[Taxacom] Were bivalves the first molluscs to evolve?

[Kenneth Kinman]

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 ...