[Taxacom] Molluscan phylogeny (overview)

[Kenneth Kinman]

Dear All,
       Someone reminded me that bivalves are technically not really
"headless".  To be more precise, what they lack is a pharynx (buccal
apparatus).  So the question really is, did bivalves : (1) lose their
pharynx; or (2) did they never have a pharynx (buccal apparatus) in the
first place.                
       I obviously lean toward the second view (which is the opposite of
what most malacologists believe).  Anyway, I more precisely reflected
this in my 2002 cladogram (which I reposted here last night) at step no.
2:  "Buccal development begins".  I guess we should all be precise, and
refrain from calling bivalves "headless".  Perhaps "pharynxless" would
be better.                                  
      MAJOR QUESTIONS.  But getting back to the issue of the radula.
Why would all bivalves completely abandon the very successful strategy
of feeding with a radula?  And doesn't it seem strange that there isn't
any trace or vestige of a radula in any bivalves?  It makes more sense
that they never had one in their ancestry, that a very simple radula
evolved at the base of Glossophora and then gradually became more
complex in various lineages. 
      Same with the torsion problem.  Torsion would begin gradually (and
didn't become extreme except in the Streptoneura gastropods).  No need
for an unparsimonious development of extreme torsion followed by
reversals.  The neural development of molluscs would follow this same
trend---euthyneuran coming first and streptoneuran coming next.  I know
that all this is the opposite of what most malacologists believe, but it
makes more sense.  In particular, it would solve a lot of problems in
gastropod evolution and systematics.  There are also much broader
implications for the evolution of Bilateria, origins of arthropods, etc.
          -------Cheers,
                        Ken


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