Fossils

What is a fossil?

A fossil is an impression, cast,

original material or track of any animal or plant that is preserved in rock after the original organic material is transformed or removed.

FOSSILS

Traces and preserved remains of ancient life

found within rock layers

• Fossils show:

– Biodiversity

– How species have changed over time

– Correlation between rock layers from around the world

– Relative ages to particular strata

– Evidence for the geological time scale

A fossil may be:

• an original skeleton or shell;

• a mold or cast;

• material that has replaced the once living thing;

• traces such as footprints or worm tubes

What conditions promote fossilization?

• Hard body parts such as skeletal bones or exoskeletons

What conditions promote fossilization?

• Rapid burial and/or lack of oxygen

What are the fossil types?

• Body fossils – actual parts of an organism, unaltered or altered

bones, shells, leaf imprints

• Trace fossils – evidence of life that is not a body fossil

tracks, burrows, casts

Body fossils

Trace Fossils

What are the modes of fossil preservation for body fossils?

Unaltered Original Material - original,

unaltered material from the living organism unaltered bone or shell

Encrustations or entombments –

material is trapped inside coating such as amber

What are the modes of fossil preservation for body fossils?

Unaltered Mummification - quickly

dried material

Freezing –

material is trapped inside ice and tissue is preserved

What are the modes of fossil preservation for body fossils?

Unaltered

Tar Seeps - original remains are trapped in tar

What are the modes of fossil preservation?

• Altered

Permineralization (petrification)– pores in tissue are filled by minerals

Replacement – replacement of tissue with minerals

What are the modes of fossil preservation for body fossils?

• Altered

Carbonization – tissue material is decomposed or reduced to a film of carbon

More on trace fossils

• Mold – reproduction of the inside or outside surface of a living thing

• Cast – duplicate of the original organism; usually formed by replacement of inside of living thing

More on trace fossils

• Burrows or borings –

Spaces dug out by living things and preserved as is or filled in

More on trace fossils

• Gastroliths – smooth stones from abdominal cavity of dinosaurs

• Coprolites – fossilized excrement; usually preserved by replacement

More on trace fossils

• Tracks – impressions of passage of living things

RELATIVE DATING & AGE

• Relative Dating: putting rocks and geological events in correct chronological order

• Relative Age: how old something is in comparison to something else

• HOW? – Use of sedimentary rocks

– Use of fossils

– Study of strata

INDEX FOSSIL

Fossil that defines and identifies geologic

periods; often in only one layer of rock

• Easily recognizable

• Short-lived (found only in a few layers of

rock worldwide)

• Wide distribution (geographic range)

Ex/ INDEX FOSSIL:

AMMONITE

– Ammonite fossils are

found worldwide, but

they existed for only a

very specific period of

time

– this means ammonites are found in very specific layers of rock

– when an index fossil is found, the age of the rocks it is preserved can be determined

LAW OF HORIZONTALITY

Sediments are originally deposited in horizontal

layers

Folds or inclines:

layers must have

been deformed after

they were deposited.

LAW OF SUPERPOSITION For undisturbed rocks, the oldest layer is on

the bottom and the youngest is on top (Supai

is oldest)

What kind of rocks

are these fossils in?

Which layer is

oldest?

Which layer is

youngest?

How do you know?

LAW OF SUCCESSION • Fossils are found in a predictable sequence

• Fossils in rock B are older then fossils in rock A

LAW OF INCLUSIONS

If a rock body (Rock B)

contained fragments

of another rock body

(Rock A),

then Rock B must be

younger than the

fragments of rock it

contained

Law of Exclusions

• An extrusion occurs when lava cools above

ground.

• The extrusion will always be younger than

the rock below it.

• Radiometric dating uses decay of unstable

isotopes.

– Isotopes are atoms of an element that differ in their number of

neutrons.

neutrons protrons

– A half-life is the amount of time it takes for half of the isotope to decay.

• Radiometric dating uses decay of unstable isotopes.

– Isotopes are atoms of an element that differ in their number of neutrons.

GEOLOGIC TIME SCALE

a series of time

intervals that

divides Earth’s

history

• Each layer of rock

represents specific interval

of time

• Index fossils help

determine specific period

• Time periods divided by

specific events like mass

extinctions