[Taxacom] Were bivalves the first molluscs to evolve?

[Kenneth Kinman]

And yet there seems to be a clade (Streptoneura , Monoplacophora, Placophora, and Aplacophora) which are united by two fundamental morphological characters (not found in Euthyneura, Cephalpoda, or Bivalvia):


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).


I do not believe that misrooted molecular phylogenies are more reliable than these morphological characters.


________________________________
From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of David Campbell <pleuronaia at gmail.com>
Sent: Thursday, November 23, 2017 8:38 PM
Cc: taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] Were bivalves the first molluscs to evolve?

Molecular data indicates closer relationship between the extant
monoplacophora and the polyplacophora than between the monoplacophora
and the other univalve/bivalve forms.  Exact relationships between the
various fossil "monoplacophora" and extant ones, and between various
early Paleozoic multi-sclerite taxa and the modern polyplacophora and
aplacophora are unclear, but there are likely early Cambrian
representatives of the polyplacophra/aplacophora/monoplacophora group.
Neither morphological nor molecular analyses have supported
euthyneurans being basal to streptoneurans, nor for cephalopods to
arise within gastropods.  Euthyneurans are well-supported
morphologically, molecularly, and by timing of appearance in the
fossil record as being relatively derived gastropods showing varying
levels of detorsion, not partiallytorted ancestral gastropods.

Molecular resolution of the relationships of major groups in the
Mollusca remains relatively poor, but the extent of genomic coverage
is increasing.

--
Dr. David Campbell
Assistant Professor, Geology
Department of Natural Sciences
Box 7270
Gardner-Webb University
Boiling Springs NC 28017
_______________________________________________
Taxacom Mailing List
Taxacom at mailman.nhm.ku.edu,
http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
Taxacom Info Page - mailman.nhm.ku.edu Mailing Lists<http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom>
mailman.nhm.ku.edu
Taxacom is an e-mail list for biological systematics. Named and brought to life by Drs. Richard Zander and Patricia Eckel, Taxacom began its peripatetic existence on ...



The Taxacom Archive back to 1992 may be searched at: http://taxacom.markmail.org
Taxacom Home - MarkMail - Community libraries<http://taxacom.markmail.org/>
taxacom.markmail.org
MarkMail is developed and hosted by MarkLogic Corporation. MarkMail is a free service for searching mailing list archives, with huge advantages over traditional ...




Send Taxacom mailing list submissions to taxacom at mailman.nhm.ku.edu
To subscribe or unsubscribe via the Web, visit: http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
Taxacom Info Page - mailman.nhm.ku.edu Mailing Lists<http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom>
mailman.nhm.ku.edu
Taxacom is an e-mail list for biological systematics. Named and brought to life by Drs. Richard Zander and Patricia Eckel, Taxacom began its peripatetic existence on ...



You can reach the person managing the list at: taxacom-owner at mailman.nhm.ku.edu

Nurturing Nuance while Assaulting Ambiguity for 30 Some Years, 1987-2017.