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Semilunates and the Digit-Homology Question



Mickey Mortimer (mickey_mortimer@email.msn.edu) wrote:

<Hou et al. (1999) argue since the semilunate carpal only fuses
to the second metacarpal, it must be derived from a single
distal  carpal. This simply doesn't follow logic. The semilunate
still articulates with metacarpal I, so it's not reduced in size
compared to Archaeopteryx and other coelurosaurs. There is no
evidence suggesting the semilunate of birds isn't formed from
distal carpals I and II, unless you want them to seem less 
maniraptoran.>

  Or that they, in fact, do not correspond the the same
elements, period. We have no paleoembryology of the LJ--EK
"birds" with which to test either Gauthier's or Hincheliff's
hypotheses, except the data from theropod digit reduction and
avian embryology....
 
Ken Kinman (kinman@hotmail.com) wrote:

<A Feducciary (not on this list) says that he "can turn around
and ask what is the evidence that suggests the avian semilunate
is formed by a fusion of distal carpals 1 and 2?">

  And essentially, his logic would be true. We have no valid
embryology for *Confuciusornis* or *Archaeopteryx* (which would
be ideal) and the Las Hoyas hatchlings have not been as studied
in depth as would be ideal. There _are_ embryos of
oviraptorosaurs and troodontids, and whatever those
Macroelongatoolithus thingies are [therizinosauroids? Your guess
is as good as the unpublished study, if it's been done or being
worked on]. But the wrist has not been studied, and I'd almost
bet you its not ossified.

<There isn't any. And if I read Feduccia (1996:70) correctly,
the avian semilunate is a single bone throughout the development
of living birds; there is no evidence that it is formed by
fusion of any two bones during development. Developmental
biologist Richard Hinchliffe identifies the avian semilunate as
distal carpal 3! (Ibid.) Developmental biology, therefore,
argues against the avian semilunate being the product of bone
fusion.">

  I'm willing to take the various papers from Hincheliff on the
subject at face value, without undergoing the studies myself
(but I have that luxury, since this isn't a field I think I have
too much experience or status on, and would not reflect my
current trend in studies: functional morphology, biomechanics).
But as Dr. Holtz has stated on this list, what seems not to be
very prevalent is the idea that _both_ concepts may be correct
and that at some unknown point the avian transition underwent a
carpal paradigm shift.

  In the transition from a 1-2-3 to a 2-3-4 wrist, the
semilunate contacting the first metacarpal would be lost or
reduced, transformed (pisiform of birds?), or not there to begin
with. Thus the carpal would be either the single 2, or even 3,
or both together, which would seem as possible, as the shift to
ectaxony in birds would neccesitate a shift in the primary
distal carpal. Now why is another matter.... :)

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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