Fossil Evidence

1
Fossil Evidence
Fossil Formation
Other Fossil Types
Dating Fossils
Permineralization
Cast Fossil

• Relative Dating
• 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.

• 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 .

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.
http//www3.interscience.wiley.com8100/legacy/col
lege/levin/0470000201/chap_tutorial/ch04/images/le
04_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

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'.
Organic Preservation

• 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.

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.ambermine.com/images/spiders/5sm.jpg
http//www.discoveringfossils.co.uk/Whatisafossil.
htm
http//evolution.berkeley.edu/evosite/lines/images
/strat_column.gif
Deb Whittington Mark Davidson, 2007