QUALITATIVE AND QUANTITATIVE ASSESSMENT OF DENTAL DIFFERENCES BETWEEN STEGOCERAS VALIDUM (ORNITHISCHIA: PACHYCEPHALOSAURIDAE) AND THESCELOSAURUS NEGLECTUS (ORNITHISCHIA: THESCELOSAURIDAE) WITH IMPLICATIONS FOR PALEOECOLOGY AND IDENTIFICATION OF TEETH FROM MICROSITES

Small herbivorous dinosaurs of the clades Pachycephalosauridae and Thescelosauridae occur in multiple Cretaceous formations in North America, their coexistence likely made possible by differences in feeding style. While fossils of these gracile taxa are generally rare, isolated pachycephalosaur and thescelosaur teeth are common at microfossil sites, though easily misidentified. We use qualitative features and 15 linear measurements to compare dentitions of the pachycephalosaur Stegoceras validum and the thescelosaur Thescelosaurus neglectus, based on teeth preserved in identifiable skeletons. S. validum and T. neglectus possess heterodont dentitions with bulbous premaxillary teeth, broad maxillary teeth and triangular dentary teeth. Principal component (PC) analysis of the measurements shows that PC 1 and PC 2 account for 41% and 18% of the total variation, respectively. T. neglectus premaxillary, maxillary, and dentary crowns cluster separately on the PC plot, whereas all S. validum crowns cluster closely and overlap with the dentary crowns of T. neglectus. Linear discriminant function analysis shows little overlap between S. validum and T. neglectus. These results indicate strong heterodonty among the premaxilla, maxilla and dentary of T. neglectus contrasting with more morphometrically uniform teeth in S. validum that resemble those on the dentary of T. neglectus. Differences in feeding mode between the taxa can at least partly explain the discrepancies between their teeth in size, shape and the occurrence of qualitative characters. The high level of heterodonty in T. neglectus, combined with the narrow rostrum, suggests a specialized, selective feeding strategy. S. validum combines its less pronounced heterodonty with a wide rostrum, suggesting more indiscriminate bulk-feeding behavior and less refined processing. This analysis of dental differences between S. validum and T. neglectus should facilitate the identification of isolated teeth from microfossil sites and will later be extended to other pachycephalosaur and thescelosaur taxa. The results will improve the ability to test hypotheses on the biogeography, macroevolution, and temporal distribution of Pachycephalosauridae and Thescelosauridae using the microfossil record.