tree Trunks and Branches– Animal Dropings

Where are Fossils FoundWhere are Fossils Found

• In layers of sedimentary rock

• The oldest fossils are found in the bottom layers

• The youngest fossils are found in the top layers

How are Fossils Formed?How are Fossils Formed?

• An animal dies and is covered by sediment

• (dirt, sand, etc.)

• The sediments build up until the body is buried.

• These layers become rock.

Continued….Continued….

• Minerals replace the bones and hard parts of the

• animal.

• These minerals form the fossil.

• Sometimes the animal decays, leaving an impression of its body.

PaleontologistPaleontologist

•a scientist who studies fossils.

Platystrophia PonderosaPlatystrophia Ponderosa

BrachiopodeBrachiopode

Mesolimulus WalchiMesolimulus Walchi

Eurypholis boissieri Eurypholis boissieri

How old are fossils?How old are fossils?

(How can we tell?)(How can we tell?)

Dasornis londinensisDasornis londinensis

Tyrannosaurus Rex Tyrannosaurus Rex

How old are fossils?How old are fossils?

• Relative dating: figure out the age of a fossil found in one layer of rock by comparing it to rock in other layers.

• Based on the law of superposition – younger layers of rock are deposited on top of older layers

How old are fossils?How old are fossils?

• Radiometric dating: uses the decay of radioactive isotopes to measure the age of rocks

• Uranium-238 used to date rock millions of years old.

• Carbon-14 used to date organic substances like bones and tissues that are thousands of years old. Ex: mummies

Which type of dating is Which type of dating is more accurate?more accurate?

Radiometric datingRadiometric dating

GEOLOGIC TIME SCALEGEOLOGIC TIME SCALE

• Describes the major geological and biological events in Earth’s history (Fig 14.5 p. 397)

Divided into Divided into eraseras which are then which are then divided into divided into periodsperiods..

Cambrian explosion:Cambrian explosion: rapid branching out of rapid branching out of

most most animal groups animal groups

• Throughout the time scale there have been many

• mass extinctions

• a large amount of plants and animals die in a short period of time

• caused by a catastrophe (fire, earthquake, etc.) and/or environmental factors