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Draco-Stygi-Pachy



-I'll be curious to see how the Draco-philes responsd to this.
Personally I am glad to see this in print and will plan on brandishing
it like a sword.  Now if we can just get the Jane, nano=juvy t.rex paper
out there.

Scott Williams

Burpee Museum

>>Thomas Holtz wrote- 

Horner JR, Goodwin MB (2009) Extreme Cranial Ontogeny in the Upper
Cretaceous Dinosaur Pachycephalosaurus. PLoS ONE 4(10): e7626.

doi:10.1371/journal.pone.0007626

Abstract Top

Background

Extended neoteny and late stage allometric growth increase morphological
disparity between growth stages in at least some dinosaurs. Coupled with
relatively low dinosaur density in the Upper Cretaceous of North
America, ontogenetic transformational representatives are often
difficult to distinguish. For example, many hadrosaurids previously
reported to represent relatively small lambeosaurine species were
demonstrated to be juveniles of the larger taxa. Marginocephalians
(pachycephalosaurids + ceratopsids) undergo comparable and extreme
cranial morphological change during ontogeny.

Methodology/Principal Findings

Cranial histology, morphology and computer tomography reveal patterns of
internal skull development that show the purported diagnostic characters
for the pachycephalosaurids Dracorex hogwartsia and Stygimoloch spinifer
are ontogenetically derived features. Coronal histological sections of
the frontoparietal dome of an adult Pachycephalosaurus wyomingensis
reveal a dense structure composed of metaplastic bone with a variety of
extremely fibrous and acellular tissue. Coronal histological sections
and computer tomography of a skull and frontoparietal dome of
Stygimoloch spinifer reveal an open intrafrontal suture indicative of a
subadult stage of development.

These dinosaurs employed metaplasia to rapidly grow and change the size
and shape of their horns, cranial ornaments and frontoparietal domes,
resulting in extreme cranial alterations during late stages of growth.
We propose that Dracorex hogwartsia, Stygimoloch spinifer and
Pachycephalosaurus wyomingensis are the same taxon and represent an
ontogenetic series united by shared morphology and increasing skull
length.

Conclusions/Significance

Dracorex hogwartsia (juvenile) and Stygimoloch spinifer (subadult) are
reinterpreted as younger growth stages of Pachycephalosaurus
wyomingensis (adult). This synonymy reduces the number of
pachycephalosaurid taxa from the Upper Cretaceous of North America and
demonstrates the importance of cranial ontogeny in evaluating dinosaur
diversity and taxonomy. These growth stages reflect a continuum rather
than specific developmental steps defined by "known" terminal
morphologies.