LAUSR

Large theropod dinosaurs are often reconstructed with their marginal dentition exposed because of the enormous size of their teeth and their phylogenetic association to crocodylians. We tested this hypothesis using a multiproxy approach. Regressions of skull length and tooth size for a range of theropods and extant varanid lizards confirm that complete coverage of theropod dinosaur teeth with extraoral tissues (gingiva and labial scales) is both plausible and consistent with patterns observed in living ziphodont amniotes. Analyses of dental histology from crocodylians and theropod dinosaurs, including Tyrannosaurus rex, further indicate that the most likely condition was complete coverage of the marginal dentition with extraoral tissue when the mouth was closed. This changes our perceptions about the appearance and oral configuration of these iconic predators and has broad implications for our interpretations of other terrestrial animals with large teeth. Description Not a toothy grin Theropod dinosaurs such as the iconic Tyrannosaurus rex have long been portrayed with their teeth fully visible, similar to extant crocodilians. This pattern of portrayal largely had to do with relatedness between dinosaurs and crocodilians and the relationship between tooth and jaw size. Cullen et al. tested hypothesized facial reconstruction in this group using histological analysis of tooth wear patterns and quantitative relationships between skull length and tooth size in both extinct and extant reptiles. Contrary to depictions that have dominated for more than a century, they found that theropods, including T. rex, had lips that covered their teeth, leaving them looking more like modern Komodo dragons than crocodiles. —SNV Giant theropod dinosaur’s teeth were covered by oral and extra-oral tissues (“lips”) when the mouth was closed, like lizards but unlike crocodilians.