The basic definition I learned was that a fossil is over 10,000 years
old and an old shell, etc. isn't.  No particular chemical or physical
change is necessary to make it a fossil, though there is probably
always some degree of alteration over such time scales.  This is
basically since the glaciers last started significant melting.

Fossil shells are under essentially all of Florida.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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