The study of the changes that occur to bodies after death is called
TAPHONOMY. If you search for "taphonomy" on the Web, you will find quite a
few references to the subject, including a research center at the
University of Arizona, and whole bibliographies in researchers' Web pages.
 
The subject is a large one, and I hope to write on it at greater length
when I have more time to spare. But to answer the question at hand,
seawater is indeed saturated with calcium carbonate in warm water and
undersaturated in cold water. This means that shells dissolve readily
outside the tropics, and also in the deep sea, which is cold even at the
equator. Shells may begin to dissolve during the lifetime of the owner if
the periostracum is broken, making it difficult to collect perfect
specimens of some species.
 
The shell deposits of the ancient past were largely laid down in tropical
waters, but not all. Some were buried so rapidly, and thus sealed off from
corrosive water, that they remain intact today. The continents have drifted
through the ages, and sea level has bobbed up and down. North America was
lying on the equator during the Ordovician and sea level was high, so much
of North America today is cloaked by Ordovician limestone, including Art
Weil's beloved Cincinnati.
 
Shells do not last forever even in the tropics. Almost all are broken or
drilled by predators, chemically eroded by burrowers, abraded by currents,
and/or bored by organisms that use the shell as a convenient place to live.
A great deal of shell destruction can be attributed to these very
interesting boring organisms, which include algae (Have you ever noticed
green-stained shells?), fungi, sponges, polychaete worms and many other
kinds of worms, bryozoans, barnacles, bivalves... the list goes on and on,
and they are especially diverse and abundant in the tropics. Here is a
primer on how to recognize them in your shells:
 
Algae and fungi: The borings are too small to see with the naked eye, but
on broken surfaces you may notice a discolored, porous zone on shell
surfaces. Living algae may be green.
 
Sponges: Network of round holes connected by narrow tunnels. Sometimes the
networks are branched; sometimes they cover the shell. Some living species
of Cliona (not Clione, that's a tunicate) are bright yellowish orange.
 
Polychaete worms: Polydora and its relatives bore a long U following the
shell surface.
 
Bryozoans: Delicate comma-shaped openings arranged in branched patterns.
Often quite beautiful.
 
Barnacles: Acrothoracican barnacles live in small slots, not arranged in
branched patterns. Usually a bit larger than bryozoan borings, and solitary
rather than colonial. The bases of larger, more familiar balanid barnacles
can also leave scars on shells.
 
Bivalves: Teardrop-shaped borings; some with double opening to accommodate
the siphons.
 
All of these borings can be recognized in fossils as well as in living
organisms. The study of borings is part of ICHNOLOGY (the science of modern
traces and ancient trace fossils), so ichnology and taphonomy are closely
related!
 
Andrew K. Rindsberg
Geological Survey of Alabama