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RE: Theropod phylogenies



> From: Richard Forrest [mailto:richard@plesiosaur.com]
>
> I'm bothered (perhaps through ignorance) about birds and theropods, and
> perhaps someone on the list can put me right on a few things.
> As I understand it, sorting out the relationships of modern birds based on
> morphology alone is tricky, and the advent of firstly molecular data, and
> more recently DNA analysis has led to some extensive revision of their
> relationships.

Well, to be fair, DNA analyses will often give wildly different topologies
depending on the particular genes and the particular subset of taxa used...

Also, be aware that until recently there were no morphological phylogenetic
analyses across all major clades of modern birds that approached the density
of data that we have in modern non-avian theropod studies (i.e., compare the
# characters/taxon).  This has changed dramatically with Liveszy and Zusi's
recent analyses (among others).  Even these, though, lack data from the
extinct members of Neornithes, and it will be interesting to see how these
data would affect the tree shape.

PLEASE NOTE: the same comments from the above paragraph can also be applied
to the comprable studies across placental mammals.  Really comprehensive
morphological studies from across Placentalia are currently lacking: the
morphological trees that are being overturned by recent DNA studies were
really preliminary, and as much a product of software and hardware
limitations of the 1980s as anything else.

> And if
> this is the case, don't we need to treat these cladograms as rather
> tentative hypotheses rather than solid phylogenies?
>
Of course all serious workers in the field acknowledge that these, like
every phylogenetic analysis, are only tentative.  However, given the growing
correspondance between the main aspects of the tree (e.g., _Coelophysis_ or
_Ceratosaurus_ are not winding up closer to birds than does _Allosaurus_),
it looks as if the basic topology is relatively secure.

                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