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Re: Digit homology



David Marjanovic (david.marjanovic@gmx.at) wrote:

<And what about the finding that the axis doesn't pass through a finger
but  through the distal carpals from V (or VI...) to I, while all
metacarpals and fingers form postaxially = distolaterally to the axis
(with another axis, apparently from the radius instead of the ulna, ending
in the praepollex)? I have never seen this mentioned in Feduccia's papers
or answers to them, but  constantly in illustrations of the evolution of
hands from fins. Or am I talking about another axis altogether?>

  One of the things I've been seeing is a dismissal of the idea of an "X"
element _and_ a "praepollex", based as they were on ideas of loss of
identification from basic principles. Identified in other animals, it is
likely, though not used because of the literature issue, that all
tetrapods have digits 5 of seven, and the praepollex is a remnant of the
condensation of this first digit, and there was a seventh unossified ("X")
element that would form part of this extraneous digit condition or
metacarpal. Similarly, there is also a lack of consideration to the basal
amniote condition of two ranks of carpals, and either element being the
core of chondrification of these distal or proximal carpals. Not all
elements should be expected to hold their positions from embryogenesis
through ontogeny, and in fact they shift about in the skull and ear as
development ensues.

<Galton has constantly been publishing on this since 2000.>

  Indeed, but this is not evidence of a frame-shift in sacral recruitment.
There are always two basic sacrals, and even closely related animals vary
in which elements from which side make the second or fourth in the series,
and so forth. The number of recruited dorsals and sacrals in taxa with
four or five elements in sauropods varies, as in theropods, and one thing
I have gotten from Galton's and Wilson's work is that there is no clear
phylogenetic utility in sacral composition. During ontogeny, it is likely
that some elements are arrested from fusing while others are enforced.

  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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