Damnit, the groove is real and only in Nanotyrannus!
When a groove is not a groove: Clarification of the appearance of the dentary groove in tyrannosauroid theropods and the distinction between Nanotyrannus and Tyrannosaurus. Reply to 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”
Schmerge et al
The occurrence of a lateral groove in the dentary of theropod dinosaurs was argued to be a key diagnostic character for establishing the validity of Nanotyrannus lancensis as a unique taxon separate from Tyrannosaurus rex by Schmerge and Rothschild (2016). The validity of this distinction has been challenged in a comment paper by Brusatte et al. (2016). The main criticisms raised in this comment address the methodology of the original study, the distribution of the dentary groove in theropods, the possibility of ontogenetic variability in the occurrence of the dentary groove, and the application of phylogenetic analysis to studying character distributions. In this reply, we clarify the definition of the theropod dentary groove, elucidate the difference between a true dentary groove and the appearance of a false “pseudo-groove”, justify our original methodology with a discussion of the errors involved in identifying grooves by Brusatte et al. (in press), and support our original findings with descriptions of additional specimens. Investigation of additional specimens of Nanotyrannus, as well as critical examination of Tyrannosaurus specimens presented by Brusatte et al. (2016), reaffirm the result of our original study that Nanotyrannus can be differentiated from Tyrannosaurus based on the depth of its dentary groove, independent of ontogenetic stage. Despite any possible ontogenetic variation in the appearance of the dentary groove that can be interpreted, all specimens of Nanotyrannus possess distinct grooves whereas Tyrannosaurus lacks a groove. The most parsimonious explanation for the different appearance of these grooves is that Nanotyrannus does not represent a juvenile Tyrannosaurus.
gimme a break.
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”
Brusatte et al
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.