Individual, ontogenetic, and phylogenetic variation in the dentition of hadrosaurids (Iguanodontia: Ornithischia)

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Phylogenetic analyses of Hadrosauroidea are generally well-resolved, but finer resolution within Hadrosauromorpha remains contentious. This lack of resolution is due in part to the inability of discrete traits to illustrate their natural variation, particularly relating to the dental tooth battery. Not only has the variation not been properly examined, but the character definitions fluctuate throughout the literature. Here, we evaluate individual, ontogenetic and phylogenetic variation in four fundamental tooth characters at the subfamily level to better establish their use in future analyses: number of tooth rows, tooth aspect ratio, secondary enamel ridges and marginal enamel denticles. Factor analysis of mixed data and linear discriminant analysis were used to evaluate how accurately these four characters together identified the phylogenetic groups. We then used these data to predict phylogenetic relationships for a select few historically problematic taxa. We found that although the number of tooth rows is phylogenetically informative, it is related to size and is only useful in separating hadrosaurids from non-hadrosaurid iguanodontians. Tooth aspect ratio was found to be highly variable, and phylogenetic groups cannot be separated reliably from one another. Secondary ridges are individually variable, and their presence or absence should be grouped into a single character instead of being separated. Lastly, the presence and shape of marginal denticles in hadrosaurids is similarly variable, though rounded mammillations are almost exclusive to Lambeosaurinae, with some variation in their presence and prevalence within the group. When used in combination, these characters can accurately identify dentary teeth below Hadrosauridae, providing new options for identifying poorly preserved specimens and isolated teeth from microsites. Taken together, these characters may still be informative, but the individual variation described here must be accounted for when constructing character states, and we recommend several modifications to their coding.

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