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Re: Birds and mosasaurs



Jeffrey Martz wrote:

> However, given the
> incompleteness of the Jurassic fossil record, how certain are you that, at
> least for a lot of Cretaceous dinosaurs, you are not seeing a group that 
> actually
> extends back a lot futher then is currently known?  

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.  Troodonts and dromeosaurs are fine dates, I just think its 
too early 
to get married.  (My ex reminds me a little of a Coelophysis, so you understand 
my 
caution ...)

>       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!" ?)

> how does the REPEATED evolution of bird-like characters
> in a group make it UN-likely that bird origens lie within that group?  If
> a group liked to evolve bird-like charatcers a bunch of times, doesn't
> that increase the likelyhood that one eventually produced a bona-fide
> bird?

Absolutely.  Apparantly quite early on.  Taking Jonathan Wagner's comments on 
Protoavis 
to heart, quite possibly *very* early on.  Too early on even for Coelosaurs? 
Perhaps.

>      When you say "at a minimum", you seem to be implying that
> all dinosaur-bird synapomorphies are found in a more general theropod
> group then Coelurosaura.  This is not the case.  Although Dinosauria,
> Theropoda, and Tetanurae have several avain synapomorphies, Coelurosauria
> has even more.  In fact, as you go through these taxa from the most to
> least inclusive, the number of characters goes up; just like you would
> expect if bird-origens lay within the Coelurosauria.  The dromeosaurs and
> troodontids are the most bird-like in a VERY bird-like group.

Remember, we're dealing with similarities that you indicate were independently 
derived 
in different lines.  What the Coelosauria indisputably had is the evolutionary 
and 
genetic potential to develop many different bird-like features.  That 
potential, if not 
the traits, may have arisen earlier, perhaps much earlier.  However, few 
archesaur 
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.

>       There ARE no other derivations, unless you want to play the same
> game as Fedducia and Larry Martin and invent COMPLETELY hypothetical
> bird-like crocodylomoph groups and very late surviving thecodonts, a
> guessing game a lot more extreme then extrapolating dromeosaurs and
> troodontids into the Jurassic.  At least we have evidence that there
> were THEROPODS then for God's sake.  As has been pointed out
> in the past, even Chatterjee proposes that _Protoavis_ is a theropod, and
> if he is right the number of bird-like convergences among
> theropod groups goes up even more. 

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 
transitional 
forms like Pisanosaurus or Eoraptor.  I agree we can't wait for fossils to be 
discovered 
to use our heads.  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).  I 
don't think 
its wrong, for example, for Rubin to ask, OK -- now what do all those 
synapomorphies 
really mean for the physiology of the critter?

  --Toby White