[Taxacom] what is a radula? (what is a mollusc?)

[Kenneth Kinman]

Dear All,
       There seems to have been a great debate over fossils like
Odontogriphus and Wiwaxia in recent years, whether their "jaws" are
radulae, and thus whether or not they are molluscs.  But as early as
these fossils are, I suspect they are actually a little too late to tell
us what stem-group molluscs actually looked like.  I'll go even further
and suggest that their even earlier possible relative (Kimberella)
doesn't help much either.  The real stem-group molluscs probably not
only had unmineralized "shells", but worse yet, such "shells" were
probably tiny.     
        The reason fossils like Wiwaxia and Odontogriphus (as well as
halkieriids, etc.) are so popular in discussions of mollusc origins is
that they are large enough to be more easily found and studied, unlike
the tiny creatures which stem-molluscs most likely were.  And whether
the "radula-like" structures of Odontogriphus and Wiwaxia are molluscan
radulae, polychaete jaws, or even something in between (which wouldn't
surprise me at all), it may be irrelevant to mollusc origins if such
radulae and jaws are all highly derived compared to what earlier
molluscan jaws were actually like.   
        I know malacologists in general don't seem to like my "radical"
idea that the earliest radula was just a single tooth (for puncturing
and feeding on single algal cells), but it still makes more sense to me.
The small forms which bore these single-toothed radulae would not
fossilize readily, except their single tiny tooth (which would be easily
over-looked or misinterpreted).  In the meantime, their bigger
descendants with bigger and multiple teeth naturally get all the
attention, and perhaps wrongly making Mollusca and Polychaeta appear to
be sister groups.    
         Actually, seems more likely to me that derived molluscs with
derived radulae (like those in Odontogriphus and Wiwaxia) are perhaps
just odd-ball, stem-polychaetes.  If so, they tell us little about
stem-molluscs and assumptions that they do is diverting us from a real
understanding of mollusc origins.  Then there is the even more vexing
issue of how other "worm" taxa, like Nemertina are related to them.  Are
nemertine worms really more closely related to them or to brachiopods?
Even the molecular evidence is still so sketchy that different
researchers come to very different conclusions.           
        Whether you put your faith more in morphological or molecular
data, it's still based on sketchy evidence.  So don't be surprised if
the phylogeny of Mollusca and its relatives becomes radically
reinterpreted in the coming years.  It could come from lots more
molecular sequences or from some lucky new fossil finds, but either way,
this will no doubt begin to unlock the mystery of how the various
bilaterian phyla are actually related to one another.  Ecdysozoa
(whether it is a true clade or not) was just the beginning of
unravelling this phylogenetic mess.     
        Of all modern phylogenies, molecular or morphological, I would
guess that those of Mollusca are by far the most poorly understood and
poorly-rooted of all major groups of organisms.  To assume that Phylum
Mollusca is holophyletic (i.e., not giving rise to certain other
"Phyla") is the probably still the most unrecognized fallacy in the
study of metazoan phylogeny.        
        -----------Ken Kinman