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Nanotyrannus valid as member of Albertosaurinae, based on dentary groove



Ben Creisler
bcreisler@gmail.com

A new paper:

Joshua D. Schmerge & Bruce M. Rothschild (2016)
Distribution of the dentary groove of theropod dinosaurs: Implications
for theropod phylogeny and the validity of the genus Nanotyrannus
Bakker et al., 1988.
Cretaceous Research 61: 26-33
Joshua D. Schmerge & Bruce M. Rothschild (2016)
http://www.sciencedirect.com/science/article/pii/S0195667115301415



This study examines the phylogenetic distribution of a morphologic
character, described as a groove containing pores, on the lateral
surface of the dentary bone in theropod dinosaurs. The nature of this
groove is a feature unique to theropods. Of the 92 theropod taxa
examined for the presence and absence of this feature, 48 possessed
and 44 lacked this feature. Distribution of this character was
compared to published phylogenetic analyses of theropods, in order to
evaluate the utility of the dentary groove as a diagnostic feature.
80% of pre-Tyrannoraptoran theropods possessed the dentary groove,
with only 6 reversals in basal theropod clades. Theropods with beaks
or edentulous jaws all lacked a dentary groove. Tyrannosauroidea is
marked by mosaic distribution of this character. Among
tyrannosauroids, the dentary groove occurs only in Dryptosaurus and
the Albertosaurinae (Albertosaurus + Gorgosaurus). Nanotyrannus
lancensis, sometimes described as representing juvenile Tyrannosaurus
rex, also possesses this groove, unlike the remainder of the
Tyrannosaurinae. Nanotyrannus lancensis was included in a phylogenetic
analysis of Tyrannosauroidea and was recovered within Albertosaurinae.
We recommend that Nanotyrannus stand as a valid taxon nested within
the Albertosaurinae, based on the presence of this groove, as well as
other features of the skull.