RE: _Achillobator giganticus_ and homoplasy

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

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Nick Longrich
>
>       No, you probably haven't heard of it. It's the recently named giant
> dromaeosaur from Mongolia (Perle, Norell, and Clark 1999), published in an
> obscure place in Mongolia.

First comment: I have not seen the paper yet, although a copy is supposed to
be on its way in the mail to me.  I will therefore refain from exhaustive
but informed comments on the critter, but address a few theoretical issues.

>       But I guess that's what they make journals like "cladistics" for. I
> should really get over to the library more often.

Indeed.  One thing I recommend to all folks out there for whom understanding
phylogenetic relationships is more than a passing fancy: read some of the
papers on other groups of organisms, and other types of data (molecular,
behavioral, etc.).  Get a feel for what procedures are used out there.
"Cladistics" is a good journal for this, as is "Systematic Biology".  The
"Biological" and "Zoological Journal of the Linnean Society" also deals with
these issues, albeit with longer articles that also go more into specimen
descriptions (not a bad thing, mind you).

>       My 2¢ for what it's worth.  Maybe Chris Brochu or Tom Holtz would
> have some more intelligent stuff to say about this. I wouldn't
> mind hearing
> about what people think about the patterns of homoplasy seen in other taxa
> besides theropods , which is all I ever think about. Are theropods really
> more homoplastic, or have we just not looked at the others in enough depth
> to really uncover the homoplasy?

There are different measures of homoplasy out there.  Theropods are NOT
particularly homoplastic compared to lots of other groups: however, because
they are a) historically significant as the clade containing birds; b)
extraordinarily popular; and c) the coolest things ever to inhabit the Earth
(hmmm, some subjective bias there... :-), they get a lot of attention.
Thus, the discordant aspects of different theropod cladograms receive more
attention than, for example, alternative marsupial or echinoderm or asterid
cladograms.

Also, Nick's comments about the strange combination of features of this
critter form an important point.  In this particular case, he stated that
some of the features suggested it wasn't a dromaeosaurid, but that it
clearly must be because it possessed a set of other features found only in
other known dromaeosaurs.

The first thing that I would then consider is "are dromaeosaurs, as
traditionally envisioned, monophyletic?"  That is, are _Dromaeosaurus_ and
_Velociraptor_ and _Utahraptor_ and _Saurornitholestes_ and _Deinonychus_
(and more fragmentary guys) really more closely related to each other than
they are to other known taxa?  Had _Achillobator_ had just one or two
discordant features a simple answer might be a reversal, but with this big a
mix of characters I suspect there is a larger solution.

That is, that some "dromaeosaurs" (_Achillobator_, perhaps) might lie
outside a clade containing birds and other "dromaeosaurs" (_Velociraptor,
for example).  An appropriate test for this?  How about a comprehensive
phylogenetic analysis of individual species or specimens of the pertinent
taxa, and sort out the taxon and character distribution.

And always remember, the results are only a phylogenetic hypothesis, subject
to overturning with yet another weird-ass taxon.

More when I get to practice what I preach, and read the damn paper... :-)

                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): 301-314-7843