Quick-write

• What is a fossil?

Fossils

• Fossils are remains, or evidence, of living organisms that have been preserved over millions of years.

Rock fossils

• Remains of living organisms that have been turned into rock.

• The hard parts of the organism like shells and bones get buried in sediment.

• Found in layers of sedimentary rock.

Molds and Casts

Petrified wood

Trace fossils

• Evidence the organism existed is preserved in rock.

• Examples include: footprints, animal homes or even feces.

Carbon films

• Every living thing contains carbon. Sometimes when an organism dies the carbon is left behind on a rock in a visible layer.

Original remains

• These are fossils that are actual bodies or parts of bodies of the organism. They are preserved by being sealed inside of a substance that keeps the organisms from decaying.

Amber (resin or tree sap)Ice (freezes the dead organism)Tar (sticky, oily substance)

Index Fossils

• Some fossils can be used as index fossils to provide information about the age of rock layers. Not all fossils are index fossils.

• An index fossil must have lived:1. For a relatively short period of time2. In many places around the world

Index Fossils

• For example: Viviparus is a common river snail that has only been around since the late Jurassic period. Rocks containing fossils of Viviparus are younger than the late Jurassic period.

Relative Age

• Relative age is the age of an event or object in relation to other events or objects.

• For example I have a younger brother, so you know he is less than 42 but you don’t know his exact age.

• If a scientist finds a new kind of fossil in the same rock as a Viviparus fossil he can determine the relative age of the new fossil.

Rock layers and relative age

Absolute Age

• To know my brothers absolute age you would have to know is birthday.

• Scientists can find out the absolute age of rocks and fossils they use a technique called radioactive dating.• Uranium 235 can be used to date igneous rocks• Carbon 14 is used to date anything that was living.

Radioactive Dating

• When a rock or living organism is formed some of its elements are unstable.

• Depending upon the element, over a period of time the element undergoes radio active decay at a constant rate.

• This decay can be measured in “half-life” which is the time it takes for half of the element to decay.

Carbon 14 Decay