Opabinia

[Ken Kinman]

Bill,
      Well, I think it is far more than just plausible.  The hose nose (of
Opabinia) and the frontal appendages (anomalocarids) have often been
homologized.  I just think instead of two frontal appendages fusing to form
the "hose nose" proboscis, that the converse is more likely:  the two halves
of the proboscis perhaps became fully separated during metamorphosis.  I
don't think the Opabinia mouth has been formally described, but would expect
something homologous to the Peytoia-type mouth.  Their mouths may not have
been as sclerotized or mineralized, and therefore less likely to fossilize.
     Bergstrom noted many of the shared characteristics of Opabinia and
Anomalocaris (Lethaia, 19:241-246), and Briggs and Whittington rebuttal
(Lethaia, 20:185-186) was so vehement that I was reluctant to even mention
my "larval" hypothesis in 1994 (and only did so in an addendum).   I believe
Briggs will admit they were wrong (not sure about Whittington), and
Bergstrom's viewpoint is now widely believed.
     Briggs and Whittington even thought it implausible that Opabinia and
Anomalocaris were related at all, but that has changed.  I think it is now
time to go the next step and examine the "form genus" hypotheses, which have
not even been discussed as far as I know.  Bergstrom stuck out his neck in
1986, and after the reaction he got, I only timidly went further in 1994.
But now I'm ready to really stick it out and see if a public discussion
can't bring some developmental neontologists into the debate, or anyone who
has some new angle on how to falsify or corroborate the alternatives I have
suggested.  If I get the guillotine, so be it, but believe this worth
pursuing.  We shall see.  I need to review Bergstrom's papers.  Also, I am
sure Anomalocaris specimens are known that are far smaller than 0.5 meters.
More tomorrow.
                         -----Ken Kinman
******************************************************
>From: Bill Shear <wshear at EMAIL.HSC.EDU>
>Reply-To: Bill Shear <wshear at EMAIL.HSC.EDU>
>To: TAXACOM at USOBI.ORG
>Subject: Re: Opabinia
>Date: Wed, 28 Feb 2001 16:45:06 -0500
>
>Ken,
>
>You also need some reasons to set up hypotheses as well as to
>disprove them.  "Why not?" doesn't constitute a very good reason for
>thinking something is so.  What are the reasons for thinking that
>Opabinia and Anomalocaris are different stages/sexes of the same
>animal?  As you state, a striking metamorphosis would be required to
>go from one to the other--the change in eye number, acquisition of
>frontal appendage, new jaw structures and loss of the "hose nose."
>Also as you state, Opabinia were very small and nearly all the
>Anomalacaris are from 0.5 to 1.0 meters long.  Where are the
>intermediate life history stages?
>
>Both Graham Budd and Simon Conway Morris think the two are closely
>related, but distinct, animals.  Both these guys as well as
>Whittington and Derek Briggs have studied the actual fossils and have
>a lot of experience in a wide range of arthropods.  What they have to
>say about the two animals is detailed and quite believable.
>
>So, since you are the odd man out, you have to prove your case.  Mere
>plausibility doesn't work...
>--
>
>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:
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