Mollusc phylogeny (bivalves never had a radula to lose?)

[Ken Kinman]

Dear All,
     Pierre and Zdenek's abstract discussions are very interesting, but when
it comes to phylogenies at higher taxonomic levels, I think we are often
still so far from a resolution that cladistic analysis can be extremely
misleading when we get into a mental rut (convinced we are resolving a
phylogeny when we are really very far offbase).
     One general consensus that I think could very well be wrong and very
damaging is that bivalve molluscs evolved from molluscs with a radula and
"head" (their absence in bivalves widely believed to be reversals).  I have
long believed that this is probably totally wrong and is preventing us from
a true understanding of molluscan evolution (and the polarity of a lot of
other character transformations).  Having "wormy" solenogasters and chitons
at the base of Mollusca has been intuitively pleasing, but I think that is
going to change once it is forcefully challenged.
      I did not place the bivalves at the base of molluscs in my 1994
classification, because I simply did not have time to even begin testing my
intuition on this subject, but now I plan to begin examining the pros and
cons more deeply.  Any comments onlist or offlist are welcome.  This is
obviously going to be controversial, but I would like to explore this
possibility before getting pulled back into arthropod phylogeny (which is a
lot messier, but at least there our lack of understanding is at least more
widely acknowledged).
      Cladistic analyses (e.g. Shultz, 1990) that show scorpions embedded
within an arachnid class have been and will continue to be challenged.  I
think it's finally time that cladistic analyses showing bivalves embedded
well within Mollusca (which are largely taken for granted) must also be
challenged.  I'm getting my mollusc "folder" out this weekend, and see if we
can't get a preliminary discussion started on this matter.
            ---- Sincerely,
                     Ken Kinman
P.S.  I obviously also think prokaryotic evolution and phylogeny has
suffered these same sort of high-level polarity problems for over 20 years,
but I'm not going to get back into arguing about those problems until more
whole genomes are sequenced.  Just don't get too attached to Three Domains
or those "presently" popular Trees of Life (really "Trees of Ribosomes")
based on them.


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