Re: smallest ANCIENT non-bird dinosaur - was what I was asking

["T. Michael Keesey" <keesey@gmail.com>]

On Thu, May 1, 2008 at 8:56 PM, Tim Williams
<twilliams_alpha@hotmail.com> wrote:
>
>  The reasons are argued by Gauthier and de Queiroz (2001).  But I don't think 
> their reasons are "well-argued".  :-)  Their logic seems to be that, because 
> Linnaeus did not include fossil taxa in his "Aves", then the name "Aves" 
> should be limited to the crown group.  Never mind that Linnaean didn't know 
> of any "fossil taxa", or even know about "evolution".

If that was their only argument, I'd agree with you. It would also
mean that they'd want to include sharks in _Reptilia_, crabs in
_Insecta_, and rhinoceri in _Glires_.

They have other arguments as well -- I'd suggest re-reading the paper.
They key one, to my mind, is this: "attaching the name 'Aves' to the
dinosaur crown maximizes the number of safe inferences that can be
made about the ancestor of the clade designated by that name; it
therefore also maximizes the number of generalizations that can safely
be made about birds as the group composed of that ancestor and all of
its descendants." (This is after establishing the fact that the name
"Aves" is by far the most-used of all the names they define in that
paper.)

>  Actually, _Archaeopteryx_ has been a very stable taxon, phylogenetically 
> speaking.  Almost every tree I have seen has it as the sister taxon to all 
> other "birds", at the base of a clade that includes it and all these other 
> "birds", but not deinonychosaurs, oviraptorosaurs, etc.  This clade deserves 
> a name, and we might as well use "Aves" - considering it has 150 years of 
> precedence for this content.

Exactly how many characters hold it there?

>  > Besides, it's only historical accident that makes _Archaeopteryx_ so 
> interesting.
>
>  That may be true.  But that doesn't detract from the fact that it *is* 
> interesting.  It's the earliest and most basal know "bird" for which we have 
> anatomical evidence for powered flight.  (Plus, there are many other reasons 
> why _Archaeopteryx_ is interesting.)

That evidence is in dispute.

You're right that _Archaeopteryx_ is interesting as a very early
member of whichever minimal clade it does share with avians. But
that's precisely why it makes a lousy specifier -- it's relatively
undifferentiated from the ancestor. (In fact, it could be an ancestor
of avians for all we know, although the odds are not good.)

>  > Really, I question whether giving this clade any name is useful.
>
>  Using that rationale, it's not much point giving *any* clade a name.  Even 
> "stable" taxa bounce around to some degree, if only because certain taxa 
> around them are unstable.  All this means is that definitions should be 
> framed very carefully - especially regarding the choice of positive and 
> negative specifers.

You're right. Let me rephrase: I question whether giving this clade
such an overwhelmingly popular name is useful.

(And actually, that clade may already be named _Eumaniraptora_,
although, as you note, this is the minority view.)

>  That second definition (Gauthier and de Queiroz, 2001) actually used powered 
> flight as the *definition* for Avialae.  Such an "all-or-nothing" definition 
> ("flight-or-no-flight") is almost impossible to apply to many fossil taxa, 
> including _Archaeopteryx_.  Sure, _Archaeopteryx_ was *probably* capable of 
> powered flight; but we are never going to be able to test for it.  The 
> converse is true for _Microraptor_ (probably incapable of flapping flight, 
> but not testable).

The fact that its content is nebulous does not mean that it isn't an
interesting clade to discuss. I find myself referring to it quite
frequently even though I have no opinion as to whether _Archaeopteryx_
does or does not belong.

Apomorphy-based clades: hard to place on a cladogram but easy to place
in a paragraph.

>  Yes, those were the examples I heard too.  Damn, now I'm hungry....

Breakfast time!
-- 
T. Michael Keesey
Director of Technology
Exopolis, Inc.
2894 Rowena Avenue Ste. B
Los Angeles, California 90039
http://exopolis.com/
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