[Taxacom] Were bivalves the first molluscs to evolve?

[Richard Zander]

Cladograms, both morphological and molecular, show continuity between nodes, which are sets of traits distinguishing the node from the next more basal node. Continuity between nodes is not continuity between taxa involved in evolution. The nodes could represent any nearby taxon even if highly supported. Molecular analysis is further confounded by the fact that the OTUs represent molecular strains, not taxa. Molecular strains of one species often separately generate different descendant species, as can be seen in cladograms with multiple exemplars of any one species. 

This is why phylogenetic models are dichotomous trees which model optimal hierarchic cluster analysis trees or Markov chains, not evolutionary trees.  Cladograms only model themselves.

Phylogenetic trees and evolutionary trees are not the same, and struggles to find common ground are senseless.


-------
Richard H. Zander
Missouri Botanical Garden – 4344 Shaw Blvd. – St. Louis – Missouri – 63110 – USA
richard.zander at mobot.org 
Web sites: http://www.mobot.org/plantscience/bfna/bfnamenu.htm and http://www.mobot.org/plantscience/resbot/ 

-----Original Message-----
From: Taxacom [mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Kenneth Kinman
Sent: Wednesday, November 22, 2017 11:33 AM
To: taxacom at mailman.nhm.ku.edu; Kenneth Kinman
Subject: [Taxacom] 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

________________________________



>>
>>\________ 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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