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RE: comments in this week's Science about the Thermopolis Archaeopteryx
Thomas R. Holtz, Jr wrote-
Two brief articles in the latest Science (actually, a Technical Comment and
Response) about the Mayr et al. article on the
Thermopolis Archaeopteryx:
Corfe, I.J. & R.J. Butler. 2006. Technical Comments: Comment on "A
Well-Preserved Archaeopteryx Specimen with Theropod Features".
Science 313:1238.
snip<
and the response:
Mayr, G., and D.S. Peters. 2006. Response to Comment on "A Well-Preserved
Archaeopteryx Specimen with Theropod Features". Science
313:1238.
Corfe and Butler (2006) find that Confuciusornis is in the
Archaeopteryx-Rahonavis clade in trees one step longer than the tree
published by Mayr et al. (2005), where it was sister to Microraptor. They
altered codings in the Buitreraptor paper after Mayr et al. and found that
it did not change Confuciusornis clading with Archaeopteryx and Rahonavis
clading with dromaeosaurids. The recoded two characters Mayr et al. changed
in Archaeopteryx, coding the scapulocoracoid and metatarsus as fused. I'm
not sure which coding is correct, as I haven't looked into either character
yet. I think Mayr and Peters (2006) are right to defend against the
assertion they suggested flight originated twice (as opposed to being lost).
However, they then state-
"We are not aware of any derived character shared by Archaeopteryx and
Confuciusornis that is not also present in the deinoychosaur Microraptor.
However, and as noted above, Microraptor shares derived characters with
Confuciusornis that are absent in Archaeopteryx, and we thus do not agree
with Corfe and Butler's comment that "the hypothesis of a polyphyletic Aves
is no better supported by available data than that of a monophyletic Aves."
One of the essentials of phylogenetic systematics, which makes it superior
to other methods of phylogenetic reconstruction, is the naming of
apomorphies of the clades in question. Purely statistical
comparisons reduce it to just another numerical approach. Unless Corfe and
Butler can present unambiguous apomorphies of a clade including
Archaeopteryx and Confuciusornis, to the exclusion of deinonychosaurs, we do
not believe that their argument poses a robust challenge to our hypothesis
of bird ancestry."
Yet the entirity of the evidence is what's important in cladistic analyses,
not just comparisons of two nodes in competing trees. While the
Archaeopterx-Confuciusornis node may only have been supported by two
characters, and the Microraptor-Confuciusornis node supported by five, the
former tree was only one step longer than the latter tree, so additional
support for the whole topology must be present at other nodes. For
instance, placing Confuciusornis in Dromaeosauridae necessitates that it
reverse dromaeosaurid synapomorphies or that Microraptor evolve them in
parallel to Sinornithosaurus and other dromaeosaurids. This isn't needed in
the Archaeopteryx-Confuciusornis tree, so probably helps it be shorter than
it would be otherwise.
Also, there are derived characters shared by Archaeopteryx and
Confuciusornis to the exclusion of Microraptor. The potential list is
shorter than if other dromaeosaurids were used, in part because Microraptor
is more aerial/birdy, but importantly also because it lacks well
preserved/described cranial remains. So the palatal and braincase
characters shared by Archaeopteryx and birds to the exclusion of
dromaeosaurids aren't known in Microraptor, though they would still count
against Mayr et al.'s topology (since Microraptor is still a dromaeosaur).
They aren't known in Confuciusornis either, as all described remains are
extremely crushed. The potential list of derived characters is also shorter
because confuciusornithids have heavily modified skulls and hands compared
to other basal birds and paravians. So characters like Archaeopteryx's
serrationless teeth can't be included to indicate it is more birdlike than
Microraptor, as confuciusornithids lack teeth. But despite these handicaps,
there are some obvious derived characters shared by Archaeopteryx and
Confuciusornis that aren't found in Microraptor. For one thing, the
external naris is larger, the dorsal premaxillary process longer and the
anterior margin of the external naris is placed further posteriorly. Also,
the tail is shorter (less than four times femoral length) and made of less
vertebrae (<24). Posterior dorsals have lateral central fossae in the two
birds, but not Microraptor. Finally, when viewed edge-on, the coracoids are
bent (at the level of the coracoid tubercle) more in Archaeopteryx and
Confuciusornis than in Microraptor.
Of course the TWG matrix isn't the best to solve this issue, as it was
designed with deeper relationships in mind. One very important point is
that this isn't a three taxon problem. The character distributions of taxa
like omnivoropterygids, Shenzhouraptor, Jixiangornis, Dalianraptor,
Jinfengopteryx, scansoriopterygids, Buitreraptor and undescribed Ukhaa basal
troodontids(?) are important to resolving the phylogeny in this area of the
tree. I wouldn't be surprised if none of these taxa (or confuciusornithids
and Archaeopteryx) were closer to birds than Dromaeosaurus, Troodon or
Oviraptor are.
Mickey Mortimer