Re: hypocleidium (an interclavicle???)

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

Ken Kinman (kinman@hotmail.com) wrote:

<So you are saying that the theropod "hypocleidium" and the pygostylian 
hypocleidium may not be
homologous (and therefore should probably be coded as separate characters)?>

  Not exactly. I am saying that the features may not be homologous to origin, 
but can be analogous
to function. The earliest theropod hypocleidium appears to be derived from the 
suture of two
diaphyses. This becomes adapted in more avian theropods, and is rather large in 
certain
maniraptorans (*Velociraptor,* *Oviraptor,* *Ingenia,* and *Citipati*). That it 
is not present in
birds until the Ornithothoraces suggests a loss. That the furcula becomes a 
single ossifying unit
in modern bird embryology can suggest that, where the hypocleidium becomes 
ossified over time,
extending distally, this single-furcular chondrosteosis can be extended back to 
Aves (including
*Archaeopteryx*) rather than limited to Ornithothoraces. Nothing else can at 
present be attributed
to this. Muscular and/or tendonitic attachment to the non-avian hypocleidia 
(*Velociraptor* and
oviraptorids) is not indicated at present, but it should be easy to examine and 
thus determine.
But not at present, based on published material.

<If so, is there any way to know which of these types is present in Protoavis??>

  The element attributed to *Protoavis* looks rather advanced, and includes a 
distinct distally
discoidal process that may very well be a hypocleidium. I do not affirm any 
homology of elements
to this taxon aside from the type braincase.

<Aren't such structures uncommon among non-avian theropods, or perhaps they are 
common but just
not often in ossified form?>

  There is a probability of about 0% of finding a hypocleidum in any other 
fircula... We cannot
know the distribution of a functional characteristic to elements of this 
nature. However, the form
of a furcula is probably certainly present in animals which use the arms in 
such a fashion as to
grasp objects, usually in tandem. Animals that use the limbs independantly do 
not possess objects
that impede swing of the coracoid against each other, such as in quadrupedal 
animals. When Greg
Paul illustrates glide-phases in the forelimbs and pectoral girdles of his 
quadrupedal dinosaurs,
it would be a good idea to follow along. Especially for those animals with 
forelimbs shorter than
the hind, thus increasing forelimb stride to equate to hindlimb stride. A 
furcula would impede
this. The abscence of such a structure in some animals, despite how well they 
are preserved
(ornithomimids, probably alvarezsaurians) suggests that the forelimbs could 
move independantly of
each other. Other than that, there isn't much else that can be derived from the 
lack of a furcula.
They can be implied not only by pectoral morphology (often with a facet on the 
scapular acromion),
but by phylogenetic braketing (Witmer, 1995, in _JVP_ 15(suppl. to 3); see also 
Witmer, 1997, in
_Encyclopedia of Dinosaurs_ for further discussions of this).


=====
Jaime A. Headden

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

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