Squamosal ontogeny and variation in the pachycephalosaurian dinosaur Stegoceras validum Lambe, 1902, from the Dinosaur Park Formation, Alberta

ABSTRACT

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.

ACKNOWLEDGMENTS

We thank W. Fitch (UCMZ), D. Brinkman, and B. Strilisky (TMP), P. Currie (UALVP), K. Shepherd and M. Feuerstack (CMN), and K. Seymour (ROM) for access to collections and loans of material. We are grateful to I. Morrison for preparation and casting of various specimens, D. Dufault for illustration of TMP 89.116.88, and C. Brown for providing line drawings of UCMZ(VP) 2008.001 and commenting on a previous version of this manuscript. We thank M. Goodwin, N. Longrich, M. Ryan, R. Sullivan, and T. Williamson for discussions regarding pachycephalosaur morphology and systematics. Finally, we would like to thank the editor and reviewers who helped to improve the final version of the manuscript.

Handling editor: You Hailu