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Re: Longisquama (was: Quo vadis, T. rex? [long])

In a message dated 96-02-07 11:57:09 EST, Robert.J.Meyerson@uwrf.edu (Rob
Meyerson) writes:

>Forgive my ignorance, but is this critter considered a dinosaur?  It
>was my understanding that when one starts getting into the Late
>Triassic, we start getting awfully close to the thecodont/dinosaur
>split.  At this range, do the associations begin to be muddled?
>Could it be argued that the likelyhood of _Longisquama_ is a separate
>thecodont lineage?  If so, then Archy and it's decendants (assuming
>for a moment that BCF is acccurate) might not be dinosaurs by any
>definition that the cladists would use.  Personally, I don't like the
>idea, but I figure it had to be asked.

I can't think of a single paleontologist who would consider _Longisquama_ a
dinosaur, BUT: _Longisquama_ had a furcula nearly identical in shape to that
of _Archaeopteryx_ (and _Allosaurus_ and _Oviraptor_), as well as the only
structures resembling feathers in any known pre-_Archaeopteryx_ vertebrate. I
consider these characters diagnostic of the clade Aves, though not of the
more inclusive clade Dinosauria. My take on _Longisquama_ is that it
represents a lineage of dino-birds descended from the common ancestor of
Dinosauria and is thus, cladistically, a primitive avian dinosaur.

It is possible that the furcula, which is the fusion of the clavicles into a
single element, appeared before the phytodinosaurs diverged from the avian
lineage, but if so one would expect at least _one_ primitive phytodinosaur to
exhibit a furcula. None is known. But _clavicles_ have been reported in some
primitive sauropod taxa. Thus, either the furcula appeared before
phytodinosaurs and was reversed in that group, or the furcula appeared after
phytodinosaurs. The latter possibility is the more parsimonious (as far as
furculae go, anyway).