Arthropod Evolution Alternative

[Jason at]

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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