EROSIVE CRANIOFACIAL STRUCTURES IN MARGINOCEPHALIANS: AN EXTANT PHYLOGENETIC BRACKET APPROACH

Marginocephalia (Ornithischia: Cerapoda) is a clade of ornithischian dinosaurs that includes Ceratopsia and Pachycephalosauria. This clade is predominantly characterized by a prominent parietosquamosal shelf extending posteriorly over the occiput in the form of a large frill in ceratopsians and a thickened frontoparietal dome in pachycephalosaurs. The function of the respective parietosquamosal shelves has been the subject of much debate in paleontological discussions. Common hypotheses of parietosquamosal shelf function include defense structures in ceratopsians, “battering-rams” in pachycephalosaurids, and display structures for both clades.

Abnormal features in the form of erosive structures on the dorsal surface of pachycephalosaurid frontoparietal domes have been casually regarded as taphonomic artifacts. However, this claim has yet to be quantified or qualified. Ceratopsians have received considerably more attention in literature regarding the function of the parietosquamosal shelf and orbital horns, as well as potential etiologies of abnormal craniofacial lesions.

Presented here are the preliminary results of a comparison between the characteristics of erosive craniofacial structures of pachycephalosaurs and ceratopsians. The Extant Phylogenetic Bracket method has been utilized to infer and compare potential parietosquamosal functions such as sexual display and defensive structures of extant archosaurs such as crocodilians and cassowaries. To investigate potential pathological etiologies in marginocephalians, this method of inference is also utilized to investigate osteological response to physical trauma in birds and crocodilians. The preliminary results suggest that similarities in the morphological characteristics of some craniofacial lesions in marginocephalians are consistent with pathologies resulting from trauma and osteological response. While these results do not show irrefutable evidence for behaviors such as “head-butting”, they do support the need for further evaluation of craniofacial lesions in margiocephalians. As such, these results will assist in the development and design of taphonomic experiments to further assess the potential etiologies of these structures.