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Introducing: Sinovenator changii



As promised, a modification of the Norell et al. (Ostrom Symposium) paper,
by means of adding a new taxon.

Xu, X., M.A. Norell, X.-L. Wang, P.J. Makovicky & X.-C Wu.  2002. A basal
troodontid from teh Early Cretaceous of China.  Nature 415:780-784.

Troodontids have "traditionally" (since 1969) been considered the sister
taxon to dromaeosaurids, in a clade Colbert & Russell called
Deinonychosauria.  This position has been found by most, but not all,
authors.

Okay, back in 1994 I published a paper which placed troodontids as the
sister taxon to ornithomimosaurs, in the new clade Bullatosauria.  This
study suggested that some features (previously observed by others, such as
Currie) shared by ornithomimosaurs and troodontids (bulbous paraphenoid,
arctometatarsalian pes, and more) were signs of the common ancestry of these
two, while the similarities between troodontids and dromaeosaurids were
convergences and/or basal features "over printed" by the derived
ornithomimosaurs.  A similar conclusion was reached by Perez-Moreno et al.
at just about the same time.

In 2000 (analysis done in early 1999) my study found troodontids to be
equally parsimoniously explained as bullatosaurs or as the sister taxon to
dromaeosaurids plus birds.  Analyses by Norell et al. (finally published in
2001) and some of the Chinese teams found troodontids to be the sister to
"enigmosaurs".

As I discussed in the Ostrom Symposium, part of this problem stems from the
lack of basal members of the Troodontidae, to resolve which features are
ancestral in that lineage, and which derived.  That lack has ended.

Enter _Sinovenator changii_, (Meeman Chang's China hunter) based on a
partial skull and skeleton and a referred skeleton from the lowest part of
the Yixian Formation.  Associated fauna are _Psittacosaursu_,
_Jeholosaurus_, and the mammal _Repenomamus_.  Sadly, the horizon it is from
does not preseve the integument of the critter, one way or the other.

The anatomy, breifly:
The promaxillary fenestra is present and in the basal position.
The parasphenoid capsule is NOT inflated.
The basipterygoid processes do not appear to be hollow.
Denticles are absent on anterior teeth and small on maxillary teeth.
Dorsals have fan-shaped neural spines (shades of _Sinosauropteryx_!)
Glenoid fossa on scap-coracoid faces laterally.
Pelvis is *OPISTHOPUBIC* (not vertically oriented, but well past 90 degrees)
Metatarsal III is somewhat constricted, but non-arctometatarsalian.

The analysis includes species-level OTUs, which I won't reproduce entirely
here.  Although they do not include all the higher level names I will use,
here is their tree:
Avetheropoda
 Allosaurus
 Coelurosauria
   Tyrannosauridae
   Maniraptoriformes
     Ornithomimosauria
     Maniraptora
       Ornitholestes
       Metornithes
         Alvarezsauridae
         ye olde classic maniraptorans
           "enigmosaurs"
             Therizinosauroidea
             Oviraptorosauria (incl. Avimimus)
           Paraves
             Avialae
             Deinonychosauria
               Troodontidae
                 Sinovenator
                 all later troodontids
               Dromaeosauridae
                 Sinornithosaurus
                 all later dromaeosaurids

You know, I am quite pleased with these results, and think we (theropod
systemicists) are reaching a consensus for this part of the tree.

So, to sum up:
Troodontids are almost certainly deinonychosaurs.
I was wrong about troodontids in 1994, but don't care.
The Yixian is the place to find cool specimens
And even more are on their way... :-)

                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