Download Fossils: a Glimpse into Earth`s History

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http://etc.usf.edu/lit2go/contents/4100/4118/4118_txt.html
Fossils:
A Glimpse
into Earth’s
History
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 fish
Sediment coming
from river
Fish skeleton slightly
covered by sediment
More recent sediment builds up.
Older sediment
becomes rock.
Fish skeleton fossilized
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 Fossil
Your parents wouldn’t like it: they’re way too old for you.
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www.evolution.berkeley.edu
•
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
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 HalfLives
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
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• 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
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?