Ontogenetic Sequence Analysis Reveals Extensive Sequence Polymorphism in Tyrannosaurus rex

The study of ontogeny in fossil organisms is vital to broader understanding of their ecology and evolution. Ontogeny in Tyrannosaurus rex (Theropoda: Coelurosauria) has been studied extensively by dinosaur researchers, documenting the transition from small juvenile to gigantic adult. Previous authors have regarded this transition as largely linear, serving as a model for understanding growth and development in other tyrannosaurids as well as theropods as a whole. However, the presence of sequence polymorphism (variation in the temporal order of developmental events) has yet to be investigated for this taxon, representing a potential source of error for published ontogenetic series. Here we present the first Ontogenetic Sequence Analysis (OSA) of Tyrannosaurus, recovering extensive sequence polymorphism in a previously published ontogenetic character matrix. In contrast to ontogenetic series of Tyrannosaurus recovered by cladistic ontogenetic methods, we find a significant correlation between maturity and body size among large adult specimens, and corroborate previous assertions of a relationship between histologic and morphological indicators of maturity. Our results highlight the prevalence of sequence polymorphism among saurischian dinosaurs, and demonstrate the importance of considering such variability when attempting to reconstruct the ontogeny of fossil vertebrates. We expect that further study will reveal similar variability across closely related taxa and recommend that this possibility be incorporated into interpretations of morphological variation in tyrannosaurids.