FOSSILS- Evidence from Once-living Organisms

Fossils found in SEDIMENTARY ROCKS

The lower the sediment layer is, the older the fossils of the layer will be.- Law of Superposition

As time elapses, more and more sediment layers form to create the layers of sedimentary rocks.

EXAMPLES OF SEDIMENTARY ROCKS

DIFFERENT ROCK LAYERS OF THE EARTH

Relative dating places rocks in their proper sequence of formation, e.g. which formed first, second etc.

Although relative dating cannot give us numeric dates for events that took place, it does provide useful information on what event followed or preceded another event.

Relative dating is still very valuable to scientists and still widely used.

The discovery of radioactive dating has supplemented relative dating techniques.

Oldest?

Youngest?

Can you put the events in order?

Relative Dating combined with Radioactive Dating

Absolute (Radioactive) Dating of Fossils using the half- life of elements

o Half Lives for Radioactive Elements

Radioactive Parent

Stable Daughter

Half life

Potassium 40 Argon 40 1.3 billion yrs

Rubidium 87 Strontium 87 48.8 billion yrs

Thorium 232 Lead 208 14 billion years

Uranium 235 Lead 207 704 million years

Uranium 238 Lead 206 4.5 billion years

Carbon 14 Nitrogen 14 5730 years

Half-Life of an element is the TIME it takes for HALF of the atoms present to RADIOACTIVELY decay into a stable element.

http://www.agiweb.org/news/evolution/datingfossilrecord.html#null%23null

Types of Radioactive Decay

-2 protons, 2 neutrons

-Helium atom

-Emitted from the nucleus

-100x more penetrating than alpha

-Neutron changes to one proton, one electron

-Electron is emitted from nucleus

-no charge or mass

-results in only lost energy

-Emitted in Photons

-most dangerous form

100%C-14

50%C-14

50%N-14

25%C-14

75%N-14

12.5% C-

14

87.5% N-14

5730 years

5730 years

5730 years

1/2 C-14 1/4 C-14 1/8 C-14

PARENT = C-14

DAUGHTER = N-14

C-14 Decay

MOLDS Are “trace fossils” Are not the organisms themselves, but is

evidence that the organism has been there Depressions in a surface

Smurf Mold

EXAMPLES OF MOLDS

Lobster Fish

Trilobite

CASTS

Three-dimensional trace of organism Protrudes out of surface of rock

EXAMPLES OF CASTS

Insect Trilobite

Eurobrontes (dinosaur footprints)

IMPRINTS

Different from molds—are not indented or depressed into its surface

Different from casts—do not protrude from surface

Thin layer of carbon which forms from where the organism was

EXAMPLES OF IMPRINTS

Leaf

Fish

BODY PARTS

Actual parts of an organism left behind from the past

Example: bones, organisms preserved in amber or ice

EXAMPLES OF BODY PARTS

Skull of horned andhornless deer

From the Dominican Republic, Miocene epoch (23.8 to 5.3 mya)

The 67-million-year-old Tyrannosaurus rex skeleton known as Sue stands on display at Union Station in

Wash D.C.

A baby mammoth has been uncovered in the permafrost of north-west Siberia.

Approx. 9,000 years old.

Gradualism- pg439

Punctuated Equilibrium-pg439