Origins of Hexapoda

[Ken Kinman]

Dear All,
      I have been seriously contemplating this hypothesis for quite some time, but rather reluctant to discuss it much since it seems somewhat out in the middle of left field (so to speak), but here goes.  Could the Ostracoda (or something closely related) have evolved into at least some of the hexapod groups on land?  Particularly, the Collembola (whether or not they are directly related to various other hexapods).  The three pairs of uniramous thoracic legs of ostracods, especially those adapted to walking (not just swimming or feeding), seem make them definite candidates for close relatives of some hexapods (whether that group is holophyletic or polyphyletic).

      THOUGHT EXPERIMENT: Just how difficult would it be to transform a freshwater or terrestrial ostracod into a collembolan?  Loss of one pair of antennae would be fairly easy (actually collembolans have a transient pair of incipient second antennae during their development!).  The three pairs of uniramous legs are already in place.  Transforming caudal furcae into the "springtail" furcula doesn't seem particularly hard to contemplate.  Loss of the shell is not difficult to imagine either (just think of sea slugs as the mollusc example).

     But what would be the really difficult tranformations that would be required?  I'm not sure they would be quite as difficult as some might imagine, but I would interested to hear what others think would be the biggest objections to this hypothesis.  Anyway, if collembolans are terrestrial descendants of a crustacean group like ostracods, it could provide important clues to the origins of other hexapods (including insects sensu stricto), and this would be particularly true if collembolans are the basal sister group of other hexapods.  But unfortunately my gut feeling is that Hexapoda is polyphyletic (2 or 3 separate clades evolving from different crustacean taxa).  However, even if Hexapoda is polyphyletic, solving the origins of Collembola would still provide indirect insight into the origins of the other hexapod taxa.
                 ------ Cheers,
                            Ken Kinman
P.S.  As I have said before, a taxon Uniramia is dead in my opinion.  Too many taxa have both uniramous and biramous appendages, and in any case, it would not be that difficult to transform biramous limbs into uniramous limbs once they were no longer needed for swimming.  I am also highly dubious of the long prevalent view that early crustaceans had a large number of repeating abdominal segments (the teloblastic generation of many segments seems more derived to me, and probably occurred more than once).   And of course, this also fits in well with my oddball idea that bivalves came first in mollusca and that bivalve taxa also came first in arthropod evolution as well.  If so, many mollusc and arthropod phylogenies have been badly misrooted, and alternative viewpoints need to be seriously explored.