David Campbell wrote,
"True, although the extinction rate for lower and mid-Pliocene species in
the eastern U.S. is exceptionally high.  In this particular case, those
researchers I know of who have looked at it agree that there are enough
differences (rib number, size, inflation) to warrant separation.  These do
not include the individual to whom you alluded, although with his
exceptionally low standard for recognizing new species, he would doubtless
agree that the two are distinct."
 
Indeed. And other people are free to knock these species down to subspecies
rank if they think it is warranted. Most biostratigraphers applaud the
subdivision of lineages into species or subspecies, since it makes their
job of distinguishing the ages of strata easier.
 
Though, when I see a paper that claims that a single bed yields six closely
related species that no one else can tell apart, I raise an eyebrow. It
does happen, of course, e.g., with species of modern Cerithium; but usually
it's a sign of oversplitting. It makes work for the next researcher, by
forcing him or her to measure the length, height, etc. of hundreds of
shells just to prove that they all belong to one variable species. All
that, just to return the taxonomy to where it had been before the splitter
did his work.
 
Andrew K. Rindsberg
Geological Survey of Alabama