RE: Microraptor and Birds

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Dino Guy Ralph
>
> How are Aves and Avialae currently defined?

How aren't they?!?!  You've stumbled onto two of the most contentious taxa
(in terms of definitions) out there.

For Aves there are generally two different alternatives, both node-based:
1) crown-group birds (i.e., the most recent common ancestor of all living
birds and all of its descendants) [the same clade is called "Neornithes" by
those who use definition #2 for Aves]
2) _Archaeopteryx_, modern birds, their most recent common ancestor, and all
of its descendants.

Avialae Gauthier 1986 was originally defined by a perfectly good stem-based
defininition:
I) Aves (using #1 above) and all taxa closer to it than to Deinonychosaurs.

However, in 1999 Wagner and Gauthier redefined it to be identical to Aves
definition #2 above.  When queried, it turns out that Gauthier had forgotten
his stem-based definition.

THEN, to make matters worse, Gauthier & de Queiroz re-redefined Avialae to
have an apomorphy-based definition: _Vultur gryphus_ (their choice for the
"type" bird: much prefer _Passer domesticus_ myself...), its first ancestor
possessing feathered wings synapomorphic with it used for powered flight.

EEEEEEEEEEEEKKKKKKKKKK!!!!!  (Incidentally, Avialae is the only taxon I know
of that has been used as a stem-based, node-based, and apomorphy-based taxon
all by the original author!!).

First off, we have the horrendously difficult (if not impossible) situation
in trying to test for powered flight in fossil taxa.  Secondly, if we change
this to simply the (easier to demonstrate if taphonomy is correct) presence
of feathered wings (and we agree that feathers require rhacis, barbs, and
barbules), then Avialae would encompass deinonychosaurs and oviraptorosaurs,
too.

>  Is either clade closer to synonymy with the colloquial term, "bird"?

Gauthier (in 1986) described "Aves" as "modern birds" and "Avialae" as
"birds".  That is actually not so bad a description...

> Now, on to _diagnosis_.  What synapomorphies -- if any -- banish
> _Microraptor gui_ from inclusion into either clade,

Check out the relevant papers.  Very few, these days...

> and -- in light of
> the unfortunate circumstance that no one has had the privilege of
> personally seeing all relevant life forms evolving and radiating over
> millions of years -- how has this set of distinguishing synapomorphies
> been determined?

You run the analyses, find the topology, and see the distribution of
characters under Delayed or Accelerated Transformation.

>  Do different taxonomists use different sets of
> synapomorphies to determine membership for these clades?

In a sense, in that different worker's data matricies may include characters
not used in anothers.  Thus, the resultant clades may (or may not) be united
by characters unique to that analysis so far.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796