MAXILLARY AND DENTARY MORPHOLOGY DISTINGUISHES THE LATE CRETACEOUS TYRANNOSAURID DINOSAUR NANOTYRANNUS LANCENSIS AS A VALID TAXON AND NOT A JUVENILE OF TYRANNOSAURUS REX

The highly controversial tyrannosaurid dinosaur Nanotyrannus lancensis, from the Hell Creek Formation (Maastrichtian) of Montana, has been the focus of ongoing debate over whether it represents a distinct species or is based on a juvenile of Tyrannosaurus rex. Currently N. lancensis is known from at least four specimens, each of which has a well preserved skull and dentaries. We examined several morphological features in the maxilla and dentary of N. lancensis and in other tyrannosaurid taxa. One of the morphological features in the maxilla is the presence of a well-developed promaxillary strut, a feature present in the albertosaurine tyrannosaurids Albertosaurus sarcophagus and Gorgosaurus libratus, and also in the alioramine tyrannosaurids Alioramus altai, A. remotus, and Qianzhousaurus sinensis. This feature is present in some tyrannosaurine tyrannosaurids: Bistahieversor sealeyi, Lythronax argestes, and Teratophoneus curriei. However, in other tyrannosaurines Daspletosaurus, Tarbosaurus, and Tyrannosaurus it is weakly developed and diminutive during ontogeny. One of the morphological features in the dentary of N. lancensis is the anterior step of the lingual bar, a feature that contributes to the interdentary symphysis. Within two subclades of tyrannosaurids, Albertosaurinae and Tyrannosaurinae, the morphology of this characteristic step differs among taxa, but seems unchanged during the ontogeny of a given taxon. The albertosaurines A. sarcophagus and G. libratus, and the alioramines A. altai, A. remotus and Q. sinensis, possess the dorsal surface of this step that covers only the first alveolus/interdental plate, whereas the tyrannosaurines have a pronounced step that covers laterally the first and second alveoli/interdental plates. The dentaries of Nanotyrannus possess the anterior step of the lingual bar, which exhibits a mixture of albertosaurine and tyrannosaurine characters, and covers only the first alveolus/interdental plate. These characteristics provide further support for recognizing N. lancensis as a taxon distinct from and not a juvenile of T. rex.