Arachnids and phylogeny

[Ken Kinman]

Bill,
      I certainly agree that the 1990 Shultz cladistic analysis was flawed
with respect to scorpions, and I obviously didn't follow it.  And as you
noted, this was repeated in the cladistic analysis of Wheeler and Hayashi.
Why the Arizona Tree of Life continues to perpetuate this phylogeny is
beyond me (I hope they will soon at very least place the scorpions at the
base of Arachnida).  Even Shultz himself noted a number of characters in
which scorpions differ from "other" arachnids, but I was particularly
surprised that he suggested the spermatozoa of scorpions could be a
"possible reversal".
      I classified scorpions and eurypterids together in Class Scorpionea in
1994, and it was rather controversial at the time, but the work of Dunlop
and others only strengthens my belief that this was the best course to
follow.
      As for mites, I should note that even though I still find Van der
Hammen's work very intriguing, I continue to code the mites (and ticks) as a
single clade, but not as a single taxon.   Although I am generally a lumper,
since there are well over 400 families of mites, I chose to recognize 7
orders, coded as follows (and ticks get their own order):
        7  Opilioacarida
        B  Holothyrida
        C  Ixodida (ticks)
        D  Gamasida
        8  Actinedida**
        9  Acarida
       10  Oribatida
  If van der Hammen turns out to be correct, and this grouping is not
holophyletic, I will simply move the last three orders (the acariforms) up
further in the classification of Class Arachnidea.  These 7 orders (often
ranked as suborders) are very stable and uncontroversial, but lumping them
all together in a single formal order (Acarina or Acari) is not advisable in
my opinion.  Cladistic analyses of arachnids with the mites divided into two
or three separate OTUs might very well yield better results.
     I have no major objections to the two order classification (Acariformes
and Parasitiformes), although it would be preferable to split
Opilioacariformes (my Opilioacarida) from the Parasitiformes sensu lato (my
compliments to the Arizona Tree of Life on their tripartite division of
mites, but their placement of scorpions should be changed as soon as
possible).
                  -------Ken Kinman

** Actinedida (including heterostigmatans, but they should perhaps be split
off as a separate order).

*******************************************************
>From: Bill Shear <wshear at EMAIL.HSC.EDU>
>Reply-To: Bill Shear <wshear at EMAIL.HSC.EDU>
>To: TAXACOM at USOBI.ORG
>Subject: Arachnids and phylogeny
>Date: Wed, 11 Apr 2001 08:59:10 -0500
>
>There is a Silurian arachnid, Eotarbus jerami, a trigonotarbid that
>is in the running for the first known fossil of a terrestrial animal.
>It comes from the Ludford Lane site in Shropshire, about 414 myo.
>
>While I would personally agree that scorpions and eurypterids are
>sister groups, or that scorpions are derived from within a lineage of
>eurypterids, most arachnologists disagree and consider scorpions to
>be the plesiomorphic sister group of all other arachnids.  Jason
>Dunlop in Berlin is accumulating evidence, however, that should make
>the eurypterid-scorpion connection clearer.  The most widely
>reprinted arachnid phylogeny is that of Jeff Schulz and Jerome
>Rieger, which places scorpions high up in the arachnid tree, as a
>sister group of Opiliones and Pseudoscorpions.  This seems absurd to
>many of us, but as it is based on a parsimonious argument utilizing
>numerous characters, it has gained acceptance.  Many of the
>characters, however, are based on leg musculature and patterns of
>joints, which may be highly adaptive and subject to homoplasy.  Leg
>joints were also the mainstay of van der Hammen's system, which was
>not cladistic, but scenario-based.
>
>Hayashi and Wheeler worked on a molecular phylogeny for the arachnid
>orders, but unfortunately their analysis gave nonsensical results.
>They combined their molecular data with the morphological data of
>Schulz for a total evidence analysis, and not surprisingly came up
>with Schulz's tree (with one or two minor differences in the
>Pulmonata).  However, because the molecular data detracted from
>rather than contributed to the robustness of the tree, this cannot be
>considered corroboration.
>
>The monophyly of the Acarina is very well supported, as Barry has pointed
>out.
>--
>
>Bill Shear
>Department of Biology
>Hampden-Sydney College
>Hampden-Sydney VA 23943
>(804)223-6172
>FAX (804)223-6374
>email<wshear at email.hsc.edu>
>Moderating e-lists:
>Coleus at http://groups.yahoo.com/group/coleus
>Opiliones at http://groups.yahoo.com/group/opiliones
>Myriapod at http://groups.yahoo.com/group/myriapod
>
>"How watchful we must be to keep the crystal well that we were made,
>clear!--that it not be made turbid by our contact with the world, so
>that it will not reflect objects.  What other liberty is there worth
>having, if we have not freedom and peace in our minds--if our inmost
>and most private man is but a sour and turbid pool?"
>
>--Henry David Thoreau, Journals, October 26, 1853.
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