[Taxacom] diphyletic/paraphyletic Bivalvia

Tue Nov 28 19:09:14 CST 2017

Dear All,

         A paper by Giribet et al., 2006, actually found a paraphyletic Bivalvia (using a combination of several different molecules).  Although it was only paraphyletic with respect to classes Gastropoda, Polyplacophora, and Monoplacophora, while Cephalopoda and the other mollusc classes were in a group just one branch out from the bivalves.

         If bivalves did evolve into Juliidae (or a unknown family that gave rise to the Juliidae of the Cenozoic), this must have happened in the earliest Cambrian or even earlier.  So why haven't we found them in the Paleozoic or Mesozoic?  Probably because they would have been very, very  small, and looked just like bivalve fossils, and their "shells" were probably uncalcified early on.  Nobody even knew Juliids were gastropods until living species were finally discovered in the 20th Century (about 1959 if I recall correctly).

      And where would one draw the line between Bivalvia and Gastropoda?  Development of the first primitive radula?  Beginning of buccal development?  Or one of the other characters I listed giving rise to Euthyneura?  Even the first tiny radula may have been uncalcified, but has anyone ever thought of trying to look for a radula associated with a tiny Paleozoic bivalve shell in rocks which show such fine structure in fossils.

Here is a weblink to the tree from Giribet et al., 2006:

http://www.pnas.org/content/103/20/7723/F2.expansion.html

________________________________
From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of Kenneth Kinman <kinman at hotmail.com>
Sent: Tuesday, November 28, 2017 9:17 AM
To: taxacom at mailman.nhm.ku.edu
Subject: [Taxacom] diphyletic/paraphyletic Bivalvia

Dear All,

       The phylogeny of Mollusca that I presented shows a paraphyletic Bivalvia (Protobranchia + Lamellibranchia).  Perhaps this is why some molecular trees for Mollusca have shown a mystifying diphyletic Bivalvia (some bivalves branching off within the Gastropoda.  The complete mitochondrial genome of genus Solemya shows it embedded within the Gastropoda.  And that same paper notes a diphyletic Bivalvia appearing other molecular trees (such as the 18S RNA)---see weblink below.

     There seems to be no explanation for such results, but noone seems to have considered mis-rooting ( choosing the wrong outgroup) to be the problem.   Perhaps more molecular data (especially for the bivalved gastropods, Family Juliidae) could shed light on these problems.

https://bmcgenomics.biomedcentral.com/articles/10.1186/1471-2164-14-409
[https://images.springer.com/sgw/journals/medium/12864.jpg]<https://bmcgenomics.biomedcentral.com/articles/10.1186/1471-2164-14-409>

The complete mitochondrial genome of Solemya velum ...<https://bmcgenomics.biomedcentral.com/articles/10.1186/1471-2164-14-409>
bmcgenomics.biomedcentral.com
The complete mitochondrial genome of Solemya velum(Mollusca: Bivalvia) and its relationships with Conchifera

________________________________

>>
>>\________ 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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  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 ...

[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>
[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 ...

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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