Nanotyrannus was a groovy baby

The title works best when read in Austin Powers' voice.

So a new paper's out, and let's just say it's not very good.  Schmerge and Rothschild's (2016) premise is that Nanotyrannus has a lateral dentary groove, so based on this and adding it to Brusatte et al.'s (2010) tyrannosauroid matrix, it's an albertosaurine instead of a tyrannosaurine, let alone a juvenile Tyrannosaurus.

The first problem is that the lateral dentary groove has never been a great character, since its depth varies so much between taxa that numerous genera have been coded both ways in different TWG analyses.  I would say for instance that the slight longitudinal depression in Dryptosaurus (Brusatte et al., 2011; fig. 3A) doesn't qualify, contra the authors.  Indeed, Carr (online) list additional tyrannosauroids he interprets as having the groove, which were not coded that way by Brusatte et al..  Of these, I'd agree with Raptorex, and the juvenile Bistahieversor and Tarbosaurus, but view e.g. the Bistahieversor type and Alioramus (altai's type) to be more like Dryptosaurus.  Thus I agree with Headden (DML, 2016) that the character shows ontogenetic variation.  But my main point is that it's so subjective that authors can't agree on which morphologies count, and that any attempt to quantify depth and continuity (many individuals have grooves that vary in depth along the dentary, most obviously visible in the CT scan of Tarbosaurus juvenile IGM 107/7) won't be easy.

Dentaries of tyrannosauroid taxa (some flipped) showing degree of lateral groove.  Left (top to bottom)- Bistahieversor juvenile paratype NMMNH P-25049 (after Carr and Williamson, 2010); Tarbosaurus juvenile IGM 107/7 (after Tsuihiji et al., 2011); Nanotyrannus BMRP 2002.4.1 'Jane' (after Schmerge and Rothschild, 2016); Raptorex juvenile holotype LH PV18 (after Sereno et al., 2009). Right (top to bottom)- Dryptosaurus holotype ANSP 9995 (after Brusatte et al., 2011); Bistahieversor holotype NMMNH P-27469 (after Carr and Williamson, 2010); Alioramus IGM 100/1844 (after Brusatte et al., 2012); Tyrannosaurus FMNH PR2081 (after Brochu, 2003).  I'd say those on the left have a strong groove while those on the right lack one, and the least Dryptosaurus has less of a groove than e.g. Bistahieversor or Tyrannosaurus, contra Schmerge and Rothschild's recoding of it.  Also note deeper grooves are common in juvenile tyrannosaurids.

The second problem is that even if Schmerge and Rothschild had an unassailable, quantifiable character difference between Nanotyrannus and Tyrannosaurus that was not known to be ontogenetically variable, it's STILL just one character.  They're being BANDit-esque in assuming one character can override the totality of character evidence.  Sure they go on to list five other characters supposedly shared between Nanotyrannus and albertosaurines, but not only are some obviously plesiomorphies  (maxillary fenestra placed posteriorly; rounder orbit; greater dentary tooth count), the authors actually got the maxillary fenestra character backwards (they claim "contact of the maxillary fenestra with the rostral margin of the antorbital fossa" unites Nanotyrannus and albertosaurines).  The juvenile Tarbosaurus and Maleevosaurus' type are sufficient to show it's expected in juvenile Tyrannosaurus anyway.  What about the cladistic analysis, you ask?  While I haven't seen the matrix yet*, I'm betting the authors coded Nanotyrannus as if it was an adult.  And we already know juvenile tyrannosaurines emerge more basally when coded as adults.  Carr (2005) found Nanotyrannus' holotype to be sister to Daspletosaurus+Tarbosaurus+Tyrannosaurus, and the Stygivenator holotype to be "in a polytomy consisting of Teratophoneus and all non-tyrannosaurine tyrannosauroids."  Tsuihiji et al. (2011) found their Tarbosaurus juvenile to emerge sister to albertosaurines+tyrannosaurines.  So this is an unsurprising result even assuming Schmerge and Rothschild's coding was competent.

Edit- the matrix is terribly undercoded.   Only one character of 27 coded for the nasal and lacrimal, the squamosal and quadratojugal are completely uncoded, the entire skull roof, palate and braincase uncoded, postdentary mandibular bones uncoded, only one postcranial character coded.  Did nobody check the matrix and notice the huge swath of question  marks?!  Seriously, Cretaceous Research, this is just sad.

