[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Alvarezsauridae splitting??



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Ken Kinman
>
>      Before I get too much of reputation as a "mad" lumper (although I do
> admit to being on the lumper side of the spectrum), I would like
> to bring up
> the subject of Alvarezsauridae.
>      I've always suspected that lumping Alvarezsaurus and
> Mononykines in the
> same family was perhaps not such a good idea.  It seems like once
> this was
> done, most people started to treat it as a unit, rather than possible
> independent entities.
>      So my two questions are:
> (1)  Is the evidence for a holophyletic Alvarezsauridae really all that
> strong?  [After all, Alvarezsaurus is kind of "scrappy" isn't it].

Yes, _Alvarezsaurus_ is scrappy and yes, the evidence for a monophyletic
Alvarezsauridae is pretty good.  The "glue" that holds it together is
_Patagonykus_: a form which retains more primitive features of the pelvis
found in _Alvarezsaurus_ and the highly derived forelimb of the mononykines.

Several different analyses have used the individual taxa of Alvarezsauridae
in phylogenetic analyses, including Novas 1996 (Mem. Queensland Mus.) and
1997 (JVP), Chaippe et al. 1998 (the _Shuvuuia_ paper), the forthcoming
Novas & Pol paper, and my own SVP 2K analysis.  The latter two, in
particular, include many additional non-alvarezsaurid OTUs, thereby allowing
for the possibility of many other potential placements of the component
taxa.

> (2)  If it isn't really holophyletic, could this perhaps explain why it
> jumps around Maniraptoriform phylogenies so much (sort of like a barefoot
> kid in an asphalt parking lot on a hot summer's afternoon).  Just
> wondering
> if splitting up Alvarezsauridae (like putting shoes on the kid)
> might make
> it less phylogenetically jumpy.

Just want to point out here that the position of Alvarezsauridae does not
seem to be too jumpy WITHIN databases (unlike certain other taxa I could
name: Troodontidae, for instance...).  Most studies find it just inside or
just outside _Archaeopteryx_ near the base of Avialae, positions just a node
or two apart from each other.  Sereno's & Martin's find it as an
ornithomimosaur relative, but these exclude a number of characters observed
in the alvarezsaurs (or at least represent an alternative interpretation of
the anatomy).  Studies in press do find an additional position or two.

However, the main problem with Alvarezsauridae is that (following the
tradition of most non-avialian coelurosaurs) the only really well-preserved
ones are from the Campano-Maastrichtian, long after the origin of the clade
and long after the often radical transformation of the anatomy.  For
instance, the forelimb anatomy of ornithomimosaurs on the one hand and
maniraptorans on the other are both different from each other and from the
basal avetheropodan condition.  More complete forelimbs of _Alvarezsaurus_
or a more basal member of its line could concieveably hold clues as to which
(if either) of these two it more resembled.  Instead, however, all we have
are the incredibly transformed forearms of _Patagonykus_, _Mononykus_, and
_Shuvuuia_.

                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