The endless recursions of Feduccia and friends

["Jonathan R. Wagner" <znc14@ttacs1.ttu.edu>]

Debby Hoffmaster wrote:
>The researchers (N. Carolina, I believe) studied the development of
>the "hand" in "birds" and "dinosaurs" (no species given in the report).  The
>results indicate that fingers 2,3 & 4 comprise the bird "hand" (loss of 1 and
>5) whereas fingers 1, 2 & 3 comprise the "theropod" hand (loss of 4 & 5). 
        I was digging through my files, and came up with this, which was
posted on my now-defunct website. Not to toot my own horn, but I think it
goes nicely with Nick Pharris' posting, reproduced at www.dinosauria.com. I
guess this is a warm-up for Feduccia's new rag, which cometh soon.

_____________________________________________________________________________

                Bird Metacarpal Homology
                by Jonathan R. Wagner

Something George Olshevsky said about a month ago got me to lookin' up stuff,
and I came across:

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.
        The latter point strikes me as weak, since he is using embryological
evidence to dispell the homologies posited by workers who are looking only
at osteological evidence, while a priori accepting the homolgies these
workers postulate for another group.  On the other hand, part of the point
of the paper is that homologies established on osteological evidence may be
weaker than is often thought.  In any case, if one were to accept Dr.
Henchliffe's findings at face value, most parsimoniously it would simply
cause us to reconsider the homologies of the theropod manus.
        [Translation: if birds' digits are II-III-IV, given the evidence,
isn't it just a simpler conclusion that dinosaurs' digits were II-III-IV?]

        The case in point is the ulnare, which was previously thought to
make up part of carpometacarpus.  Hinchcliffe demonstrates that an element,
which he considers the ulnare (he's got me convinced), actually disappears
during ontogeny, and an element, which he considers the pisiform, fuses with
the carpometacarpus.

        For those of you out there who really groove on this stuff, there's
also a bone identified as X for which the homologies are unclear.  This bone
fuses to the carpometacarpus in the place where the ulna was thought to attach.

        Hinchliffe's first major point involves the location of the bone he
identifies as the pisiform.  He identifies it based on it's location with
respect to the radius, ulna and the "postaxial" border.  Based on it's
location between a small splint he identifies as a vestigial metacarpal
(which he assigns as V), he declares that the minor metacarpal of birds is
homologous to the IV digit of other tetrapods.

        Which leads to questions:

        1) Is there evidence that the bone Hinchliffe identifies as the
pisiform is in fact homologous to that bone in other animals?  Either Paul
or Bakker states that dinosaurs lack a pisiform.
Of course, given Hinchliffe's statements on osteologically determined
homologies, can we be sure of anything...

        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?
In the paper, Hinchliffe does not consider the possible effects reduction of
digit IV/V/whathaveyou would have on the pisiform.  

        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/alular,etc) and distal
ends of the wing bud.  He interprets this anterior cell death as possibly
being the "mechanism for  eliminating the tissue which... formed digit I."
He cites evidence (Ede 1971, Summerbell 1981, refs below) that the width of
the mesenchyme is related to the number of digits formed.  He interprets
this anterior cell death as possibly being the
"mechanism for eliminating the tissue which... formed digit I."  He does not say
whether this necrotic zone is present in other other tetrapods (except the
mouse), but points out that mice (no species given), which have a
antero-posteriorly longer digital plate, do not have have necrotic zones,
and grow five digits.  Not really negative evidence, but perhaps questionable.

        This second point is more interesting, but leads to questions:

        1) Is there evidence for such a necrotic zone in any non-avian
tetrapods, especially animals which lose digits in ontogeny?

        2) Is his association of the necrotic region with the loss of digit
I consistant, considering 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, while all extant avians have experienced the
loss of two digits?  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.  Could this be related to the fact that there seems to
be a vestigial metacarpal still present on the posterior (pinkie) side?

        3) Conversely, 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.  Is this not as contradictory as it seems?

        4) Does anyone know the argument made in the references cited above
(full refs below) for the width of the limb bud and digit reduction?  Is the
evidence only from birds?

        I should add that, to the untrained eye, in the pictures presented,
it certainly *looks* like the alular digit is digit II.

        I must say that, for all that, this paper is very readable for
someone with little experience in embryology, although the terminology can
be a bit confusing.  Anyone with any knowledge of osteology or embryology,
please feel free to help me better understand this paper.

Refs cited by Henchliffe:

Ede, D.A. 1971
Control of form and pattern in the vertebrate limb - In:
Control mechanisms of growth and differentiation (Davies, D.D. and Balls, M.
eds.) pp 235-254 Symposium 25 of the Society of Experimental Biology.
Cambridge University Press
Summerbell, D. 1981
The control of growth and the development of pattern
across the antero-posterior axis of the chick limb bud - J. Embryol. exp.
Morph. 63:161-180. Cambridge
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    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
        "Chimp here does the killing." - Doug Mackenzie