Arthropod Evolution Alternative

[Ken Kinman]

Hi Jason,
     Thanks for the questions.  Manton believed Arthropods were
polyphyletic, but I do not (never have).  Manton believed in a taxon
Uniramia, while I have come believe uniramians are polyphyletic (it seems
relatively easy to form a uniramous limb by loss of one branch of a biramous
limb, especially when a group become terrestrial).   Any direct relationship
between the uniramian insects and myriapods is becoming more and more
doubtful all the time.  See the recently published paper by Dove and
Stollewerk, 2003 (Development, 130:2161-2171).  So the polyphyly of
Atelocerata seems increasingly certain.
     The traditional view of the Ur-Crustacean having lots of identical
(teloblastic) segments (with minimal regionalization) is based on the model
of typical annelids as being ancestral to arthropods.  I no longer believe
annelids form a natural group, and only the clitellates seem to be
particularly close to arthropods.  I therefore think it is Annelida that's
probably polyphyletic, not the Arthropoda.  And it strikes me as extremely
odd that with all the interest in Ecdysozoa, that there has been little (if
any) mention that some clitellates also have ecdysteroidal moulting of their
cuticle (is that a gasp I hear from some of the molecularists?).  Ecdysozoa
might be okay if it includes clitellates (especially leeches), but I still
think the taxon Lophotrochozoa is worse than useless, it's looking more and
more like a misleading conglomeration of molecular plesiomorphies
masquerading as a clade.
      As for parsimony and bivalve shells, I was dumfounded several years
ago when Simon Conway Morris (one of the world's brightest paleontologists)
actually proposed that halkieriids (which have completely separated anterior
and posterior shell valves) evolved into brachiopods by gradually folding up
and forming a hinge between the valves.  To me this sounds not only
improbable but even dangerously teleological.  Doesn't it seem far more
likely brachiopods evolved into halkieriids the other way around, by
unhinging and the body growing larger (sort of like what an enrolled
trilobite could have evolved into if it could have dispensed with a hinge
instead of just adding more elements to it (although vulnerabilities of some
sort may have prevented that from happening in that group).
      Anyway, it seems like zoologists are still having trouble completely
shaking free of the Vermes concept (vermiform animals as primitive animals
that need to be pushed as far to the base of the tree as possible).  But to
me, filter-feeding inside a pair of shells seems like a pretty primitive
life style, whereas many worms had to develop better nervous systems to
detect and evade predators to which they were more vulnerable (or develop
chemical deterrents and warning colorations, as many "slugs" have done).
For a clam it's often a much simpler two-step process:  sense danger, clamp
shut.  Period!!!  It's also generally easier to be a gatherer than a hunter
(even in the sea).  Anyway, I would rather have one huge paraphyletic
"Bivalvia" than a polyphyletic Vermes, not that I am advocating either.
         ---- Cheers,
                   Ken Kinman

*****************************************
>From: Jason Mate <jfmate at HOTMAIL.COM>
>Reply-To: Jason Mate <jfmate at HOTMAIL.COM>
>To: TAXACOM at USOBI.ORG
>Subject: Re: Arthropod Evolution Alternative
>Date: Mon, 5 May 2003 19:00:11 +0200
>
>Dear Ken,
>
>I find the scenario that you write about interesting but I am confused as
>to
>what you mean exactly:
>
>>It is my belief that the first arthropods lived within bivalved shells
>>somewhat similar to those of modern ostracodes (or their bradoriid
>>ancestors). I believe that all arthropods probably ultimately evolved from
>>related bivalved arthropods also growing beyond their shells.
>
>What do you base this assertion on? All of the evolutionary scenarios that
>I
>have seen place the Cephalocarida, Remipedia or Mystacocarida in a basal
>position and "shelled" crustaceans in a more derived placement. Arthropod
>evolution as a whole has tended to move from more to less segmentation and
>Crustaceans appear to follow this pattern, with basal forms being
>multisegmented and lacking body regionalisation.
>
>>What I am saying is that mineralisation of shells occurred independently
>>in
>>many different lineages, but that the evolution of the original
>>unmineralised bivalved shells was an extremely rare event.  The notion
>>that
>>various arthropods evolved bivalved shells many times >convergently is not
>>only unparsimonious, but to me has always seemed >rather nonsensical.
>
>This appears to be a reinterpretation of Manton's view with the addition of
>Dollo parsimony. How does your hypothesis differ from Manton's? Why should
>the arthropod "shell" be so unique?
>
>Cheers
>
>Jason Mate
>
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