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Reassessment of Stegoceras novomexicanum

Ben Creisler

A new paper:

Thomas E. Williamson & Stephen L. Brusatte (2016)
Pachycephalosaurs (Dinosauria: Ornithischia) from the Upper Cretaceous
(upper Campanian) of New Mexico: A reassessment of Stegoceras
Cretaceous Research 62: 29–43
http: // www.sciencedirect.com/science/article/pii/S019566711630012X

Pachycephalosaurs, a group of ornithischian dinosaurs with distinctive
cranial ornamentation and skull domes, underwent dramatic changes in
cranial morphology during ontogeny. This has caused debate about
whether some specimens belong to juveniles or adults, which impacts
studies of pachycephalosaur phylogeny and evolution. One such debate
concerns a small skull roof specimen from the Campanian (Upper
Cretaceous) of New Mexico, NMMNH P-33898, which was originally
described as an indeterminate juvenile but later regarded as a mature
adult and erected as the holotype of a new small-bodied species,
Stegoceras novomexicanum. We restudied NMMNH P-33898 using computed
tomography scanning, morphometric and phylogenetic analyses, and
comparisons to growth series of other pachycephalosaurs (Stegoceras
validum, Pachycephalosaurus wyomingensis). We conclude that two
purported paratype specimens of Stegoceras novomexicanum cannot be
referred to the same taxon as the holotype, that the holotype and
controversial paratypes all belong to immature specimens and not
aberrant small-bodied adults, but that current evidence cannot clearly
determine whether NMMNH P-33898 is a juvenile belonging to its own
diagnostic species (S. novomexicanum) or is a juvenile of Stegoceras
validum, Sphaerotholus goodwini, or another known taxon. We review the
pachycephalosaur record of New Mexico and demonstrate that
pachycephalosaurs were important components of dinosaur faunas in the
southern part of Western North America during the ~15 million years
before the end-Cretaceous extinction, just as they were in roughly
contemporaneous northern localities.