Dentary groove morphology does not distinguish 'Nanotyrannus' as a valid taxon of tyrannosauroid dinosaur. Comment on: "Distribution of the dentary groove of theropod dinosaurs: implications for theropod phylogeny and the validity of the genus Nanotyrannus Bakker et al. 1988"
Abstract There has been considerable debate about whether the controversial tyrannosauroid dinosaur Nanotyrannus lancensis from the uppermost Cretaceous of North America is a valid taxon or a juvenile of the contemporaneous Tyrannosaurus rex In a recent Cretaceous Research article, Schmerge and Rothschild (2016) brought a new piece of evidence to this discussion: the morphology of the dentary groove, a depression on the lateral surface of the dentary that houses neurovascular foramina. They argued that an alleged Nanotyrannus specimen, which possesses a groove, cannot be referable to Tyrannosaurus rex which they considered as lacking the groove, and they hypothesized that Nanotyrannus is closely related to albertosaurine tyrannosauroids, which also are said to possess the groove. However, we show that the groove is a widespread feature of tyrannosauroids that is present in T rex and many other specimens, and that it is an ontogenetically variable feature that changes from a sharp, deeply impressed groove to a shallower sulcus as an individual matures. As a result, the presence or absence of a dentary groove does not clarify the validity of Nanotyrannus or its phylogenetic position among tyrannosauroids. We consider it most parsimonious that Nanotyrannus' specimens belong to juvenile T rex