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Horner JR, Goodwin MB (2009) Extreme Cranial Ontogeny in the Upper
Cretaceous Dinosaur Pachycephalosaurus. PLoS ONE 4(10): e7626.
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.
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.
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.
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Thomas R. Holtz, Jr.
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Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Faculty Director, Earth, Life & Time Program, College Park Scholars
Faculty Director, Science & Global Change Program, College Park Scholars
Mailing Address: Thomas R. Holtz, Jr.
Department of Geology
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College Park, MD 20742 USA