Bivalves (molluscan phylogenies upside down?)

Ken Kinman kinman at HOTMAIL.COM
Mon Aug 12 16:40:33 CDT 2002


Dear All,
      Although it might seem presumptuous of me to challenge the views of
professional malacologists (yes, I'm going to "pick on them" now), I am
still trying to find hard evidence that disproves my long-held suspicion
that bivalves were the first molluscs (and that their headless and
radula-less condition is primarily primitive, rather than secondarily
derived).
      On the contrary, I am going to begin discussing evidence that runs
counter to presently held views (see below).  The long-held notion of
aplacophoran and polyplacophoran primitiveness may well be due to the
assumption (perhaps a very false assumption) that molluscs evolved from
"worms", and a perhaps fruitless fight over whether those worms were
flatworms, annelids, sipunculans, or some other vermiform group of
metazoans.  If this "worm" rooting is a false assumption (and I strongly
suspect it is), our understanding of mollusc evolution is being thwarted by
cladistic misrooting and circular reasoning.  Unfortunately, this could
further damage the reputation and potential benefits of cladistic analysis.
      (1) Bivalves have unequal cleavage, while all other molluscs
(including scaphopods) have equal cleavage.  Why would bivalves be the only
mollusc group to lose its head, lose its radulae, and switch to unequal
cleavage?  This makes little sense to me.
      (2) The developmental evidence being investigated by workers like
Robert Guralnick and David Lindberg (Evolution, 55:1501-1519) shows bivalves
popping up all over the molluscan tree.  They even go so far as to call
bivalves "putative molluscs".  That might be a little extreme, and I am not
advocating removal of bivalves from Mollusca, but I find this very
interesting, and I can certainly understand why someone might call bivalves
"putative molluscs".
     (3) Then there is the molecular evidence, which again often has
different bivalves popping up here and there with various molluscs,
polychaetes, and other spiralians.  A common reaction is that a
non-holophyletic Bivalvia can't possibly be correct, and the results are
explained away using long-branch attraction or any other excuse that will
return them to holophyly.  My assumption at the present is that Bivalvia is
probably not holophyletic, but that this phylogenetic scattering could very
well be due to paraphyly (not necessarily polyphyly).
     These kinds of things are why I think malacology has gotten into a rut
with regard to the evolution of molluscs overall, and that we have to take a
very hard look at bivalves as possible primitive molluscs.  This has
ramifications beyond malacology, since molluscs are a key group in our
understanding of the evolution of Bilateria as a whole.
              ----- Cheers,
                      Ken Kinman
P.S.  It is also interesting to note that bivalves show up very early in the
Cambrian, while chitons (polyplacophorans) don't show up until the Upper
Cambrian.  The lack of aplacophorans in the Cambrian could be blamed on very
poor fossilization potential, but if I am right, the aplacophorans may not
have existed in the Cambrian at all.



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