[Taxacom] Taxacom Digest, Vol 55, Issue 11

[Ronald Guimarães]

No it isn't a mollusc, but it's the tipe of tongue of molluscs. This
tongue have teeth that can scrape the surface of the animal or plant species
which provide food for molluscs



2010/10/12 <taxacom-request at mailman.nhm.ku.edu>

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> Today's Topics:
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>   1. what is a radula? (what is a mollusc?) (Kenneth Kinman)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 11 Oct 2010 23:14:57 -0500
> From: kennethkinman at webtv.net (Kenneth Kinman)
> Subject: [Taxacom] what is a radula? (what is a mollusc?)
> To: taxacom at mailman.nhm.ku.edu
> Message-ID: <18207-4CB3E0C1-4821 at storefull-3253.bay.webtv.net>
> Content-Type: Text/Plain; Charset=US-ASCII
>
> 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
>
>
>
>
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> End of Taxacom Digest, Vol 55, Issue 11
> ***************************************
>



-- 
Ronald R. Guimarães, M. Vet, M. Sc.