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Bird Metacarpal Homology (Hinchliffe 1985)



        George got me to lookin' up stuff a few weeks ago, and I came up
with  Hinchliffe 1985: "One, two, three" or "Two, three, four": An
Embryologist's View of the Homologies of the Digits and Carpus of Birds, in
_The Beginnings of Birds_ (Bronner and Drentler, Eichstatt, 1985).
        Hinchliffe attempts to demonstrate, using two different lines of
embryological evidence, that the digits of the avain carpometacarpus are
II-III-IV, then proceeds to use this as evidence that birds are not derived
dinosaurs.  While the latter point strikes me as weak, since he is using
embryological evidence to dispell the homologies posited by workers who are
looking at osteological evidence, while a priori accepting the homolgies
these workers postulate for another group, the basic premise of the paper is
very interesting.
        Hinchliffe's first point involves the location of what he identifies
as the pisiform bone.  Based on it's location between a small splint he
identifies as a metacarpal which is lost later in ontogeny, he declares that
the minor metacarpal of birds is homologous to the IV digit of other
tetrapods.  He does not consider the possible effect reduction of digit IV
would have on the pisiform, and whether the pisiform might have migrated
after the ontogenetic elimination of a digit IV.  Which leads to two
homology questions for the group:
        1) Is there a large body of evidence that the bone Hinchliffe
identifies as the pisiform is in fact homologous to that bone in other
animals?  Either Paul or Bakker (sorry GSP!) states (in PDW or DH) that
dinosaurs lack a pisiform, which immediately makes me question any argument
based on homology.
        2) If it is the pisiform, is it wholly improbable that this bone
might migrate to support the carpometacarpus if digits IV and V were lost?

        Hinchliffe's second point, which I will admit is way way over my
head (my apologies to all if I mangle the terms...), involves a region of
mesenchymal cell death on the anterior (thumbward?) and distal ends of the
wing bud.  He cites evidence (Ede 1971, Summerbell 1981) that the width of
this mesenchyme is related to the number of digits formed.  He does not say
whether this mesenchyme is present absent in other tetrapods (except the
mouse), but points out that mice (no species given), which have a longer
digital plate, do not have have necrotic zones, and grow five digits.  This
seems to be negative evidence.
        This second point is more interesting, but leads to questions:
        1) Is there evidence for such a mesenchyme in any non-avian tetrapods?
        2) How correlative is this evidence, when he points out that while
there is a short-lived posterior (pinkieward) necrotic zone in the species
he was studying, this is not present in other avian species.  How certain
can we be of the theory that the anterior zone is associated with digit loss
when the posterior zone, in an area which certainly loses at least one digit
(so we think...) has no necrotic zone in other avian embryos. 
        3) Even more confusingly, in this one species where there is a
posterior necrotic zone, there is in fact a vestigial metacarpal,
interpreted in the article as V.  This seems to muddy the association of a
mesnchyme with digit loss.  Or is this not as contradictory as it seems?
        4) Does anyone know the argument made in the references cited above
(full refs available...) for the length of the mesenchyme and digit
reduction?  Is the evidence only from birds?

        I must say that, for all that, the paper is very readable for
someone with little experience in embryology.

        Anyone with any knowledge of osteology or embryology, please feel
free to help me better understand this paper.

        Thanks,
        Wagner
  
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| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
| Lubbock, TX 79409                               and if they don't clade, |
|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
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