[Taxacom] Were bivalves the first molluscs to evolve?

[John Grehan]

So if the molecular phylogeny was 'correctly rooted' and got the same
answer you would accept it?

John Grehan

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On Thu, Nov 23, 2017 at 10:27 PM, Kenneth Kinman <kinman at hotmail.com> wrote:

> And yet there seems to be a clade (Streptoneura , Monoplacophora,
> Placophora, and Aplacophora) which are united by two fundamental
> morphological characters (not found in Euthyneura, Cephalpoda, or Bivalvia):
>
>
> Radulae become more complex,
> >>         with increasing numbers of
> >>         teeth per transverse row.**
> >>     MESENTOBLAST (4d) formation
> >>         begins to come after the
> >>         24-cell stage (usually 40-63).
>
>
> I do not believe that misrooted molecular phylogenies are more reliable
> than these morphological characters.
>
>
> ________________________________
> From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of David
> Campbell <pleuronaia at gmail.com>
> Sent: Thursday, November 23, 2017 8:38 PM
> Cc: taxacom at mailman.nhm.ku.edu
> Subject: Re: [Taxacom] Were bivalves the first molluscs to evolve?
>
> Molecular data indicates closer relationship between the extant
> monoplacophora and the polyplacophora than between the monoplacophora
> and the other univalve/bivalve forms.  Exact relationships between the
> various fossil "monoplacophora" and extant ones, and between various
> early Paleozoic multi-sclerite taxa and the modern polyplacophora and
> aplacophora are unclear, but there are likely early Cambrian
> representatives of the polyplacophra/aplacophora/monoplacophora group.
> Neither morphological nor molecular analyses have supported
> euthyneurans being basal to streptoneurans, nor for cephalopods to
> arise within gastropods.  Euthyneurans are well-supported
> morphologically, molecularly, and by timing of appearance in the
> fossil record as being relatively derived gastropods showing varying
> levels of detorsion, not partiallytorted ancestral gastropods.
>
> Molecular resolution of the relationships of major groups in the
> Mollusca remains relatively poor, but the extent of genomic coverage
> is increasing.
>
> --
> Dr. David Campbell
> Assistant Professor, Geology
> Department of Natural Sciences
> Box 7270
> Gardner-Webb University
> Boiling Springs NC 28017
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