Fossils: A Glimpse into Earth

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Fossils A Glimpse into Earths History
http//etc.usf.edu/lit2go/contents/4100/4118/4118_
txt.html
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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

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Sediment coming from river
Living fish
Fish skeleton slightly covered by sediment
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More recent sediment builds up.
Older sediment becomes rock.
Fish skeleton fossilized
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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

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Where is the oldest rock?
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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?v75Wn38jADB8

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Dating a FossilYour parents wouldnt like it
theyre way too old for you.

• Relative age by looking at a fossils 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

www.evolution.berkeley.edu
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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

Isotope Half-Life
Carbon-14 5730 years
Potassium-40 1.3 billion years
Uranium-235 4.5 billion years
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• 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 simplicitys sake,
  lets say it is 10 times as abundant).

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• At moment that an organism dies, it stops taking
  in carbon, and has a set ratio of 12C14C (101).
  
• (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
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• Ratio of 12C14C found in a fossil is measured
  and compared to the atmospheric ratio (101) 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
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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

etc.usf.edu
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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

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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

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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?