Arachnids and phylogeny

Bill Shear wshear at EMAIL.HSC.EDU
Wed Apr 11 08:59:10 CDT 2001

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
FAX (804)223-6374
email<wshear at>
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"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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