Pachycephalosauridae from the San Carlos and Aguja Formations (Upper Cretaceous) of West Texas, and observations of the frontoparietal dome

ABSTRACT

Several fragmentary frontoparietal domes from the San Carlos and Aguja Formations (Campanian) of Brewster and Presidio Counties, Texas, are referable to Pachycephalosauridae (Dinosauria: Ornithischia) and represent a southern extension of the known range of these dinosaurs in North America. Although the specimens are insufficient for confident generic identification, the size, relative dome height and width, continuity of frontal and parietal portions of the dome, and obliteration of sutures between these bones suggest affinity with pachycephalosaurids as Stegoceras and Gravitholus. More than one taxon is probably represented in the collection. Sections taken from one of the domes reveal histological tissue zonation comparable to that found in other pachycephalosaurids and growth lines indicating that at least 5 years were required to attain adult size. The bone tissue is well suited for strengthening the dome to resist compressional loading, and is compatible with the hypothesis that these animals engaged in head-butting behavior.

ACKNOWLEDGMENTS

I thank W. Langston Jr. for his advice and assistance in studying these specimens, and for his comments on an early version of the manuscript, R. Pascal of the Coal Mine Ranch for his hospitality while I conducted fieldwork in the San Carlos Formation, and S. Kelley and F. Zimmer for their assistance in the field; their care and dedication helped bring these specimens to light. D. Evans and an anonymous reviewer helped improve the content and presentation, and the Department of Geosciences at Texas Tech University provided funds for the preparation of the manuscript. Illustrations are the work of the author.