Re: How do scientists know if it is a freshwater or saltwater fossil?

Kelly:

This is an interesting question and it has many answers. The simplest way
to identify a fossil as freshwater is to recognize it as identical to a
living species that lives only in freshwater. Failing this, it may belong
to a group that has living representatives, all of which live in
freshwater. Or, the fossil may always be found in association with some
other fossils that are already known to be found only in freshwater.
Sometimes we know enough about the rocks that contain the fossils to
identify the deposit as a freshwater deposit. For instance, if the fossils
are found in the bed of an ancient river, at a point far from the river
mouth, they are likely to be freshwater fossils. Fossils found in lakes are
commonly freshwater fossils, though some lakes are salty. This method is
not 100% reliable so one must use good judgement. For instance, coal is a
freshwater deposit, but many coals form near the sea. The gradual back and
forth movement of the shoreline can result in interlayering of coals and
marine shales. The best way to answer this kind of question is to use
multiple lines of evidence. That way, if one piece of evidence is
misleading, the other pieces of evidence will reveal that fact. This method
works as long as more than half of your lines of evidence give the expected
results. 

One of the most reliable methods of identifying fossils as freshwater only
works if the specimens have not been changed too much (for example, turned
to silica or pyrite). Usually, it is obvious that a fossil is pristine
(good as new) because the microscopic structure is the first thing to be
lost during changes that affect the fossil underground. If the
microstructure looks normal, then the isotopic ratios (for example, amount
of light and heavy carbon atoms) and trace element geochemistry (for
example, amount of manganese atoms in the fossil) of the fossil are like
they were when the organism was alive. For example, seashells can contain
different amounts of light and heavy oxygen atoms depending on whether the
shells formed in salt water or freshwater. Trace elements like Boron and
Sodium also are present in different amounts in shells formed in salt- and
freshwater. Sodium is part of salt (sodium chloride) and it is very common
in salt water. This kind of analysis can be expensive and difficult, but it
has the potential to show a lot about how the fossil formed (and what
happened to it after burial, if indeed it has been altered underground). 

Paragraph one is pretty much common sense. Geochemistry, the subject of
paragraph two, is very complicated, but you can learn a little about it by
reading an undergraduate geology textbook (a modern one; old ones don't
have much about geochemistry because it is a new field). I know that you
are still pretty young, and you might have difficulty with a college-level
textbook. Another possibility is to talk to a local geochemist. If you can
find one who is good at explaining his or her work, you may learn a lot!

Good luck,

David Kopaska-Merkel
Geological Survey of Alabama
PO Box 869999
Tuscaloosa AL 35486-6999
(205) 349-2852
fax (205) 349-2861
www.gsa.state.al.us