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Re: Coracoid/scapula (was Re: flying Archie)

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

<Ah, then we aren't talking of exactly the same character. I am talking of the
joint, which I interpret as having 3 possible states: flat (plesiomorphic),
coracoid in scapula, or scapula in coracoid. I may be wrong in which of these
is the plesiomorphy, but the reversals I mentioned all require independent
losses of the joint.>

  I beleive it is possible to derive mobility between two surfaces in an
unfused contact between two bones (aka, what David is calling a "joint"), but
it is equally possible that such movement would be impossible based on the
shape of those surfaces. For example, if the margins were flat and the surfaces
straight from one side to the other (i.e., glenoid to acromion) then one could
possible pull the two bones into an angle away from their natural position, and
would pull on the ligaments connecting the bones equally along the entire
surface with a point of pivot being an entire edge. This could be given as a
"joint" regardless of the surface anatomy. Again, if that surface was, perhaps,
curved along its length, the pulling of the surfaces would occur at only two
points on either side or one point in the middle, and the distance the other
margins must move relative to the others and the strain on the ligaments
connecting them greater, such that movement might be impossible without
effectively "breaking" the joint. Yet both of these are "joints". There should
be some specification on the meaning of this (mobile versus immobile for one),
as well as that even if such movement is possible, it might be so mild as to be
ineffective, and the ligament strain in even the straight margin could be too
much to make an effective "joint" movement possible.

  Another issue that has been raised is that if an animal is to have two of
these bones separated in anyway, they could be said to be mobile relative to
one another. This seems to be the null hypothesis when it comes to avian-like
shoulders, but I see no reason why this should be true, though it has been
argued as a probable feature in non-avian dinosaurs argued as avian-ancestors
(for example, Paul in _Dinosaurs of the Air_ and elsewhere). I beleive Ostrom
also mentioned this in his *Deinonychus* works, but I could be mistaken.

  Flexure of this region is used NOW in birds for assisting respiration and, in
part, is coupled with the flight stroke (which are interdependant actions). But
this may not have been the function in the past, or may have been developed
relative to breathing BEFORE development of the flight stroke. Fusion of the
should can occur to prevent this, or force the entire should as a unit to move
in a single action, instead of more than one. A small sternum is coupled with
an immobile scapulocoracoid suture in oviraptorids and dromaeosaurids, appears
to be the case in troodontids and microraptorians, but not the case in [at
least some] caenagnathids. The opposite is to be said of birds within
Ornithuromorpha,  where a large sternum is coupled with a mobile
scapulocoracoid joint. Between these two extremes, are a host of large
sternum/fused or "locked" suture animals.

  So, I think the issue of the shape of the joint may be related, in any way,
to its function, but the shape must be clarified relative to mobility possible,
and clarification of use of the term "joint" in this case, be made in order to
better understand this transition.


Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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