Origins of Cephalopoda

[Ken Kinman]

Dear All,
      Although my studies of gastropods are still in the earlier stages, it
has become increasing clear to me from which part of the gastropod "tree"
the cephalopods evolved.  They certainly are NOT sister groups as
conventional wisdom maintains.
      Whether you believe traditional phylogenies (archaeogastropods evolved
into mesogastropods), or if you believe my hypothesis that mesogastropods
evolved into archaeogastropods----either way, the Class Cephalopoda appears
to have split off during the transition between these two groups.
     Among the transitional forms are Neritimorpha, Architaenioglossa, and
perhaps family Neomphalidae.   Most likely Cephalopoda split off between
Neritimorpha and Architaenioglossa, although they seem closer to the former.
  I based this on various morphological characteristics at first, but being
a non-malacologist I thought I must be missing something that would indicate
that they were perhaps just convergences.
     Well, convergence pretty much evaporated as an obstacle when I learned
of two very important additional pieces of evidence.  First, that Colgan et
al., 2000, found molecular evidence that connected neritimorphs with
Nautilus (although they also had neogastropods in the same clade).  Second,
and this was the "clincher", at least some neritimorphs have inner
partitions (proto-septa) in their shells (although they are usually
reabsorbed).
     It seems pretty clear to me that a neritimorph (or a very closely
related clade of gastropods) stopped reabsorbing such internal partitions,
and began using them as permanent (and more complete) septa that could
increasingly be used for buoyancy purposes.  The perfection of cephalopod
septation (i.e., shell "chamberization") and controlled buoyancy could have
been rather gradual during the Cambrian, but once achieved, cephalopod
diversity could really take off around the beginning of the Ordovician.
      Hopefully cephalopod specialists can take it from there and explore
this hypothesis in more detail, but the old idea that Cephalopoda and
Gastropoda are sister taxa seems to be another cladistic "simplistication"
in need of a strong challenge.  The question remains what gastropod taxa may
have paraphyletically give rise to other molluscs clades (Scaphopoda; and
one of more clades of "spiculates") and also to non-mollusc clades (such as
gnathiferans and platyhelminths).
      If it is as massively paraphyletic as I think it is, Gastropoda is a
grade that is much in need of splitting.  But you wanna bet that many
gastropod cladists will resist this?  Just you watch, as some will cling to
the one-character classification of gastropods as a clade united by torsion
(plus some plesiomorphies thrown in for good measure, although they will be
labelled synapomorphies).  Half the time, strict cladists aren't there when
you really need them.  How frustrating.
   ----- Less than cheerful tonight,
                 Ken Kinman

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