Tetrapoda (and the new genus _Tiktaalik_)

Ken Kinman kinman2 at YAHOO.COM
Thu Apr 6 23:10:24 CDT 2006


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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