Nancy Smith asks how you can distinguish modern and fossil shells.

The first thing to realize is that time is continuous, so all stages exist
between a mollusk at the moment of death and a shell that has been dead for
millions of years. There are plenty of meaningful events that can change a
shell, but not all of them are as sudden as death or burial. Some of them
can be very slow, like recrystallization. We had a big discussion about
what "fossil" meant a few months ago and I have no intention of encouraging
that to start up again. It does not have a definite answer. What most
people mean by "fossils", most of the time, are remains that are OLD
(generally older than 10,000 years), somewhat ALTERED from their original
state, and BURIED (the word "fossil" means "dug up"). Often they are the
remains of extinct animals, but this is not necessary.

Old? Radiocarbon testing can determine if a shell is about 50,000 years old
or less, since shells are made of calcium carbonate and contain quite a lot
of carbon. After about 50,000 years, the radiocarbon has decayed to the
point where not enough is left to measure accurately. I take it that you do
not have a mass spectrometer and about a hundred dollars per sample? And
you want the sample to be intact afterward? <sigh> Well, I don't have one
either. Other methods exist (amino acid racemization, for example), but not
available without a lot of technical equipment and knowledge.

Altered? You can look at a shell and see whether it has been "aged" by
dissolution, abrasion, drillholes, borings, breakage, cementation,
recrystallization, alteration to other minerals, etc. But some of these
processes are quick and can happen even in living shells. A shell washed
out of a Miocene deposit can sometimes be fresh enough to be mistaken for a
modern shell. If it belongs to a species that has changed little for
millions of years, it may be very difficult to determine that you have a
fossil.

Let's return to that general concept of "fossil": Old. Altered. BURIED.
Context is important in archaeology; it is just as important in
paleontology. A seashell buried in loose sand on the beach is not likely to
be a fossil. A shell found in road metal a hundred miles inland is likely
to have been transported; it is out of context. But a seashell buried in
firm sand a hundred miles inland is, particularly if it is extinct and so
are a lot of the other species along with it. Context is important.

Clear as mud? So is the concept of "fossil"!

Andrew K. Rindsberg
Geological Survey of Alabama