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Ornatotholus is juvenile Stegoceras (PLoS ONE)

From: Ben Creisler

New in PLoS ONE:

Schott, R.K., Evans, D.C., Goodwin, M.B., Horner, J.R., Brown, C.M., et al.
Cranial Ontogeny in Stegoceras validum (Dinosauria: Pachycephalosauria): A
Quantitative Model of Pachycephalosaur Dome Growth and Variation. 
PLoS ONE 6(6): e21092. doi:10.1371/journal.pone.0021092

Historically, studies of pachycephalosaurs have recognized
plesiomorphically flat-headed taxa and apomorphically domed taxa. More
recently, it has been suggested that the expression of the frontoparietal
dome is ontogenetic and derived from a flat-headed juvenile morphology.
However, strong evidence to support this hypothesis has been lacking. Here
we test this hypothesis in a large, stratigraphically constrained sample of
specimens assigned to Stegoceras validum, the best known pachycephalosaur,
using multiple independent lines of evidence including conserved morphology
of ornamentation, landmark-based allometric analyses of frontoparietal
shape, and cranial bone histology. New specimens show that the diagnostic
ornamentation of the parietosquamosal bar is conserved throughout the size
range of the sample, which links flat-headed specimens to domed S. validum.
High-resolution CT scans of three frontoparietals reveal that vascularity
decreases with size and document a pattern that is consistent with
previously proposed histological changes during growth. Furthermore,
aspects of dome shape and size are strongly correlated and indicative of
ontogenetic growth. These results are complementary and strongly support
the hypothesis that the sample represents a growth series of a single
taxon. Cranial dome growth is positively allometric, proceeds from a
flat-headed to a domed state, and confirms the synonymy of Ornatotholus
browni as a juvenile Stegoceras. This dataset serves as the first detailed
model of growth and variation in a pachycephalosaur. Flat-headed juveniles
possess three characters (externally open cranial sutures, tuberculate
dorsal surface texture, and open supratemporal fenestrae) that are reduced
or eliminated during ontogeny. These characters also occur in putative
flat-headed taxa, suggesting that they may also represent juveniles of
domed taxa. However, open cranial sutures and supratemporal fenestrae are
plesiomorphic within Ornithischia, and thus should be expected in the adult
stage of a primitive pachycephalosaur. Additional lines of evidence will be
needed to resolve the taxonomic validity of flat-headed pachycephalosaur

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