what is a fossil

["Bonnie Blackwell, x 3332" <bonn@qcvaxa.acc.qc.edu>]

you have asked the 64 k$ question there.   there are currently two
working definition as to what constitutes a fossil:
1.  remains from an animal or plant that have been remineralized.
2.  any dead animal or plant

obviously, these are two ends of a very long very complex spectrum of
processes that turn fossils type 2 into type 1 (you might think of
them as sensu stricto and sensu lato).  I have personally found
remains that fully qualify as #1 that are less than 5 YEARS OLD!!
SOME AS YOUNG AS 3 MONTHS!!! so age plays little role.  the speed
at which fossilization (sensu stricto i.e. remineralization occurs)
can be very rapid in some environments, such as saline lakes.
if you examine the fossil record, preservation is enhanced in areas
where the fossilization is very rapid. 

conversely, there are 200,000 year old fossils in which the 
bone mineral and its organic component are 98% pristine.  
so again age is not a guideline.  

no should remineralization alone be a factor.  there are some
40 different processes that affect animal bones and teeth (the stuff
i am most familiar with) during their fossilization.  remineralization
may be the first step, the last, anywhere in between, or not occurring
in many remains that most geologists and archaeologists would
recognize as being fossils.  in molluscs for example, few molluscs
younger than 1 Ma show any remineralization, and many as old as
10 Ma still have little remineralization. 

i personally like definition #2. 
b
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Bonnie A.B. Blackwell,                          bonn@qcvaxa.acc.qc.edu
Dept of Geology,                                off: (718) 997-3332
Queens College, City University of New York,    fax: (718) 997-3299
Dept of Earth \& Environmental Sciences,        messages: (718) 997-3300
The Graduate Center, CUNY,                      
Flushing, NY, 11367-1597, USA