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Re: Birds and mosasaurs
> Not sure at all. My argument is for agnosticism, not for any particular
> lineage. My
> personal preference is for a pretty basal Theropod, but that's just a hunch
> or a bias,
> not a deduction.
Putting bird ancestry among the coelurosaurs IS a deduction.
Tetanurans have all the general theropod bird-like characters, plus a
few more. Coelurosaurs have all the tetanuran AND general theropod
bird-like features, plus a few more. Dromeosaurs and troodontids have
all the coelurosaurian AND tetanuran AND general theropod bird-like
features, plus a few more (even if they are different for the different
groups). Nailing down the family may be complicated by convergence, but
nailing down the coelurosaurs as the theropod group closest
to birds is not.
> > This reminds me of Fedduccia's argument
> Bah! Guilt by association? (I'm supposed to break down and, like Little
> Buttercup, cry
> out: "That name! Remorse! Remorse!" ?)
I wasn't trying to make a personal comment, but Fedduccia DID
say that, and you seem to me to be following a similar line of
reasoning. Fedducia says the tendancy of coelurosaurs to evolve
bird-like features repeatedly implies that they were not ancestral to
theropods, and you seem to be implying the same.
> Remember, we're dealing with similarities that you indicate were
> independently derived
> in different lines.
Yes, but the common ancestror of those independantly derived lines are
nestled well in the Coelurosauria. If you see the independantly
derived bird-like characters between dromeosaurs and troodontids, or any
of the bird-like features in coelurosaurs as a problem, you are still
starting out from a common ancestor that is a coelurosaur, not a basal
theropod. Convergence causes problems INSIDE the Coelurosauria; not so
much between the coelurosaurs and other theropods.
> the traits, may have arisen earlier, perhaps much earlier. However, few
> clades are as diverse or prolific as the Coelosaurs, so what we see is the
> working out
> of bits and pieces in the relatively large sample of Coelosaurs of features
> which may
> have been united in another, more distantly related species.
Some are and some aren't; see the first paragraph. The cladistic work
on theropods in the past decade or so has been coming up with lists of
coelurosaur apomorphies that because they are called "apomorphies" are
implied NOT to be present in other tetanurans, or theropods in general.
> I agree that its very likely that Mr Bluebird is a Theropod, though I'd buy
> an earlier
> type. Jeez, think how little we know about Therizinosaurs, Prosauropods or
> forms like Pisanosaurus or Eoraptor.
And think about how much we DO know about _Archaeopteryx_,
dromeosaurs and troodontids. Again, if ALL of what we are calling
synapomorphies between birds, _Archaeopteryx_, dromeosaurs and troodontids
in general are in fact analagous rather then homologous, it is a pretty
amazing case of evolutionary convergence. Even if the theropod fossil
record outside of the Coelurosauria was so impoverished that we couldn't
be sure other theropods didn't have the features we are calling
coelurosaur-bird "synapomorphies" (and since it isn't that impoverished,
we know that they didn't), the case for a coelurosaur-bird link would
still be very good. It isn't just the closest fit to date, it is a GOOD
> However, it reaches a point of diminishing returns to keep counting
> synapomorphies dancing on the head of a pin (particularly if, as I'm
> beginning to
> suspect, its hard to agree on what the appropriate outgroup should be).
If those synapomorphies are really synapomorphies, what do you
think that means?
On the subject of _Protoavis_ I know admittedly little, so am am in
no position to debate on it. However, if I am not mistaken _Protoavis_
seems to have a lot of bird-like features not seen in _Archeopteryx_, so
if Chatterjee is right the bird-like features of _Archaeopteryx_, like its
very bird-like flight feathers, are convergent with birds. Also, I am
willing to lean a little more heavily on the opinions of paleontologists
who have been studying the case for a coelurosaur-bird link in detail then
those who have not, especially considering most (including Phil Currie)
who have examined _Protoavis_ still consider a link will coelurosaurs to
be more stable. Finally, if _Protoavis_ IS ancestral to birds, it still
means the bird-like features of coelurosaurs and the bird line are
convergent, not ancestral to both. The theropod fossil record is
good enough to recognize that a lot or most of the really compelling
coelurosaur-bird similarities are NOT present in most theropods, and
therefore do not extend as far back down the theropod family tree as
you've been implying. If coelurosaurs are not ancestral to birds, they
evolved more then once.