[Taxacom] Were bivalves the first molluscs to evolve?

[Kenneth Kinman]

Dear all,

      I forgot to mention that this phylogeny gets support from the fossil record.  The Polyplacophora-Aplacophora clade (usually called Aculifera) is the last clade to arise in my tree.  There is no fossil record for this clade until the Upper Cambrian.  All the earlier branches appear as fossils in the Lower Cambrian (if not earlier).  Thus if my phylogeny is correct, I predict that there will never be any fossils of Aculifera found in the Lower Cambrian.

             --------------Ken Kinman

________________________________
From: Kenneth Kinman <kinman at hotmail.com>
Sent: Wednesday, November 22, 2017 11:32 AM
To: taxacom at mailman.nhm.ku.edu; Kenneth Kinman
Subject: Were bivalves the first molluscs to evolve?


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

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>>
>>\________ 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.

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Here's a weblink to the article on the evolution of brains:
  https://io9.gizmodo.com/how-did-brains-evolve-1653897356