Fossils: A Glimpse into Earth’s

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http://etc.usf.edu/lit2go/contents/4100/4118/4118_txt.html

Fossil Formation

• In layers of sedimentary rock• Organism is buried by sand, dirt, ash,

sediment• Over time, as more and more layers

build up, the bottom layers become hardend through pressure

Living fishSediment coming from river

Fish skeleton slightly covered by sediment

Older sediment becomes rock.

Fish skeleton fossilized

More recent sediment builds up.

Where did fossils form?

• In aquatic environments, settle to bottom of ocean

• Where terrestrial (land) organisms get swept in to a river, then to the ocean

• On land, covered with sand, ash, or sediment

Where is the oldest rock?

Process of Fossilization

• Soft body tissues decay, but bones and teeth remain

• Minerals may take the place of organic matter, resulting in petrifaction

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Rare fossil cases

• In rare cases, organic material is preserved. Example: plant leaves found in Idaho, millions of years old, still contained chlorophyll

• Entire organism is preserved if trapped in area without fungi and bacteria to decompose– Ice (mammoth, bison, human)– Tree sap

• Could Jurassic Park really happen?• http://www.youtube.com/watch?v=75Wn38jADB8

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Dating a FossilYour parents wouldn’t like it: they’re way too old for you.

• Relative age: by looking at a fossil’s position in the rock layer, one can tell that it lived before some organisms but after others

• Absolute age: the actual age of a fossil can be determined by a method called radiometric dating or radioisotope dating.

– Examining the amount of radioactive decay in a fossil or surrounding rock

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www.evolution.berkeley.edu

Radiometric Dating• Some elements exist as unstable isotopes.

They lose protons from their nucleus, decaying (at a known rate) as time goes on

• Half-life – length of time it takes ½ of radioactive material to decay

• Different isotopes decay at different rates

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Isotope Half-Life

Carbon-14 5730 years

Potassium-40 1.3 billion years

Uranium-235 4.5 billion years

Carbon Dating•Organisms take in two different isotopes of carbon by eating plants and breathing:

•12C is stable, does not decay•14C is unstable, with a half-life of 5,730 years

• (12C is about 1 trillion times more common in the atmosphere than 14C. For simplicity’s sake, let’s say it is 10 times as abundant).

•At moment that an organism dies, it stops taking in carbon, and has a set ratio of 12C:14C (10:1).

•(For example: 100 g of 12C and 10 g of 14C.)•The amount of 12C does not change (remains 100 g)•The amount of 14C decreases as a result of radioactive decay

•After 5730 years, it is reduced to 1/2 its original amount (5g)

Years after death

# of Half-Lives

Amount of 12C (g)

Amount of 14C (g)

0 0 100 10

5,730 1 100 5

11,460 2 100

17,190 3 100

•Ratio of 12C:14C found in a fossil is measured and compared to the atmospheric ratio (10:1) in order to determine the age of a fossil.

Does carbon dating work for all fossils?•Carbon dating can be used for fossils up to about 50,000 years old•For older fossils, different isotopes are used

–i.e. 40K (potassium 40), ½ life = 1.3 billion years

40K #Half-lives Years

100 g 0 0

50 g 1 1.3 billion

25 g 2 2.6 billion

12.5 g 3 3.9 billion

Geologic Time Scale• The history of the earth

has been divided into eras, periods, and epochs

• Dividing lines are marked by major fossil shifts– Example: Beginning of

Paleozoic Era is marked by first time fossils of animals with hard parts are found in the fossil record

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etc.usf.edu

Continental Drift

• Knowledge of how landmasses have moved have helped solve biological puzzles• Matching fossils on two continents across

the ocean from each other• Plants and animals of Australia are so

different from those in other places

Major events in continental drift

• 250 million years ago, landmasses brought together into Pangaea

• Amount of shoreline was reduced• Lea levels dropped• Shallow water environments destroyed which were home to many

marine species• Interior areas have drier and more extreme climates• Species come into contact with species they would not have

otherwise• Lead to extinction of many species

• 180 million years ago Pangaea began to break up– Living things separated geographically

Mass Extinctions

• 5 or 6 distinct periods of mass extinction in last 600 million years

• End of Permain period 90% of species died• Extinction of dinosaurs 65 million years

ago caused by meteorite in Mexico?