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*Utahraptor* and Polyphyly of Recent Dromaeosaurids



Jordan Mallon wrote:

<< Well, while on the subject of _Utahraptor_, and taking into
account that it may be a valid genera, have there been any
further developments on its affinities within the
Dromaeosauridae? Last I heard, its placement within the
dromaeosaurinae was still being questioned.>>

<I've been rereading Phil Currie's (1995) description of
Dromaeosaurus and let me tell you the family Dromaeosauridae is
in deep trouble. Almost all the theropods everyone has been
calling "dromaeosaurids" probably aren't; they should be
referred to as "velociraptorids" instead (with Velociraptorinae
raised to family level as Velociraptoridae). Dromaeosaurus is
represented by a single specimen that is mainly a few skull and
mandible bones with teeth, and these are quite different from
comparable elements of velociraptorid theropods such as
Deinonychus and Velociraptor.>

  Certainly the material of *Dromaeosaurus albertensis* leaves a
lot to be desired ... a skull, hyoid, some various elements of
the postcrania. However, aside from several missing pertinent
elements, the skull is well enough preserved for comparison with
each velociraptorine known, and Currie's research reflects this.
They do form a monophyletic whole, simply because there's
nothing out there that seems closer to *Velociraptor* than to
*Dromaeosaurus* that is also closer to the avian stem to other
groups (e.g, troodontids) based on recent research. Some of this
is personal research. I have to say, Pete Buchholz got me hooked
on dromaeosaurid phylogeny when I was avoiding it just because
I'd have to involve looking at basal _avian_ phylogeny, and that
was scary.... However, even given the so, so similarity with
velociraptorines and other taxa considered closer to birds (and
forcing a polyphyletic interpretation of Dromaeosauridae) there
are still a few considerations to be had for a monophyletic
group that has to be tested, so let me, at the end of this post,
suggest some features for resolution.
 
<Phil noted these differences in segregating Dromaeosaurus in
its own subfamily Dromaeosaurinae, but I think the differences
are even stronger than that. For a few hours I contemplated the
possibility that Dromaeosaurus is based on remains of a very
immature Albertosaurus, but this is almost certainly not the
case.>

  At one point, I, too, considered it to be a young or basal
tyrannosaurid, for certainly this is what prompted Matthew and
Brown to place it in "Deinodontidae" with *Albertosaurus* and
other tyrannosaurids. The teeth are especially similar in this
regard, as is the robusticity of the jaws. Two years ago I made
a survey of animals most likely to be either "head-first" or
"hand-first" killers, and betted on whether *Dromaeosaurus* or
*Chirostenotes*, on the Judithian floodplain, would win out in a
confrontation that, hypothetically, would involve combat. It
happens in similar forms of small birds and reptiles that aren't
trying to eat each other, and some that do (king snakes are
notorious for this).

<Perhaps the closest relative of Dromaeosaurus is Mongolia's
Bagaraatan, but since the only comparable elements are
dentaries, this is a very tentative guess.>

  The jaw of *Bagaraatan* is interesting because the alveoli are
much longer than wide, the jaws are superbly wide, deep, there's
a neomorph antarticular, paired surangular foramina, deep
retroarticular process with elongate ventral portion. Whatever
*Bagaraatan* was, it was doing its own thing (headwise) and the
femur is just plain strange.... But it does appear to be
maniraptoran.... <shrug>

-------

  Arguing for a monophyletic Dromaeosauridae:

  1. fused interdental plates;
  2. lateral frontal process turned at an angle to meet
postorbital;
  3. vertical posterior process of the articular posterior to
the retroarticular process;
  4. squamosal bears a lateral ridge from the postorbital to the
paroccipital process;
  5. quadratojugal "T"-shaped, the posterior process about as
long as the anterior process;
  6. the prefrontal is fused with the lachrymal, forming a
"T"-shaped lachrymal (not analogous to the T-shaped lachrymal of
ornithomimids, whose prefrontal is separate);
  7. form of the denticles of the teeth;
  8. dorsal vertebrae with pedunculate parapophyses;
  (largely from Currie, 1995; Xu, Wang, & Tang, 2000).

