More On The Dino-bird Link

From: Thomas_R_HOLTZ@umail.umd.edu (th81)

>Among archosaurs, did birds originate the kinetic skull (the front of the
>skull can be bent upward)?

Although some have tried to identify kinetic structures in the crania of nonavian theropods, they do not unambiguously possess the detailed structures found in birds.

>Does Protoavis really have a kinetic skull? (Chatterjee says yes. Is
>that a part of his analysis that is in question?)

ALL of the analysis is in question, as the "monophyly" of the specimen has not been established. The braincase looks theropodan (even coelurosaurian), but the reconstruction of the rest of the skull is questionable (to be polite).

>Does Archeopteryx have a kinetic skull? (I think not--yes?)

Some have tried to identify these features (paper in the Archaeopteryx conference volume; can't recall the authors right now). They do seem to be there, at least incipiently.

>If Protoavis did, and Archeopteryx did not, might that suggest that
>Archeopteryx is a flying theropod rather than a flying bird? That
>would surely explain why Archeopteryx plots out very close to
>theropods.

It might if a) Protoavis was found to be a real organism, and not a chimera; b) in a phylogenetic analysis, Protoavis was found to cluster with true birds; c) in the same analysis, Archaeopteryx was found NOT to cluster with birds (and presumably would be clustering with Dromaeosauridae).

However, your last sentence does not read correctly. Archaeopteryx does not plot out "close to theropods"; it plots out DEEPLY NESTED within coelurosaurian tetanurine theropods. In the same analyses, however, it shares more derived features with modern birds than it does with nonavian dinosaurs. Thus, the last sentence should read that" Archaeopteryx plots out close to birds": not a surprising thing, after all.

>What is a bird, really? Can skeletal features provide a
>characterization, since feathers normally won't be preserved? I mean
>without assuming a particular phylogeny first, since I'm not sure we know
>whether BCF or BADD.

Depends on whether you choose anatomical or phylogenetic definitions of taxa. If by "birds" you mean 'Aves', then some definitions include "all descendants of the most recent common ancestor of ratites and neognaths" (Gauthier, 1986) or "all descendants of the most recent common ancestor of Archaeopteryx and modern birds" (used by Chiappe). As Olshevsky will surely point out in his response, his formulation of Aves includes many forms not traditionally called "birds".

>Of course, if we assume that Archeopteryx is a bird, that really stacks
>the deck in favor of maniraptoran ancestors for birds. What should I
>know that would show me why the claim of theropod ancestry for birds is
>not this transparently circular?

If Archaeopteryx is not included in these analyses, it doesn't necessarily rip out the structure. As has long been established, birds and the traditional dinosaurs (especially theropods) uniquely share many features of the pelvis, hindlimb, feet, pneumatic structures, tails, necks, etc. Archaeopteryx nicely preserves a mosaic of primitive and derived characters, and is known from many great specimens, which is why it is central to many phylogenetic studies. If, however, we were to use Sinornis, Iberomesornis, Cathyornis, and other Mesozoic birds but excluded Archie, these derived features would still be there.

Furthermore, Archaeopteryx doesn't "stack the deck" for the maniraptoran position of birds. Several paleontologists (including some on this net) have reconstructed phylogenies where traditionally nonavian theropod groups (troodontids, oviraptorids, etc.) occupy a position closer to modern Neornithes than does Archaeopteryx.

Where do you see the circularity here? Archaeopteryx shares derived features with modern birds, although it lacks many advanced features of later birds. It has long been considered a bird because of this (and more recently, because the most parsimonious distribution of these characters place it closer to unambiguous birds than to all/most "theropods").

From: "Nicholas J. Pharris" [pharrinj@PLU.edu]

> If Protoavis did, and Archeopteryx did not, might that suggest that
> Archeopteryx is a flying theropod rather than a flying bird? That
> would surely explain why Archeopteryx plots out very close to
> theropods.

Well... Yes. But if Archaeopteryx is a "flying theropod," then the very similar Confuciusornis and Sinornis are, too. And if THEY are theropods (and not birds), then the same is true of Concornis, Iberomesornis, Ambiortus, and the enantiornithians. I think you can see where I'm going with this one.

Suffice it to say, nowadays we really have a pretty good morphological sequence of fairly well-preserved animals linking Archaeopteryx securely with the modern birds. Hence, if Archaeopteryx was not a bird, then neither is a house sparrow.

> What is a bird, really? Can skeletal features provide a
> characterization, since feathers normally won't be preserved?

Certainly. Birds are fairly unusual critters. And practically every skeletal peculiarity in birds can be traced to non-avian dinosaurs.

> Of course, if we assume that Archeopteryx is a bird, that really stacks
> the deck in favor of maniraptoran ancestors for birds. What should I
> know that would show me why the claim of theropod ancestry for birds is
> not this transparently circular?

Non-archaeopterygian birds also show distinctly maniraptoran characters, including the carpal block, deep coracoids, large sterna, and the form of the hand. I particularly like the illustration by Bakker in The Dinosaur Heresies of the hands of a hoatzin hatchling, Archaeopteryx, and Deinonychus. Wow. Check it out.