Re: fused/unfused scap/coracoid

["Jaime A. Headden" <qilongia@yahoo.com>]

Dave Peters (davidrpeters@earthlink.net) wrote:

<I notice in some primitive diapsids (Protorsaurus and kin) that the
scapula and coracoid are fused. This pair becomes unfused then fused again
a few times in daughter taxa. Any thoughts on why this does or doesn't
happen, especially in these early diapsid sprawlers?>

  Fused scapcoracoids occur in crocodilians as well, and also in many
lacertilians. This is either an example of translating forces from one to
the other (the coracoid is "in line" with the scapula and related to the
abductor muscles that worked in direct, parallel opposition to the
adductors of the scapula above the humeral glenoid) or by resisting
stresses during stroke. This occurs in other quadrupeds, including
sauropod dinosaurs, some terrestrial crocodilomorphans, and iguanodont
dinosaurs. The L- or V-shaped conjoined scapulocoracoid complex of some
pterosaurs has some relation to that of crocodilians, but not that of
birds, and may not relate to flight per se, bus specialization related to
mechanical loading and muscle effect angles and effects on bone when
exerted.

  However, is there a correllation between angle and fusion in pterosaurs
for which partial fusion, complete fusion with obliterated sutures (as in
azhdarchids and pteranodonts/nyctosaurs), or even simple loss of the
synovial joint? This may have NOTHING to do with phylogeny if this is a
mechanical feature relating to flight, or loading.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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


                
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