ASSESSING THE PRESENCE OF BASAL NEORNITHISCHIAN TAXA WITHIN MICROVERTEBRATE LOCALITIES

Microvertebrate sampling methods are often employed to assess various properties of paleoenvironments and their associated faunas, including overall taxonomic diversity. Such studies often depend upon the accurate referral of isolated teeth to species or at least higher-level taxa. The referral of isolated dinosaurian teeth to specific clades is generally feasible due to the relative homogeneity of their dentition. However, basal neornithischian taxa (i.e., ‘hypsilophodontids') display marked heterogeneity across their dentition, from their ‘peg-like' premaxillary teeth to their highly modified ‘cheek teeth' (i.e., maxillary and dentary teeth). The plesiomorphic structure of their premaxillary teeth makes them difficult to distinguish them from other archosaurian teeth, especially given the relatively high abundance and diversity of form of crocodylian teeth in many microvertebrate localities. This similarity, combined with the fact that premaxillary teeth in general remain poorly known from most basal neornithischian taxa, could result in the misidentification of isolated premaxillary teeth as crocodylian teeth. To explore this issue, an examination was conducted of fossil material collected from exposures of the Hell Creek Formation within Montana over the past ten years to determine if teeth from the relatively well known basal neornithischian Thescelosaurus were accurately identified and how often premaxillary teeth are recovered. This study revealed that forty percent of the Thescelosaurus teeth collected were premaxillary teeth, and the majority were misidentified as crocodylian teeth or simply not referred to a higher level taxon. Though premaxillary teeth make up around thirteen percent of the total dentition in Thescelosaurus (based on NCSM 15728), they were recovered at a rate three times higher than expected. Thus, the ability to accurately identify and refer isolated premaxillary teeth at least to the clade Neornithischia is critically important, especially when examining material from formations where these taxa were not locally abundant. To address this problem, a list of characteristics is presented that can be used to accurately identify isolated basal neornithischian premaxillary teeth from microvertebrate assemblages.