Finally, the authors don't understand how to compute character changes, when they compare the steps necessary to move Nanotyrannus to Tyrannosaurus' sister OTU.  Note we're never presented with the actual number of steps such a move adds to the analysis, let alone told if that number is statistically significant.  Instead, Schmerge and Rothschild just show the distribution of their dentary groove character if the move is made.  Even accepting their coding is correct, the correct answer is that moving Nanotyrannus to Tyrannosaurus would add a single step, namely Nanotyrannus evolving that character in convergence with albertosaurines.  But the authors claim "this tree requires 5 more independent losses of the dentary groove than the tree proposed in this study."  Wha?!  How do they justify this?  Because for some never explained reason, the authors assume the groove is present along the entire 'backbone' of the cladogram, so that every tyrannosauroid that lacks the groove independently loses it.  How terribly unparsimonious!  This takes 11 steps in their tree 3B, whereas the most parsimonious phylogenetic reconstruction would take only four steps assuming the groove is present in the outgroup as the authors believe.  All that's necessary is the loss of the groove for taxa more derived than proceratosaurids, then independent development in Dryptosaurus, albertosaurines and Nanotyrannus.  I can only suppose the authors view the groove as being incapable of parallel development.  It's just formed by a branch of the (?)trigeminal nerve being more depressed into the dentary's lateral surface, hardly the "dramatic change (e.g., metamorphosis) [that] would need to be invoked to explain the loss of this feature through maturation" that Schmerge and Rothschild claim.

Cladogram of tyrannosauroids with deep lateral dentary grooves indicated with bold lines.  Middle- figure 3B from Schmerge and Rothschild (2016) showing their proposed evolution of dentary grooves.  Top- number of steps assumed by Schmerge and Rothschild, with steps indicated by circles.  Bottom- most parsimonious distribution of steps given an outgroup with dentary grooves.

In conclusion, I don't see how this paper made it to publication.  The anatomical structure in question has a controversial distribution, Tsuihiji et al. (2011) destroys the entire premise but is never addressed or referenced, the authors never even consider that coding Nanotyrannus as an adult is problematic (Carr 1999 showed that the holotype has immature bone grain, so even if it's an albertosaurine, it's a juvenile), and they don't understand how character state reconstruction works.  Using 'key characters', presumptions about reversability, an ignorance of the recent literature, misunderstanding cladistics... did Schmerge inherit BANDit-style biases working at the University of Kansas?

References- Carr, 1999. Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Theropoda). Journal of Vertebrate Paleontology. 19(3), 497-520.

Brochu, 2003. Osteology of Tyrannosaurus rex: Insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull. Society of Vertebrate Paleontology Memior. 7, 138 pp.

Carr, 2005. Phylogeny of Tyrannosauroidea (Dinosauria: Coelurosauria) with special reference to North American forms. PhD thesis. University of Toronto. 1170 pp.

Sereno, Tan, Brusatte, Kriegstein, Zhao and Cloward, 2009. Tyrannosaurid skeletal design first evolved at small body size. Science. 326(5951), 418-422.

Carr and Williamson, 2010. Bistahieversor sealeyi, gen. et sp. nov., a new tyrannosauroid from New Mexico and the origin of deep snouts in Tyrannosauroidea. Journal of Vertebrate Paleontology. 30(1), 1-16. 

Brusatte, Norell, Carr, Erickson, Hutchinson, Balanoff, Bever, Choiniere, Makovicky and Xu, 2010. Tyrannosaur paleobiology: New research on ancient exemplar organisms. Science. 329, 1481-1485.

Brusatte, Benson and Norell, 2011. The anatomy of Dryptosaurus aquilunguis (Dinosauria: Theropoda) and a review of its tyrannosauroid affinities. American Museum Novitates. 3717, 53 pp.

Tsuihiji, Watabe, Tsogtbaatar, Tsubamoto, Barsbold, Suzuki, Lee, Ridgely, Kawahara and Witmer, 2011. Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia. Journal of Vertebrate Paleontology. 31(3), 497-517.

Brusatte, Carr and Norell, 2012. The osteology of Alioramus, a gracile and long-snouted tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia. Bulletin of the American Museum of Natural History. 366, 197 pp.

Carr, online 2016. http://tyrannosauroideacentral.blogspot.com/2016/01/by-way-that-groove-is-also-seen-in.html

Headden, DML 2016. http://dml.cmnh.org/2016Jan/msg00074.html

Schmerge and 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.