COMPARATIVE CRANIOLOGY OF THE CERATOPSIA

The Ceratopsia or horned dinosaurs demonstrate great diversity of skull shape that makes them ideal subjects for morphometric investigation. Skulls of ceratopsians are subjected to landmark-based morphometric analysis using the robust mapping technique, RFTRA (resistant-fit theta-rho analysis). This technique elucidates morphological trends in skull shape in a series of pairwise comparisons. Morphological distance data among taxa from RFTRA comparisons are subjected to cluster analysis. Clusters formed by this technique compare favorably with higher taxa recognized by both classical and cladistic methodologies. Furthermore, deconstruction of the skull by elimination of horns, frill, and the facial region of the skull demonstrates that taxonomic cluster patterns remain substantially stable even when only selected regions of the skull remain intact. The correlation of display characters with functional complexes suggests that no characters of the skull are genuinely trivial. Major morphological trends in the skull involve reorganization of the cheek region, including forward rotation of the ventral end of the quadrate, caudal movement of the jugal, reduction of the quadratojugal, caudoventral expansion of squamosal caudal to the quadrate in centrosaurines, and further expansion of the squamosal at right angle to the previous growth axis in chasmosaurines. Muscles of mastication probably occupied the base of the frill but did not occupy the entire frill, where they would have been subject to injury during intraspecific combat. Ceratopsids may have consumed a woody diet, with small angiospermous trees a probable food source.