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WHEN IS A QUADRATE NOT A QUADRATE?
I have just received a most interesting paper:
Welman, J. 1995. _Euparkeria_ and the origin of birds. South African
Journal of Science. 91: 533-537.
Of the many interesting points inside the paper, Welman says that
theropods and crocodylomorphs are too specialized to be the ancestors of
birds (the possession of a derived median eustachian system is one
feature; last time I checked, it was considered homologous to the
basisphenoidal sinus and basisphenoid recess) and _Euparkeria_ is the
quintessential avian ancestor based on braincase features. I don't buy
it for a second, but it is rather compelling and the paper is a good
mine of information on avian braincases (with a few errors).
Anyway, to the title of the post... Welman also makes a most interesting
interpretation of the 'unidentified skull bone' of the London
_Archaeopteryx_; it is a broken piece of the left side of basicranium.
This is of interest because many recent authors (Walker and Elzanowski
and Wellnhofer) have identified it as a quadrate. The 'pterygoid
flange'-like structure is a basipterygoid process, the broken base is a
alaparasphenoid with a rostral tympanic recess (a very birdlike
feature), and the dorsal head is the basiparasphenoid (compare to
braincase floor in Fig. 2A of Elzanowski and Wellnhofer 1996). Most
interesting is this interpretation because it reopens the issue of
whether or not _Archaeopteryx_ has a bipartite quadrate articulation;
the seventh specimen, contrary to some people, is indeterminate in the
matter of a single-headed or double-headed quadrate:
"There is no indication of the medial (prootic) head although its
presence cannot be positively ruled out as the otic process is damaged
in this area." Elzanowski and Wellnhofer; 86.
Elzanowski and Wellhofer say that the _Archaeopteryx_ quadrate is
single-headed only because of the interpretation of the London
'unidentified skull bone'.
So, from my viewpoint, the issue of a single or double head in the
_Archaeopteryx_ quadrate is up in the air.
MORE INTERESTING STUFF
I know from first hand experience that it is very hard to impossible to
tell whether a bird quadrate is double-headed on the skull is viewed in
lateral view. Heck, they don't call it the medial quadrate head for
anything. Also, the medial head is not easily viewed even in a frontal
view of the avian skull, since it is occasionally very high on the avian
quadrate and sometimes hidden by the laterosphenoid and ectemoid.
Occasionally, the quadrate of ratites appears single-headed and only a
close examination shows that the quadrate is double-headed.
Another interesting bit of evidence for the presence of a double-headed
quadrate in _Archaeopteryx_ is the weird flat threshold to the caudal
tympanic recess on the London paraoccipital process (the structure of
the threshold to the caudal recess is different from various birdlike
theropods); Whetstone interpreted it as a quadrate articulation area but
Walker suggested otherwise since there was evidence for a fairly typical
archosaurian quadrate articulation rather than the articulation
suggested by Whetstone. Anyway, it can still be interpreted as a
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