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Re: At long last! Turner, Makovicky & Norell on dromaeosaurids



Jaime Headden <qi_leong@hotmail.com> wrote:

>   I do not think the current evidence suggests the sterna of 
> *Protarchaeopteryx robusta* present
> a good base for which large pectoralis muscles may be supported. I, of 
> course, haven't done the modeling
>  -- I am willing to be corrected, and viciously. My understanding is that 
> they *might* be stable if there was
> some method which kept the plates from moving relative to one another, rather 
> than kept them working
> together. There may be such a novelty as a hinge between them, controlled by 
> unknown ligaments or
> muscles, and which would thus bring them from states of instability (being 
> skewed) into stability (mirrored).


Yes, I see where you're coming from.  I was thinking a little more
broadly, and referring to the architecture of the entire shoulder
girdle, rather than to the sternum and pectoralis muscles
specifically.


In common with Ornithothoraces, certain basal avialans such as
_Jeholornis_, _Zhongjianornis_, _Jixiangornis_ and confuciusornithids
show features tied to increased reinforcement (stabilization) of the
pectoral girdle (e.g., ossified sternum; longer, strut-like coracoid).
 Although in these basal forms there was not yet a triosseal canal to
thread the tendon of the elevating (upstroke) m. supracoracoideus
through.  The pectoral girdle was modified to help brace the wing, but
fusion/coossification of the sternum into a single unit doesn't appear
to have been necessary at this point (although confuciusornithids have
it).  The next step was making the fused sternum much bigger and
keeled for attachment of the expanded supracoracoideus and pectoralis
muscles (the keel/carina being more important for the wing-elevating
supracoracoideus).


One biomechanical study came to the conclusion that
non-ornithothoracean avialans could not lift their wings higher than
their back, because of the primitive orientation of the glenoid.  This
means no upstroke, and no flapping.  If this interpretation is
correct, then the features associated with stabilization of the
shoulder region (ossified sternum; longer, strut-like coracoid, etc)
might be tied to the need to keep the arms outstretched during
"passive" (non-sustained) flight, rather than for flapping.


The point is... there is nothing especially "bird-like" about the
shoulder anatomy of _Microraptor_ or _Archaeopteryx_ or even
_Sapeornis_.  It's pretty much the same as _Protarchaeopteryx_
(although the orientation of the glenoid is unknown in this guy), or
any non-avialan paravian.  In terms of flight abilities, the shoulder
anatomy of _Protarchaeopteryx_ was no worse (or better) than
_Archaeopteryx_.






Cheers

Tim