Arthropod Evolution

[Jeff Shultz]

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