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Fossil Evidence
Fossil Formation
Other Fossil Types
Permineralization
Cast Fossil
Phase 1 – Death
Having died, the ammonite slowly
sinks to the sea floor. Scavengers
feed on the fleshy body of the
creature, and after only several weeks
all that remains is the shell.
Phase 2 - Deposition
Several months after death the shell
gradually becomes covered with silt and
sand. These layers continue to build,
providing a shield around the shell and
protecting it from damage. Time
continues to pass and more and more
layers are deposited. After a few hundred
years the shell is several feet beneath the
surface.
Phase 3 - Permineralization
Gradually the chemicals in the shell
undergo a series of changes. As the
shell slowly decays, water infused with
minerals passes through it, replacing the
chemicals in the shell with rock-like
minerals (Calcite, Iron or Silica). This
process is scientifically known as
'permineralization'.
Over millions of years the original shell is
completely replaced by the minerals and
what remains is a rock-like copy of the
original shell. The fossil has the same
shape as the original object, but is
actually rock. This process also results
in loss of original colour.
http://www.discoveringfossils.co.uk/Whatisafossil.htm
Deb Whittington & Mark Davidson, 2007
Dating Fossils
Relative Dating:
Some fossils (called cast fossils)
are not the remnants of organisms
but rocks that have formed around
the impressions of the organism in
the sediment.
 A dead organism that is captured
in sediment completely decays, but
leaves an empty mold in the
sedimentary rock.
 Water filled with minerals fills the
mold.
 The minerals crystallize, forming
a cast of the ancient organism .
Younger strata (layers of rocks) are deposited on top of older
ones. Layers which are deeper are older.
Absolute Dating:
Gives scientists the approximate date an ancient life form lived.
 Radiometric dating, the measurement of certain radioactive
isotopes in fossils or rocks is the method most often used.
 Because radioisotopes break down at a constant rate, the
ratio of the parent isotope to its break down products found
in rock or fossils can be used to determine their ages.
http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch04/images/le04_05.gif
Trace Fossil
Another type of fossil (called a
trace fossil), consists of a footprint,
animal burrow or other impression
left in the sediment by the activities
of animals.
 Trace fossils give paleontologists
information about the activities of
ancient life forms.
Iguanodon track fossil: http://storeforknowledge.com/images/IMG_1100.JPG
Organic Preservation
http://www.ambermine.com/images/spiders/5sm.jpg
If an organism happens to die in a
place where bacteria and fungi
cannot decompose the corpse, the
entire body, including soft parts
may be preserved. Examples
include:
 An insect stuck in a drop of
resin from a tree 30 million years
ago.
 Mammoths frozen in ice.
 Saber-toothed cats etc.
preserved in La Brea Tar Pits.
 Prehistoric humans preserved
in acid bogs.
http://evolution.berkeley.edu/evosite/lines/images/strat_column.gif