Gijs, Peter, Paul, Art...

Oops, I goofed! I was listening to a taxonomist, who will remain unnamed,
about subspecies, without checking it out. But no excuses. Yes, subspecies
ARE of course valid, as even  a quick peek into the ICZN code will show.

To be more than a synonym a subspecies must be consistently different from
the other subspecies. If they intergrade they are synonymous, no more than
ecomorphs or local variations. Thus gradual evolution from one species to
another does not involve anything that could be called subspecies; we must
arbitrarily draw a line and say "before this line all are Species A, after
the line all are Species B". We don't break such an evolutionary lineage
into a series of subspecies; they intergrade. In the case of punctuated
equilibrium a new species arises geologically suddenly from its parent
species, without apparent intermediate forms.

Chronostratigraphic or geographic distributions are not reasons for naming
subspecies (or species). If fossil shells of different ages are
indistinguishable they must be considered to be conspecific. Some species
have a very long timerange (Divaricella huttoniana from New Zealand springs
to mind, with a range of middle Eocene to Recent, 45 million years);
however some such long-lived species might be subdivided on anatomical, DNA
etc grounds if the animals of fossil specimens were available for study.
Without the soft tissues such studies cannot be made and we must go by
shell features alone. Most species seem to last only about 1.5 million
years before extinction or evolution into a new species.

Now please excuse me while I go wipe the egg off my face...

Andrew