Re: hypocleidium (an interclavicle???)

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

Ken Kinman (kinman@hotmail.com) wrote:

<Is the hypocleidium found in nearly all Pygostylia (or are there a lot of 
exceptions?), and is it
homologous to the "hypocleidium" found in Oviraptor and Bambiraptor (any other 
non-pygostylians
with it)?
  And are either or both of these  homologous to the interclavicle (or is that 
an old idea that
has definitely been disproven)?>

  No memeber of the Dinosauromorpha possesses an interclavicle. This is a 
distinct osseous element
that interjects between coracoids and between each of the clavicle bones; the 
interclavicle has a
tri-radiate morphology, essentially Y-shaped in form, leading to the confusion 
I'm sure that
spawned the question. In the period of time that the dinosaurian furcula of 
*Oviraptor* was
offered as an interclavicle since Osborn's 1924 description, it has since been 
the prevalent
scientific conclusion that this element is in fact a fusion or collection of 
two clavicles without
an interclavicle, and no the interclavbicle itself. Even as recent as 1986, the 
fusion of the
clavicles was regarded as "fused clavicles" and not a "furcula," and is still 
regarded as such by
the more resistant of the ornithologists (see Feduccia, 1996, for example).

  The hypocleidium is a structure that appears early in the development of 
theropods with
furculae, appearing in allosauroids and tyrannosauroids, as a distinct process. 
It appears to be
an extension of the fusion of two diaphyses, such as the ramal shafts of the 
paired clavicles. It
is lost in early birds, such as *Archaeopteryx* and *Confuciusornis*, but is 
derived in higher
groups, so that nearly all modern birds possess it. Rather than re-expression, 
it is likely that
the hypocleidium in higher birds is probably a neomorph structure adapted based 
on tendon
attachment, rather than the ends of two fused diaphyses.


=====
Jaime A. Headden

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

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