Arthropod Evolution Alternative

[Ken Kinman]

Dear All,
      The morphology of the first arthropods is a long-standing mystery that
is critical to the proper rooting of arthropod phylogeny, not only of the
many living taxa, but also placing its rich fossil record in its true
context.  Therefore, I would like to encourage anyone with interests in this
subject to consider the following alternative rooting of arthropods, even
though it may sound a bit bizarre on first hearing.
      It is my belief that the first arthropods lived within bivalved shells
somewhat similar to those of modern ostracodes (or their bradoriid
ancestors).  Just as I believe all molluscs arose from the bivalved molluscs
with bodies gradually "evolving beyond their shells" (and some even shedding
the shells for good), I believe that all arthropods probably ultimately
evolved from related bivalved arthropods also growing beyond their shells.
      And I am using the term "shells" in a broad sense, since there was
little if any mineralization of such shells before the latest PreCambrian.
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.  It
seems far more likely that multiple losses of shells would  have occurred.
Again this parallels my view of mollusc evolution (gradual shell reduction
and loss).
     If this hypothesis is true, it will require the rerooting of arthropod
phylogenies as well (crustaceans in particular, and this is one reason I did
not recognize a formal taxon "Crustacea" in my 1994 classification).  That
the insects arose from a paraphyletic "Crustacea" has become increasing
clear.  The relationship of myriapods is much less clear, but they may have
simply evolved from a different group of crustaceans (as opposed to a direct
relationship with either insects or chelicerates, both of those proposed
hypotheses seeming much less likely to me at the present time).
     The arachnomorphs (trilobites, chelicerates, and allies) arose so early
from some sort of protocrustacean group that they COULD (for most purposes)
be considered sister group of a mandibulate clade.  However, if
protocrustaceans evolved from protomolluscs (early bivalves), their shared
orientation within the shell seems to be the plesiomorphic state, while the
trilobitic body reorientation (resulting in dorsal-ventral shells rather
than left-right shells) could be an arachnomorph synapomorphy (depending of
course on one's particular definition Arachnomorpha).
      Interestingly, my view is somewhat intermediate to the traditional
view of agnostids (as closer to trilobites), and on the other hand some more
recent views that regard agnostids as closer to crustaceans.  I simply
regard agnostids being neither one nor the other (along with other
"protrilobites" like helmetiids, naraoiids, mollisoniids, etc.).
       But then, I am always on the lookout for solutions that lie between
what I see as opposing extreme viewpoints.  Once again I reiterate that such
intermediate viewpoints are not always simplistic "lowest common
denominators" that lack merit, but real solutions that perhaps seem just too
good to hope for.  I only latch onto those that I believe represent the most
probable hypotheses, even if they are presently viewed as improbable from a
conventional perspective.
          ---- So it continues,
                      Ken Kinman
P.S.  Does anyone know if the term "De-arthropodization" has ever been used?
  This is a term I plan to start using for arthropod descendants that have
lost various arthropod characteristics secondarily.  Pentastomids are
perhaps the best example.  Who knows, perhaps tardigrades and onychophorans
are also NOT protoarthropods, but secondarily de-arthropodized taxa instead.
  And wouldn't it be really weird if nematodes turned out to be tardigrades
that had secondarily lost their legs (analogous to snakes evolving from
lizards).  Something to think about.

_________________________________________________________________
Add photos to your messages with MSN 8. Get 2 months FREE*.
http://join.msn.com/?page=features/featuredemail