Abstract

A small, morphologically variable collection of pachycephalosaur cranial material from the Judith River Formation includes Stegoceras, the first record of Ornatotholus in Montana, and an unnamed full-domed pachycephalosaur represented by a partial skull.

The wide range of variation among skulls in this assemblage demonstrates the need for a repeatable system of measurements using anatomical landmarks. Toward this end, fifteen measurements are defined as a basis for gathering morphometric data from the pachycephalosaur cranium.

Introduction

The suborder Pachycephalosauria (Maryańska and Osmólska 1974) is characterized by the thickened frontoparietal region of the skull. While most taxonomic studies of pachycephalosaurs are based on isolated skull material, skulls with associated postcranial remains have been described from Alberta (Gilmore 1924) and Mongolia (Maryańska and Osmólska 1974; Perle, Maryańska, and Osmólska 1982). Wall and Gallon (1979) and Galton and Sues (1983) reported on this diverse group of ornithischian dinosaurs with emphasis on North American specimens. Sereno (1986) offered a new classification of the pachycephalosaurs in which he arranged several genera into nested groups, from primitive flat-headed to more derived fully-domed forms. Sues and Galton (1987) reviewed North American representatives of the Pachycephalosauria in their recent anatomical and systematic study.

The Pachycephalosauria are divided by some authors (Perle, Maryańska and Osmólska 1982; Sues and Galton 1987) into two families: the domed Pachycephalosauridae Sternberg 1945 and the flat-headed Homalocephalidae Dong 1978.