[Taxacom] Were bivalves the first molluscs to evolve?

[John Grehan]

This nicely points to the generally insoluble (in principle where both are
robust) problem of conflicting DNA sequence and morphological trees.

John Grehan

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On Wed, Nov 22, 2017 at 12:32 PM, Kenneth Kinman <kinman at hotmail.com> wrote:

> Dear all,
>
>        I was reading an article on the evolution of brains, and the author
> says "Meanwhile, bivalves possess just simple nerve nets. You might think
> that bivalves came first, while the cephalopods emerged later, because
> simple brains should, in theory, predate complex brains."  But he then says
> that DNA tells a different story.
>
>        I'm still of the opinion that cladograms of Phylum Mollusca would
> very well be mis-rooted, and that bivalves (with their simple nerve nets)
> did come first.  Below is the cladogram that I presented here on Taxacom
> way back in August 2002.   And below that the characters upon which this
> branching pattern is based.   Are there any obvious errors in this
> branching pattern, and if not, could it be that molecular phylogenies for
> Mollusca are still mis-rooted?
>
>                        ----------Ken
>
> ________________________________
>
>
>
> >>
> >>\________ Protobranchia
> >>1\
> >>   \________ Lamellibranchia
> >>  2\
> >>     \___________ EUTHYNEURA
> >>       \               \________Cephalopoda
> >>      3\
> >>         \___ STREPTONEURA (real torsion)
> >>        4\
> >>           \_______ Monoplacophora
> >>          5\
> >>             \______ Polyplacophora
> >>            6\
> >>               \_______ Aplacophora
> >>
> >>
> >>1.  Veliger larvae evolve.**
> >>    Gills become lamellate.
> >>    Crystalline style evolves.**
> >>
> >>2.  Simple radulae evolve.
> >>    Thus abandon filter-feeding.
> >>    Cleavage becomes unequal.
> >>    Buccal development begins.
> >>    Adductors reduced from 2 to 1 (or 0).
> >>    Unskeletonized gills?
> >>    Shells more "opisthobranch"-like.**
> >>    Gastropodan muscle fine structure.**
> >>
> >>3.  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).
> >>
> >>4. "Segmentation" arises.**
> >>     8 pairs of pedal retractors.
> >>     More than 6 pairs of dorsoventral
> >>          muscle bundles (DVM).
> >>     Over two pairs of ctenidia.**
> >>     Radula bolster vesicles increase.
> >>
> >>5.  Single conch ---> multiple plates.**
> >>     Spicules develop (7 rows**).
> >>     Even more pairs of DVM (is not
> >>        fission as likely as fusion!?)
> >>     Musculature develops beneath mantle.
> >>     Head appendages lost.
> >>     Statocysts lost?
> >>     MESENTOBLAST formation occurs
> >>         after 63-cell stage.
> >>
> >>6.  Radular reduction.
> >>     Plates lost.
> >>     Muscular foot reduced.
> >>     Numbers of ctenidia reduced.
> >>     More than 16 prs. of DVM (certainly
> >>           no reversal here).
> >>     Gametes usually exit via pericardia
> >>        (those which don't are probably
> >>         basal aplacophorans).
> >>
>   NOTE: ** indicates that the synapomorphy is subject to reversals or
> other modifications down the line.
>
> ------------------------------------------------------------
> --------------------
> Here's a weblink to the article on the evolution of brains:
>   https://io9.gizmodo.com/how-did-brains-evolve-1653897356
>
>
> [https://i.kinja-img.com/gawker-media/image/upload/s--
> qbRAJ4mk--/c_fill,fl_progressive,g_center,h_450,q_
> 80,w_800/tgtftblcjzufzhgapq4u.jpg]<https://io9.gizmodo.com/
> how-did-brains-evolve-1653897356>
>
> How Did Brains Evolve? - io9<https://io9.gizmodo.com/
> how-did-brains-evolve-1653897356>
> io9.gizmodo.com
> Humans have asked where we come from for thousands of years, across all
> cultures. But only recently have we started to address the mystery of the
> evolution of the ...
>
>
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