Dear Kévin,

The names Bivalvia, Lamellibranchia and Pelecypoda are indeed often
synonimized, causing major troubles. I'm not sure if this was already discussed
in the ICZN, nor if the problem is already cleared by this commission.
Although I will try to answer your question:

1. I think that the Class Bivalvia is a polyfyletic group (as it is seen in
broad context), meaning that (like you told us) it includes Classes
Lamellibranchiata, Brachiopoda and Ostracoda. "Polyfyletic" means that the
grade which we are dealing with includes taxa that share
certain 'characteristics' which are not valuable to obtain a correct
phylogenetic tree. In this example this character is the fact that either
Lamellibranchiata, Brachiopoda and Ostracoda have a BIVALVED shell. At the very
moment we are convinced that these taxa don't belong to the same clade
(Lamellibranchiata are a class in Mollusca, Phylum Brachiopoda are lophophorate
not symmetrical bivalved organisms and Ostracoda are a class of
marine/estuarian crustaceans with a bivalved carapax).
2. "Parafyletic" means that the discussed groups are characterised by the lack
of certain advanced features or by a plesiomorfism (a feature in its unadvanced
state, not evolved). For example: Gymnosperms and Pteridophytina can be grouped
in a single grade because they do not have flowerlike structures.
3. In phylogenetic analysis we try to find monophyletic groups (called clades)
based on genuine features (i.e. no plesiomorfisms, no convergent
evolution, ...).
4. Like Dr. Campbell says: Linnaeus used the name early, therefore you can
pretend that, after the nomenclatural priotity rule, we have to use this name.
But, like Dr. Campbell mentionned too: the name Bivalvia was also used for
certain Brachiopods by our Carolus. The name Lamellibranchiata has the same
problems: not only bivalved molluscs are equiped with lamellar guils.
5. I like to use the name Pelecypoda (but, who am I to say this?). Although not
all 'bivalved molluscs' do have an axelike foot, it seems the most appropriate
name to me. The axelike foot is normally a typical feature; sometimes it is
absent but it could be proved that it was secondary reduced. After my opinion
this name characterises the best the monofyletic group of molluscan bivalves.

Hopefully I helped you some further, many regards,

Thomas


> Hello Conchlers,
>
> I received this letter from a gentleman in France and was wondering if
> anyone can help!
> Avril Bourquin
> _______________________________________________________
> "I've just a quetion for you. Please excuse my english , I'm french and
> I
> don't speak well english.
> In your writting on "class bivalvia"
>  http://manandmollusc.net/advanced_introduction/moll101pelecypoda.html
> )you
> wrote that bivalvia, pelecypoda and lamellibranchia  are the same.
> But I've heard that bivalvia is a paraphyletic ( I don't know if it's
> the
> same in english) group which contains lamellibranchias, brachiopodes
> and
> ostracodes.
> Can you explain me the difference
>
> Thanks a lot
>
> Kévin Lelorieux  [log in to unmask]
> Faculty of Science of Poitiers ( France)
>