The late Lyman Toulmin, one of the grand old stratigraphers of the U.S.
Gulf Coastal Plain, used to say, "Stay away from young oysters!" Mature
oysters are difficult enough to identify; young oysters, even more so. But
a current expert on fossil oysters, Nikolaus Malchus (Barcelona, Spain),
finds that he has to study the earliest, larval shells of oysters in order
to understand their phylogeny (ancestry). The youngest shells preserve some
archaic features that are lost in mature shells. They make very good
indicators of which family a species belongs to.
 
This is true of other mollusks as well. There is considerable interest,
among malacologists who are interested in evolution, in investigating the
larval stages of fossil gastropods. Occasionally, a genus that was long
placed in one family has to be removed to another on this basis. Of course,
as we all know, it is not easy to find a modern snail with a perfectly
preserved apex, let alone one that is 40 million years old, but enough of
them have been discovered to make the studies possible.
 
Andrew K. Rindsberg
Geological Survey of Alabama
"Oysters R Us"