Taxonomic Reassessment of Juvenile Tyrannosaurine Specimens From Asia Reveal Large Biogeographic Ranges in Tyrannosaurids

The recently named tyrannosaurid, Asiatyrannus xui , from the Nanxiong Formation of southeastern China bears a striking resemblance to juvenile specimens of Tarbosaurus bataar , warranting a reassessment of taxa named based on material with known juvenile characteristics. Similarly, the long contested Raptorex kriegsteini also possesses numerous cranial morphologies that are shared with both juvenile specimens of Tarbosaurus bataar and Asiatyrannus xui . Here the taxonomic affinities of Asiatyrannus xui and Raptorex kriegsteini are reassessed based on proposed diagnostic characters. Morphological evidence suggests that both “ Asiatyrannus xui ” and “ Raptorex kriegsteini ” are likely junior synonyms of Tarbosaurus bataar . The referral of “ Raptorex kriegsteini ” to Tarbosaurus bataar is supported by the stratigraphic work of previous authors, the presence of Tarbosaurus bataar autapomorphies in “ Raptorex kriegsteini ”, and the widespread occurrence of previously reported Raptorex kriegsteini autapomorphies in specimens of Tarbosaurus bataar. The referral of “ Asiatyrannus xui ” to Tarbosaurus bataar is supported by the presence of Tarbosaurus bataar autapomorphies, by morphologies shared in juvenile and adolescent specimens, and by a lack of apomorphies in “ Asiatyrannus xui ” that are not present in Tarbosaurus bataar . The identification of a Tarbosaurus bataar specimen from the Nanxiong Formation of China drastically expands the known biogeographic range of the species and aligns with the reported ranges of some North American Tyrannosaurids. The evidence suggests that the known ranges for many tyrannosaurids are vastly underestimated and likely rivalled ranges of modern carnivorans. • “ Asiatyrannus xui ” and “ Raptorex kriegsteini ” are likely junior synonyms of Tarbosaurus bataar. • Results support that the biogeographic range of Tarbosaurus bataar is larger than 2300 km. • Comparison to modern species suggests that we underestimate the ranges of theropod dinosaurs.