The evolution of facial keratin and tooth distribution: implications for the coevolution of traits in dinosaurs
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Research on birds indicates that the occurrence of beak keratin and teeth are mutually exclusive, and both traits are widely distributed across Dinosauria. The macroevolution of the antagonistic relationship between keratinous beaks and teeth has only recently been explored, and these studies focus on Aves and non-avian Coelurosauria. Currently, many hypotheses posit an adaptive advantage for the trade-off of these two traits but have not been tested on a macroevolutionary scale. Examples in non-coelurosaurian dinosaurs and non-adaptive alternatives, such as trait evolution correlation and phylogenetic history, have yet to be explored thoroughly-. This study assesses the current evolutionary assumptions of beak-like keratin and tooth distribution across dinosaurs using recently-developed variable rate phylogenetic comparative methods. The diagnostic bone surface used as a proxy for the keratin cover in fossil jaws is smooth grooved textures with foramina at the jaw tips. The effect of a keratin cover on the tooth distribution within individuals is tested with Markov Chain Monte Carlo (MCMC) multinomial regression models. Coevolutionary rates for these structures are evaluated with Bayesian RJ-MCMC algorithms. The best-fit model is determined using Bayes Factors. The taxonomic breadth increased the evolutionary sample size and thus analytical power but forced a simplified coding scheme for beak and tooth traits. My analysis shows that the presence of a keratin cover influences the evolution of partial toothrow reduction but not complete tooth loss. Another factor could have driven the evolution of toothless beaks in some dinosaurs. In addition, the evolution of a reduced toothrow only precedes the presence of a keratin cover in the Theropoda lineage. Furthermore, the evolutionary rate of toothrow distribution was higher and more diverse when a keratin cover was absent; this suggests that the keratin cover imposes a functional constraint on tooth diversity at macroevolutionary scales.