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Re: tetrapoda




On Thu, 13 Nov 1997, T. Mike Keesey wrote:

> On Thu, 13 Nov 1997, Jack wrote:
> 
> > Anyone have the formal cladistic definition of Tetrapoda handy?
> 
> Judging from what I see at the On-Line Tree of Life, it's something like
> "all descendants of the most recent common ancestor of amphibians (narrow
> definition -- all animals sharing more recent ancestry with frogs,
> salamanders, and caecilians than with amniotes) and amniotes". This
> excludes a lot of primitive forms like _Ichthyostega_. The more inclusive
> clade Tetrapodomorpha would contain these. 
> 

Yes the narrow phylogenetic definition of the Tetrapoda does exclude a
number of basal taxa such as Ichthyostega, Acanthostega etc.. If Laurin's
new tetrapod phylogeny is correct, then most of the Palaeozoic "tetrapods"
(eg. temnospondyls, baphetids, embolomeres, gephyrostegids and
Seymouriamorphs) would actually be excluded from this taxon. Nevertheless
there is great resistance amongst early tetrapod workers to applying this
crown group definition of the Tetrapoda, it is analogous to kicking
Archaeopteryx, Ichthyornis etc. out of Aves because they don't belong to
the crown group. I think what will settle is either a stem based
definition like "all taxa more closely related to modern Amphibia and
Amniota than to Elpistostegids" (Elpistostegids are the closeset known
taxa to Tetrapoda that still lack limbs and digits) or a node based
definition like "Elginerpetontidae and all modern amphibians, amniotes and
all descendants of their common ancestor". The problem with the crown
group definition of Tetrapoda is that a) people are very used to calling
limbed vertebrates such as Ichthyostega, temnospondyls and
seymouriamorphs," tetrapods" and b) there isn't anything else we can call
them at the moment beyond limbed, near-tetrapod osteolepiforms.
Tetrapodomorpha does not just apply to these limbed
near-tetrapods + tetrapods, it is
infact phylogenetically defined as a stem group including all vertebrates
more closely related to modern tetrapods than to modern lungfish.
Therefore such "fishy" taxa as Eusthenopteron are tetrapodomorphs. Whew! I
seem to have gone on far longer than I intended, I hope this helps clear
things up.

cheers

Adam Yates