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The Ornithodira: A New Hypothesis, or a Revived One?



  If Dave Peters' character analysis is not robust or
flown fast to produce the results, it does not
determine the unlikelyhood, for instance, that the
pterosaur + dinosaur clade (Ornithodira) was exclusive
of such taxa as rauisuchians, *Ornithosuchus*, etc. I
haven't run the analysis, lacking a cladistic engine
to do so, so I have little opinion on the results as
detirmined. However, I do have somethign to say on the
prolacertilian + pterosaur hypothesis, and its
implications for dinosaur + pterosaur relationships.

  Prolacertilians are funny little lizard-like animals
with strange pelves, shoulders, and skulls. Even the
vertebrae have some unusual features, and a possible
prolacertilian may soon be described with _very_
unsual vertebral morphology. Prolacertilians have, for
instance, ischia extended posteriorly in a rectangular
plate, which pterosaurs have, but dinosaurs lack. This
would usually bespeak change in dinosaurs from the
condition, and one could draw an evolutionary chain
from *Tanystropheus* to *Sharovipteryx* to
*Preondactylus* to *Marasuchus* to *Eoraptor* to
Saurischia and Ornithischia (yeah, I think it's that
primitive) based on changing pelvic morphology. The
evolutionary chain of either ornithodiran sensu
stricto or sensu lato demonstrates that sternal keels
are convergent, aquired in avetheropods and pterosaurs
at the earliest.

  One could also then argue that some of the features
shared between pterosaurs and dinosaurs were acquired
uniquely from a common ancestor that neither lost in
their evolutionary lineages. Antorbital fenestrae may
or may not be a unique feature, but were present in
archosauromorphs, and possibly more previously; Witmer
(1993) demonstrated that an antorbital fenestra can be
formed from an enlarged diverticular sac, and
crocodiles have these. Loss of expression of this
feature does not mean it wasn't there ancestrally,
however. Same for clavicular arches, or furculae,
which really must be understood functionally, rather
than just an expressed feature. They may be unique to
an avian + prolactertilian lineage, as hypothesized by
some (I can mention the Ruben abstract cited recently,
but will not engage the discussion), but expression in
maniraptoriforms outside the consideration of direct
avian outgroups demonstrates that this feature is not
directly related to flight. That *Allosaurus* lacks
furculae, but *Sinraptor* possesses them shows that
the structure does not preserve as a result of loss of
noise suppression in the geneaology, but suggests that
perhaps there was a functionary purpose nesseccary to
ankylosis of the clavicular halves or ossification of
the intraclavicular cartilage or tendons that allowed
*Sinraptor* to do something, or prevented something
else from occuring. Some suggestions are: reduction of
adverse force in the shoulder; as a spring mechanism;
a brace for preventing intragirdle movement; etc.. In
short, to consider the evolutionary significance (and
presence) of the furcula, the function (or functions)
of the structure must be understood, even if it
differs among taxa. A better understanding of the
thing would be nice, and perhaps someone will
undertake a Gatesy-like effort to fully strip away the
furcula's mystery. ;) ;)

  Meanwhile, what the sensu stricto hyposthesis would
mean? If pterosaurs and dinosaurs share a more recent
common ancestor than either does to prolacertilians or
crocodiles (an emendation to the original intent
behind Sereno's 1990 diagnosis), what does this mean
for prolacertilians? The distribution of argued
"avian" or "arboreal/glissant/scansorial" features in
some prolacertilians and related taxa, like
*Cosesaurus*, suggest an avian relationship; however,
these have been only marginally quantified. So,
similarly, an analysis of the avian similarities,
rather than generalities such as shape, length of
neck, etc. must be made to ascertain function,
evolution, and possibility of homology.

  I'm done.

=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

Qilong---is temporarily out of service.
Check back soon.

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