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.
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