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Stegoceras squamosal ontogeny

From: Ben Creisler

Also in the new JVP:

Ryan K. Schott & David C. Evans (2012)
Squamosal ontogeny and variation in the pachycephalosaurian dinosaur
Stegoceras validum Lambe, 1902, from the Dinosaur Park Formation,
Journal of Vertebrate Paleontology 32(4): 903-913

The pachycephalosaurian squamosal is one of the most diagnostic bones
in this enigmatic group of dinosaurs, but little is known about
variation in its morphology. Despite this, features of squamosal
morphology are often used in diagnoses and phylogenetic studies. The
recently proposed hypothesis of an ontogenetic transition from
Dracorex to Pachycephalosaurus implies a large amount of ontogenetic
variation in squamosal ornamentation that has not been well documented
due to the small number of squamosals for these taxa. Stegoceras
validum provides an important model for examining variation in
squamosal morphology due to the large number of squamosals available.
Here we examine the morphology of 14 S. validum squamosals both
qualitatively and quantitatively. We find that although both node
number and shape vary considerably, there is no clear ontogenetic
pattern, which suggests a high degree of individual variation. The
ornamental pattern, however, is maintained throughout the sample.
Increase in node size is isometric for the enlarged medial node and
normal posterior nodes, but negatively allometric for the vertex node.
The squamosal bar is taller laterally than medially in nearly all
specimens and increases in height with positive allometry at the same
rate medially and laterally. We found no correlation between the width
of the supratemporal fenestra on the squamosal and the size of the
squamosal itself. Rather, this feature appears to exhibit a large
degree of individual variation. Overall, these findings have
implications for previous hypotheses of ontogenetic growth and
function of the ornamentation, as well as the construction of
phylogenetic characters.