Arthropod Evolution

[Ken Kinman]

Jeff,
     I would agree with some of what you said.  Pancrustacea is so well
supported by the evidence that I now automatically think of insects as very
derived terrestrial crustaceans.  I'm still not completely convinced that
they form a single clade (pterygotes and some apterygotes could have arisen
from different crustaceans in parallel).  In any case, I regard Crustacea as
a paraphyletic mother group, so I tend to cringe when Jason (like many
others) suggests that insects and crustaceans could be sister groups (which
would have us predicting the existence of insects in the Cambrian and
Ordovician).
     I do strongly disagree with Jeff that the bivalve hypothesis for
arthropod origins *unnecessarily* complicates the issue.  My purpose is to
point out this possibility as a potentially fruitful area of enquiry, not to
be casually dismissed as many malacologists continue to do regarding a
bivalved origin of molluscs.  Indeed, it is my belief that taken together,
these two hypotheses could help explain the similarities between molluscs
and arthropods (similarities which are most often attributed to
convergence).
     And don't be too surprised if basal members of Class Anomalocarea (=
"Dinocarida") turn out to have bivalved carapaces.  It really is a shame
that bivalved anomalocareans like Hurdia and Proboscicaris still have not
been formally described beyond a very brief mention in a 1993 abstract.
     Anyway, it is embryology and development that has largely shaped my
views of arthropod evolution, and less on comparison of adult morphologies.
Perhaps my *far* more speculative idea that Opabinia could be an immature
Anomalocaris is thus just wishful thinking, but who knows.  The idea that
they were even closely related was vigorously attacked at first (debated in
the journal Lethaia).  That's one of many reasons I put off publicly
expressing my bivalve hypotheses for so many years, even well after the
Internet would have made it easier to do so.  The only reason I posted my
ideas on arthropod evolution now (even though poorly organized) is because I
will be offline this summer and not sure when I will be joining this forum
again.  Only time will tell.  And besides, brainstorming isn't worth much if
you don't share it with somebody (and the Taxacom "veranda" can be a good
place to bounce such things around).
         ----- Cheers,
                    Ken
***************************************
>From: Jeff Shultz <js314 at UMAILSRV0.UMD.EDU>
>Reply-To: Jeff Shultz <js314 at UMAILSRV0.UMD.EDU>
>To: TAXACOM at USOBI.ORG
>Subject: Arthropod Evolution
>Date: Thu, 8 May 2003 11:30:00 -0400
>
>Hi,
>
>A few comments about arthropod evolution.
>
>1. Pancrustacea or Tetraconata (= Crustacea + Hexapoda) is extremely well
>supported by many different kinds of evidence (mostly molecular). The
>appearance
>of continuing debate is due largely to certain traditionalists who continue
>to state the
>same arguments over and over, but now in capital letters and exclamation
>points
>along with a blanket rejections of results from molecular data.  A real
>debate would
>entail two or more camps who are continually marshalling new evidence. This
>isn't
>happening. Frankly, I would like to see more original morphological
>research in this
>area, but the traditionalists don't seem interested in doing original
>morphology, just
>rehashing the literature.
>
>2. Manton must be congratulated for stimulating so much research. She
>erected a
>huge edifice that had to be knocked down brick by brick. She gave relevance
>to a lot
>of phylogenetic and morphological research. Hypotheses exist to be tested,
>and hers
>were tested and largely rejected.  Arthropods are monophyletic, centipedes
>try to
>wiggle when they run, spiders use hydraulic pressure for locomotion,
>millipedes and
>centipedes got longer during evolution not shorter, xiphosurans don't chew
>the way
>she thought, hexapod legs are not primitively uniramous and their jaws are
>not
>"whole limbs." The edifice was so grand that people still marvel at it. But
>grandiosity
>should not be mistaken for veracity.
>
>3. I think it is reasonable to say that "arthropodization" of the dorsal
>surface and
>appendages of extant arthropods occurred once, although stabilization in
>podomeres
>did not occur until long after arthropodization.  Arthropodization of the
>ventral surface
>is another story. Sclerotization and consolidation of ventral structures
>occurred many
>times independently in many different ways; all sorts of combinations of
>appendages
>and the body wall.  Yet, tergites seem to have pretty much the same
>organization
>throughout the extant arthropods.
>
>4. The idea that arthropods were primitively bivalved is also a hypothesis
>to be
>tested. My impression at the moment, though, is that it doesn't really
>explain anything
>and, in fact, complicates the issue unnecessarily. Based on the
>evolutionary
>tendencies noted in Point 3, I would think that a trilobite-like grade of
>organization
>(extensive dorsal sclerotization, soft ventral cuticle) is an adequate
>model for the
>ancestral arthropod. But it is all speculation.
>
>For what it's worth.
>Jeff ShultzJeffrey W. Shultz
>Membership Secretary, American Arachnological Society
>Dept of Entomology, Univ of Maryland
>College Park, MD 20742
>Voice: (301) 405-7519, Fax: (301) 314-9290

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