Could Gastropoda be polyphyletic?

[Ken Kinman]

Dear All,
     I am now seriously entertaining the possibility that Gastropoda is not
paraphyletic, but is actually polyphyletic.  Even I find this thought rather
disquieting, so I won't be surprised if this causes some gasps, groans, or
even shrieks of incredulity----it's just something to think about.
     The neritimorph, Neritodryas dubia, closes its operculum with a sort of
"hinge" made of a forked apophysis and a corresponding "tooth".  I am
proposing that this may actually be homologous with the similar sort of
hinges found in bivalves.
     If so, it supports my long-held suspicion that the operculum is
homologous to one shell of a bivalve, while the main snail shell is
essentially a conically drawn-out (and coiled) version of the other bivalve
shell.  Sort of like pair of cymbals, where one of the cymbals is greatly
stretched out to form something like a simple "french-horn" shape.  The same
might be true of a Nautilus shell and its hood.
     ANYWAY, if some bivalves evolved into neritomorphs (and various other
streptoneuran snails), while other bivalves evolved into bivalved
sacoglossans (and other euthyneuran snails)------well, we would have a
polyphyletic Gastropoda.  Unless, of course, sacoglossans and neritomorphs
are far more closely related than we realized (which would be interesting).
I haven't sorted out all the possibilities, but it's something I thought I
would throw open for discussion.
           -------- Cheers,
                        Ken Kinman
P.S.   By the way, this hypothesis might also help explain why Rosenberg's
(1994) molecular analysis placed neritoids among the clams.  Maybe neritoids
only "appear" to be long-branched due to an extremely skewed perspective of
molluscan phylogenies in general.  And even more importantly, this might
help explain why gastropod phylogeny and classification has been so
controversial for so long.


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