  In these, *Dromaeosaurus* reflects other dromaeosaurids
(velociraptorines) in being nearly identical. Other features
that separate velociraptorines are based largely on the form of
the teeth and the form of the premaxilla, which suggest a high,
round external nares, longer premaxilla than high, large (or
not) first premaxillary tooth position, form of the anteriormost
premaxillary teeth, and relative slenderness of the jaws. For
all its size, *Dromaeosaurus* is more "robust" than
*Deinonychus*, an animal about 30% smaller (based on linear
projections, probably closer to 20% based on mass). There are a
lot of other "dromaeosaurids" that have been named, the majority
of them very gracile looking animals, and for the most part,
these all have features of Dromaeosauridae. What's up? Lately,
the AMNH team have been using Velociraptorinae as their OTU in
maniraptoran phylogenetics, which is strange, but keeps the OTU
from being polymorphic in some cases. On to all those _other_
"dromaeosaurids...."

  Arguing for a polyphyletic Dromaeosauridae:

  *Sinornithosaurus*, for the most part, appears to be the most
basal dromaeosaurid. I lacks the pelvic modifications that
velociraptorines and *Archaeopteryx* share. The large degree of
birdiness in it are shared by velociraptorines, *Unenlagia*,
*Rahonavis*, *Archaeopteryx...* and so on. Very funny animal.

*Bambiraptor* is even weirder, but my observations on both these
are based off the papers, rather than anything I could seriously
say is a "pers. obs." of the material. The skull exhibits,
unlike *Sinornithosaurus* but like other "dinobirds" including
*Archaeopteryx* and pygostylians, a slender jugal below the
orbit ennervated by extensive foramina, a dorsally bulbous
braincase, and other features that seem to be indicated by the
holotype's immaturity. I am reassured that a checklist of all
the material is being prepped, so I can be patient. The lower
jaw is very hooked, and the orbit very long compared to height,
the jaws very slender, etc., etc. I'd like to see how the adult
material reflects or tarnishes this before I'd use it
phylogenetically.

*Unenlagia* was suggested by Norell and Makovicky (1999) in
their second report on *Velociraptor* from Mongolia as being
similar to dromaeosaurids. This results from the from of the
femur, vertebrae (pedunculate parapophyses, short transverse
processes), and some pelvic features (form of the pubic boot,
form of the pubic apron, extent of the ischiadic symphysis). The
ischium is strange in its extreme brevity, and seems to be
similar to other dromaeosaurids more than it is to such like
*Rahonavis* (as reflected by Forster et al., 1998),
*Bambiraptor* (Burnham et al., 2000), and *Sinornithosaurus* (Xu
et al., 2000).

  Most peculiarly among these other forms is the shape of the
pelvis, which seem more similar to each other than to
velociraptorines (for instance) in the following manner:

  1. pubis _not_ expanded into a distinct boot attentuated at
the shaft, but gradually expanded distally;
  2. pubis curves caudally along the shaft at a distinct flexure
point;
  3. pelvis mesopubic, with vertically strait lateral aspect;
  4. ischium has tiny triangular distal process;
  5. obturator process at distal end of ischium, not proximal on
the shaft;
  6. postacetabular blade of the ilium without a dorsal
extension, tapering and with a strongly pointed lateral aspect;
  7. jugal shallow along length, not triangular with a deep
suborbital ramus;
  8. scapulae strap-like without distal expansion, transfered
from a lateral to a dorso-lateral position;
  9. coracoid with a dorsal concavity perhaps analogous to the
triosseal canal in pygostylians.

  *Sinornithosaurus* has 1-6, and 8-9; *Bambiraptor* has 1-9;
*Unenlagia* has 2-6, and 8 (7 and 9 not preserved); *Rahonavis*
has 1-6, and 8 (7 and 9 not preserved); *Archaeopteryx* has 1-2,
4-9, and I'm not certain about 3 (need to look into this,
personally); *Velociraptor* and *Deinonychus* have 8,
*Velociraptor* has 9; *Dromaeosaurus,* *Utahraptor,* and
*Saurornitholestes* cannot be adequately compared;
*Achillobator* freaks me out. This all suggests a gradational
arrangement, and my preliminary observations are that
Dromaeosauridae is monophyletic, but that *Sinornithosaurus* and
the rest are closer to birds than *Velociraptor* is, thus are
avialian; then we find *Sinornithosaurus*, and *Bambiraptor*,
and apparently *Unenlagia*, then we get *Rahonavis*, then Aves.
I'll not try to form a phylogeny until I get my hands of a
phylogenetic package. But my results suggest to me that
gracility in recent "dromaeosaurids" has led to lumping of
dromaeosaurids, rather than producing a phylogenetic signal, and
what we se might be a result of plesiomorphies being used as
synapomorphies, possibly also the problem with *Achillobator.*

  Because I'm not sure about the results, I've abstained from
incurring Jon's wrath in publishing this as fast as I could, and
want to ask the list's informed opinion on the gradational
hypothesis. I'm sure Mickey and David will chime in.

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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