Tetrapoda (and the new genus _Tiktaalik_)

John Grehan jgrehan at SCIENCEBUFF.ORG
Fri Apr 7 08:25:36 CDT 2006


I could not contribute to this discussion since I do not have knowledge
of the characters used and how they were analyzed. But if Ken want's a
discussion on this I would think he needs to start of by presenting the
character evidence by which he supports his various characterizations,
and not just make what are otherwise rhetorical assertions.

John Grehan



> -----Original Message-----
> From: Taxacom Discussion List [mailto:TAXACOM at LISTSERV.NHM.KU.EDU] On
> Behalf Of Ken Kinman
> Sent: Friday, April 07, 2006 12:10 AM
> To: TAXACOM at LISTSERV.NHM.KU.EDU
> Subject: [TAXACOM] Tetrapoda (and the new genus _Tiktaalik_)
> 
> Dear All,
>       I would like to see a discussion on what the taxon Tetrapoda
should
> include (and this is not "baiting", as some might view some of my
posts on
> controversial subjects).  I personally reject very broad or very
strict
> circumscriptions of Tetrapoda.  The strictest versions are a crown
group
> (which excludes many groups of primitive amphibians).  The broadest
> versions of Tetrapoda include the rhizodontiform and osteolepiform
fish
> (and thus equivalent to what is generally called Tetrapodomorpha).
> 
>      In between those extremes, however, there are several possible
> consensus circumscriptions of Tetrapoda.  The most restrictive would
> include Acanthostega and Ichthyostega, but not more primitive forms
(this
> is the more traditional view, mainly because the immediate outgroups
were
> still undiscovered).  A good case can be made for also including two
or
> three successive outgroups, Ventastega, the Elginerpetonidae, and
perhaps
> even Livoniana.  Beyond that, I *might* even consider including
> Panderichthyidae (which had lost all but the four "tetrapod" fins and
> evolved some other "tetrapod" characters).
> 
>       But now we also have _Tiktaalik roseae_ which was described in
> today's issue of Nature.  I haven't been able to read the article yet,
but
> understand that this form was apparently more derived than
> Panderichthyidae, but less so than Elginerpetonidae.  Therefore,
Tiktaalik
> will probably become (to many biologists) either a very primitive
member
> of Tetrapoda or a very close outgroup.  So my question is whether we
> should stick with the traditional Tetrapoda (with Acanthostega and
> Ichthyostega at the base, along with the related Sinostega) or expand
it
> to include outgroups like Ventastega and Elginerpetonids.  Or even
include
> Livoniana, Tiktaalik, or Panderichthyidae.  I do not advocate going
> further to include osteolepiforms or rhizodontiforms, but perhaps
someone
> might want present arguments for doing so.
>   -----Cheers,
>            Ken Kinman
> P.S.  I can't say I like the generic name Tiktaalik (which is actually
a
> common name for the unrelated living species Lota lota), but I guess
we
> are stuck with it.  Also rather unusual that the species name (roseae)
is
> for someone who wants to remain anonymous (or at least partially so).
I
> assume her first or last name is Rose.




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