What living species of mollusk has the longest fossil record? That is an
interesting question that can be examined from several points of view.

Some shells are so simple that several populations that do not reproduce
together have much the same shell. Since they do not reproduce together,
they are considered separate species and this can be tested by DNA
analysis. More detailed study of the shells often shows subtle differences
between these hidden species, but not always.

Also, some species have such variable shells that forms overlap between
species, as in the oysters.

The result is that living populations can be separated into species (true
lineages of breeding populations, more or less) in a way that fossil
populations cannot, even in principle. So we may reasonably suspect that a
long fossil range of a simple or highly variable shell represents several
real species, but cannot test or prove this speculation without new kinds
of evidence that are not currently available.

Thus, we have extremely long-ranging genera like Nucula, Nuculana, Solemya,
all of which are hundreds of millions of years old. I suspect that the
genetic code of these organisms has changed with time, if only to resist
new diseases. Still, the shell forms are remarkably stable, and the laws of
statistics do support the idea that some taxa should be much longer-ranging
than others. Also complicating this picture is the fact that simple animals
really do survive major extinctions better than complex ones, so you can
expect Nucula to be a survivor.

When we look at the older literature on mollusks, we find many more
long-ranging genera. But as time goes on, people examine the details and
realize that the genera should be split up. So genera that formerly had
long ranges, like Nautilus and Ostrea, now are split into tens or even
hundreds of genera that represent whole families and orders of mollusks.
You will still find paleontologists referring to Ostrea in Cretaceous
rocks, for instance, but this represents a situation where the researcher
does not know what genus to place the shells in, and is merely using
"Ostrea" as a temporary holding tank.

What we see in the fossil record is that some species disappear and others
appear. There are always some new appearances and extinctions, even in the
quietest of times, though of course there are also mass extinctions. So if
you went backward through time, you would see fewer and fewer familiar
species. In the 19th century, Charles Lyell developed the idea that you
could determine the age of a fossil deposit by the proportion of modern
molluscan species it contains. This is the original concept of the Cenozoic
epochs, which were then modified a bit and can now be listed from oldest to
youngest as Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene,
and Holocene. It turned out later to be a better idea to base the epochs on
particular sequences of rock layers at particular "type sections", but this
was the original concept and it was based on Mollusca!

In Pleistocene rocks, almost every shell is of a familiar species, though
not always of a current subspecies. In Pliocene and Miocene rocks, many of
the shells are indistinguishable from those of modern species, and most
genera are familiar. In Oligocene and older rocks, there might be one or
two species out of hundreds that cannot be distinguished from those of
modern kinds, and they typically belong of one of those simple or variable
forms mentioned above, like Calyptraea aperta. I don't know of any
Cretaceous forms that are still extant, but would not be very surprised if
someone has an example.

So, how long can a species having a complex shell last without substantial
change? Let's say a few tens of millions of years.

Incidentally, abalones and Nautilus were not very good guesses. The
abalones have a very imperfect fossil record, because they live on rocky
coasts where shells tend to be pounded into bits and then eroded away. And
the living species of Nautilus, so far as I know, have no fossil record at
all. Of course, modern Nautilus can still be called a "living fossil"
because it is very similar to fossil forms, but it is not identical to any
of them.

Andrew K. Rindsberg
Geological Survey of Alabama