JUVENILE PACHYCEPHALOSAURUS SQUAMOSALS AND IMPLICATIONS FOR PACHYCEPHALOSAURINE CRANIAL ONTOGENY

Several specimens collected from microvertebrate localities in the Hell Creek Formation, Carter County, Montana, represent isolated squamosal fragments of small pachycephalosaurs. The specimens, approximately 10 mm in length, are smooth and concave ventrally, but have distinctive pyramidal nodal ornamentation on their dorsal, lateral, and caudal surfaces. The nodes are arranged in horizontal rows, increase in size laterally, and have a large, pronounced corner node. One specimen also preserves the sutural surface for the postorbital. Comparison with the squamosal of Dracorex hogwartsia, which probably represents an immature Pachycephalosaurus wyomingensis, suggests that there is a one-to-one correspondence of the larger nodes of the new squamosal specimens. This suggests that the clusters of hypertrophied nodes are derived ontogenetically and phylogenetically from transverse rows of more uniformly-sized nodes. The specimens are among the smallest known squamosals reported for Pachycephalosauria.