[Taxacom] Synapomorphies and Centuries of malacology

[Kenneth Kinman]

Dear All,
      Today I was thinking about my frustration that almost nothing has
appeared in recent years to challenge traditional views of Molluscan
classification and phylogeny.  It might seem strange that I often defend
traditional classifications against recent cladifications, and yet I
criticize malacology for clinging to traditional notions of
relationship.  But if you really want to follow a moderate path,
sometimes you have to fight battles on sides at the same time (depending
on which taxa one is studying).  In other words, there is plenty of
excess in both extremes: (1) clinging to outmoded tradition; and (2)
jumping on a popular new bandwagon.          
        I believe that malacology often suffers from both.  It clings to
the outmoded idea that aculiferan ("worm-like") molluscs are probably
primitive.  It's a holdover from the "Vermes" concept of Linnaeus that
seemingly primitive, wormlike forms are always primitive.  And those who
cling to that notion and also have a compulsion to strictly cladify
Molluscan taxa are at a double risk of producing unstable
classifications.  Synapomorphies touted for so-called clades like
"Conchifera" are most-likely just simplistic ways to justify and
perpetuate old-fashioned ideas about what constitutes the Ur-mollusc.
This is also further perpetuated by molecular systematists who choose
"vermiform" taxa as out-groups in their analyses.  Choose an
inappropriate out-group and you get non-sensical results.        
     When bivalve molluscs (Pelecypoda) are actually shown to be
paraphyletic (or sometimes even polyphyletic) in molecular analyses, the
common reaction is to explain it away, perhaps because it conflicts with
their faith in morphology (not realizing that the morphological
synapomorphies are rather simplistic or subject to
homoplasies---especially reversals).  The majority of those who teach
and research malacology continue to teach their college students that
"Bivalvia" is monophyletic, but only toss out a few morphologies that
could easily be subject to loss or other reversals.  No wonder they
can't agree what is the sister group of Bivalvia.  Have they considered
that maybe different subgroups of bivalves are actually two or more
different sister groups of other major molluscan taxa.      
      My question is this.  Why does a bivalve mollusc have to be
regarded as more derived than a vermiform mollusc (such as the
aculiferans)?  It's much like my question about arthropods.  Are
tardigrades (or even some crustacean taxa) necessarily more derived than
nematodes?  In my opinion, the notion of "Vermes" and its primitiveness
continue to haunt us to the present day.  Incorporate
vermiform-connected synapomorphies into your phylogenetic thinking, and
you are likely to be prone to simplistic thinking and unstable
classifications when it comes to Phylum and Class level taxa of
invertebrates.  In the case of molluscs, the instability could easily
extend down to ordinal level.                             
          ----------Ken Kinman