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Journal of Paleontology Papers



Previous cited are

  Barrett, P. M. and Xu X. 2005. A reassessment of *Dianchungosaurus
    lufengensis* Yang, 1982a, an enigmatic reptile from the Lower Lufeng
    Formation (Loer Jurassic) of Yunnan Province, People's Republic of China.
    _Journal of Paleontology_ 79(5):981-986.

  Gangloff, R. A., A. R. Fiorillo, and D. W. Norton. 2005. The first
    pachycephalosaurine (Dinosauria) from the Paleo-Arctic of Alaska and its
    paleogeographic implications. _Journal of Paleontology_ 79(5):997-1001.

  We begin with Barrett and Xu. Reanalysis of the type and paratype of
*Dianchungosaurus lufengensis* as described by Yang (IVPP V4735a and V4735b,
respectively) indicate that the material is largely inconsistent with a
heterodontosaurid interpretion. To begin with, heterodontosaurids have largest
third premaxillary teeth, while in *D. lufengensis* it is the first. Yang's
criteria for heterodontosaurids are based primarily on the dentary, and the
narrowness of the premaxilla, caniniform premaxillary teeth, and flat dorsally,
are not confined to heterodontosaurids even among dinosaurs of Triassic or
Jurassic age. Barrett and Xu also indicate that it is possible the external
nares were confluent on the dorsal premaxillary surface, as the medial, nasal
process of the premaxilla appears to be absent without signs of taphonomic
removal. They also use these features to argue it is not dinosaurian, but
pertains to a distinct mesoeucrocodylian, and if so, a basal one as in
*Calsoyasuchus* and thalattosaurs (e.g., *Askeptosaurus* and kin).

  The dentary, on the other hand, lacks features of the teeth such as a strong
primary ridge, denticles confined to the apical third of the crown, and
lanceollate shape. The authors cite the presence of nutrient foramina on the
dentaries laterally and no emargination of the anterior tooth row as further
indications of its affinities as a dinosaur, and moreso as a prosauropod. This
is further supported by the labiolingually symmetrical teeth. Further features
of the teeth include both a slight lingual and distal (rearward) inclination of
the crown apices and coarse denticles at 45 degrees.

  Thus the authors revise the hypodigm of *Dianchungosaurus lufengensis* to the
type and refer the paratype to an indeterminate prosauropod. The only other
ornithischians from the Lower Lufeng Formation are *Tatisaurus* and
*Beinosaurus*.

---

  Gangloff et al. have discovered a nearly complete squamosal of a
pachycephalosaurid from the Prince Creek Formation, Campanian, Late Cretaceous,
along the Colville River; the specimen is UAM AK-493-V-001, and measures a
little over 6.75cm wide and 7cm high when the posterior surface of the
squamosal is planar. In articulation the width in posterior view would be
somewhat less due to rotation of the bone. The rear bears two distinct rows of
nodes, the lower being horizontal and larger than the oblique row, and both are
in complete contact with adjacent nodes rather than spaced slightly. The lower
row nodes are largely pentagonal and higher than wide, the fifth corner
dorsally oriented and have nodal apices pointing caudodorsally; the upper row
is more irregular to hexagonal in area with apices centered and resemble
nothing so much as a Roman _scutellum_ (a small circular buckler-like shield
with a spike in the center). The squamosal is complete enough and so well
preserved to show the nuchal crest ventrally, a portion of the smooth dome
dorsally, and a region of the dorsal supratemporal chamber including the
quadrate articulation. Foramina in the dome surface are irregularly arranged
rather than forming a clear "net-like" pattern as in most juvenile
pachycephalosaur domes such as "Ornatotholus" or even the smaller, less
"perfect" *Stegoceras* domes. The parietal suture is straight and, based on
level orientation of the nuchal crest, nearly vertical. The authors use the
unique shape of the quadrate articulation, which produces a flange that firmly
sutures the quadrate to the squamosal, to suggest close affinities of the
squamosal with *Pachycephalosaurus* and indicate the specimen may pertain to
that genus based on the flange. 

  However, regular rows of nodes do not define the clade including
*Pachycephalosaurus* and *Stygimoloch*, Pachycephalosaurinae, as studied by
Williamson and Carr (2002). Instead, while *Sphaerotholus* possesses a single
row of nodes and an offset ventrolateral node, *Pachycephalosaurus* and
*Stygimoloch* do not, and in fact possess an irregular bundle of pentagonal or
greater-sided nodes, or circular. As in *Pachycephalosaurus*, the nodes are
broadly in contact with one another. The margin of the parietal suture is
vertical in *Pachycephalosaurus*, but also in *Sphaerotholus*, and
*Stegoceras*. A slight ventral widening of the parietal wedge is found also in
*Stygimoloch*, *Tylocephale*, and *Prenocephale* (contra Sullivan, 2003). Other
domed pachycephalosaurids have a single or regular sets of nodes, and a
parietal wedge that expands dorsally rather than ventrally between the
squamosals, including *Hanssuesia*, *Colepiocephale*, and the *Stegoceras*
species *S. brevis* and *S. edmontonense* [feminine gender of the time
formalized revision, and later ICZN editions supported this, contra Sullivan,
who supported the masculine ending -ensis]. This affirms and supports Gangloff
et al. and places pachycephalosaurs in Campanian Alaska along with other
typical "Albertosaurus fauna" taxa equivalent to Dinosaur Park Formation time
(see other work from Gangloff and Fiorillo on the region).

  Cheers,

Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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