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Re: Feduccia and dino-hand questions



Jeff Poling wrote:

<snip>

>    Another question has to do with a paragraph in the paper that I've read a
> dozen times and am not sure I understand:
> 
>    "As the primary axis invariably gives rise distally to digit IV in
> amniotes, it serves as a consistent marker of digital identity and assigns
> the homologies of the reduced bird hand as digits II-III-IV.  A variation of
> this pattern wherein the primary axis runs through digit III, would
> eliminate any phylogenetic significance from the morphological and molecular
> similarities in amniote limb development.  If such a condition could be
> demonstrated, patters of limb development would have to be decoupled from
> phylogeny, and this stereotypic pattern of development accepted as 
> convergence."
> 
>    Uh, what?  Does he really mean that if bird digits are I-II-III,
> developing with III as the primary axis, then there is no phylogenetic
> significance in the development of any amniote limb?  Is he really saying
> that if this is the case, the development of every other amniote limb from a
> primary axis through IV is simply convergence?  Despite the fact that the
> "aberrant" development occurs in only a single lineage?
> 
>    Does that make no sense to anybody else?

It sounds like to me like a fancy way of saying "if this modification of
the avian hand is possible, then _anything_ is possible, which means you
can't draw any solid phylogenetic conclusions from studying morphology."

Which in turn sounds like an attempt at setting up a boomerang trap: if
dino->bird advocates argue that the bird hand was modified this way,
then they neutralize their own strongest argument for the dinosaur->bird
link: the highly detailed similarities between the forelimb of
_Archaeopteryx_ and that of dromaeosaurs like _Deinonychus_.  At least,
that's where I think Feduccia seems to be going with this line of
thought.  Poor science, IMHO, but a very good rhetorical technique.

-- JSW