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Re: Bird Metacarpal Homology (errata and addenda)
The following is a series of errata and addenda for the earlier
posting on Hinchliffe 1985. Many thanks to Mickey for suggesting this
format, and for his patience and time.
One seriously embarrassing mistake was the use of "mesenchyme" to
mean area of "mesenchymal cell death". I think I got all of these points
below, but if I missed one, rest assured that I have been properly flogged.
>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, [...]
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.
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
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 point involves the location of what he identifies
>as the pisiform bone.
He identifies it based on it's location with respect to the radius,
ulna and the "postaxial" border.
>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.
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.
>He cites evidence (Ede 1971, Summerbell 1981) that the width of
>this mesenchyme is related to the number of digits formed.
The antero-posterior (thumb-pinkie) width of the limb bud is
interpreted as being related to the number of digits.
He interprets this anterior cell death as possibly being the
"mechanism for eliminating the tissue which... formed digit I."
>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.
mice (no species given), which have an antero-posteriorly longer
digital plate, do not have have necrotic zones, and grow five digits. Not
really negative evidence, but perhaps questionable. Any comments?
> 1) Is there evidence for such a mesenchyme in any non-avian tetrapods?
Necrotic zone on the limb bud, not mesenchyme.
> 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?
Necrotic zone, not mesenchyme.
> 4) Does anyone know the argument made in the references cited above
>(full refs available...) for the length of the mesenchyme and digit
Limb bud, specifically.
I should add that, to the untrained eye, in the pictures presented,
it certainly *looks* like the alular digit is digit II.
Sorry for any confusion,
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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