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 .