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Digit homology (was Forthcoming theropod paper of note on the manus)



>From our response to Alan Feduccia's and Larsson-Wagner arguments regarding 
>digit homology:

>While the idea that the three manual digits of extant birds represent
digits 2-3-4 is apparently supported by some embryological evidence,
it is not supported by all of it. Furthermore, the contention that birds differ
fundamentally from theropods in this respect is untestable as we cannot
examine embryology in Mesozoic theropods. Given that tetanuran
theropods exhibit only three digits and that a shift in the axis of
condensation from digit 4 to 3 has been suggested for
some vertebrate groups (frogs and salamanders), it could be that all
tetanurans have shifted the axis of condensation.
How can the axis has passed through  embryological digit 4 when the
animals only have 3 digit?
Alarm bells should be ringing given that saurischians differ
fundamentally from all other reptiles in exhibiting a
topographical second digit (rather than third digit) which is longest in
the hand.

Finally, both the fossil record and some aspects of embryology indicate
that digit formation is relatively flexible in some tetrapod groups.
Frame shifts can occur, apparently without severe pleiotropic effects,
as is demonstrated by vertebral counts in some sauropodomorph dinosaurs
(where there are only two sacrals but whether the sacrals are derived from
caudal or dorsal vertebrae differs between genera: this is a new
discovery reported at the 2002 SVPCA meeting).
Furthermore, the hands of basal birds and the most bird-like theropods
are near identical down to the smallest feature. Even if  bird and theropod 
hands
are developmentally different, why should
this one difference 'trump'the many other shared features seen in birds
and bird-like theropods but not in other animals? The paroccipital
processes and braincases of Archaeopteryx and dromaeosaurs are near
identical in many tiny detailed features - these are significant
because such features cannot be explained away by convergence.<<

Yes...while many people were having fun SVP guess who was talking to Feduccia 
himself?
What have I done to have such kind of fun in my life?

Luis Rey

Visit my website on http://www.ndirect.co.uk/~luisrey






"Jaime A. Headden" wrote:

> Stephan Pickering (stefanpickering2002@yahoo.com) wrote:
>
> <The numbering of theropod hands -- the best paper, thus far, on some
> aspects of the question, remains that by G.P. Wagner and Jacques Gauthier
> -- is problematic, given the poor fossil record of basal taxa. It is
> possible that avian dinoosaurs exhibit embryogenetic reduction of manus
> digit I earlier than some have thought vs. the idea of (as Frietson et al.
> write in their mss.) "a repatterning of the initial embryonal Anlage
> followed by the development of the digits that are present later on". Much
> further work is needed, indeed. But, unless I am misreading the
> literature, it is presumed theropod hands consist of I, II, III. The
> recent embryological research may revise this, as avian embryos show five
> digits, with I and V lost. Could it be that theropod hands are not I-III,
> but II-IV? Feduccia & Nowicki, earlier this year, beat the mantra drums of
> avians being anything but Dinosauria on the basis of embryology -- but
> they are out-of-tune.  The subject of the manus is, actually, quite
> interesting. The Wagner/Gauthier Frame Shift Hypothesis proposes that,
> from theropod I-III, there transpired the shift to II-IV among immediate
> clades ancestral to extant avian theropods.>
>
> Larsson and Wagner (2002: _Journal of Experimental Zoology_ 294), oddly
> enough, contradict the idea of a frame shift occuring between lacertilians
> and birds and destroy the idea of frame-shift entirely; they employ how
> the development of the digits II-IV really are II-IV in birds, not a
> transformed set of I-III, and it is not likely for non-maniraptoriform
> theropods to have II-IV. Wagner and Gauthier are faulted by the fact that
> their work has a lot of speculation in it, aside from embryological
> observation, and that linking one to the other required a few steps not
> encounted by science yet. Larsson and Wagner, on the other hand [eh,
> heh...] argue that a frame shift between lacertilians and birds did not
> occur, and that birds just arrested the development of digit I, improved
> digit IV, and the "signature" of each medial digit was transfered
> laterally, so that digit II had two phalanges, one of which was lost, and
> digit III has three, and so forth; this contradicts the normal pattern of
> a 2-3-4-1-0 pattern seen in *Herrerasaurus* and no fourth digit in all
> other known theropods. Whatever your phylogeny, theropod finger
> transformation is not see easily solved without early stage _in ovo_
> embryos, which we lack for the moment beyond any identification.
>
>   Cheers,
>
> =====
> Jaime A. Headden
>
>   Little steps are often the hardest to take.  We are too used to making 
> leaps in the face of adversity, that a simple skip is so hard to do.  We 
> should all learn to walk soft, walk small, see the world around us rather 
> than zoom by it.
>
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