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Re: Nanotyrannus valid as member of Albertosaurinae, based on dentary groove

Not so fast!

Just checking with some recently described putative juvenile theropod
skulls, and the thesis can be confirmed or refuted, as these skulls
pertain loosely (indirectly) to the question of whether CMNH 7541
pertains to a tyrannosaurine, or a juvenile of its own taxon, or
juvenile of a known taxon - specifically, whether it is a juvenile
*Tyrannosaurus rex*. Expect this paper to be touted excessively by
those who argue vociferously (many of whom are in the fossil
distribution practices) that Nano is "real."

First, the question of the presence of the groove.

In CMNH 7541, a shallow sulcus or "groove" extends along the posterior
half of the dentary, and it is within this sulcus a series of foramina
are located. These become more numerous rostrally, and the sulcus is
shallower, until it is apparently absent anteriorly. Exposure of this
structure and its apparent extent is obscured due to preservation:
both mandibles are occluded to the skull, and reconstruction of the
middle sections of each dentary obscure actual preserved details. The
sulcus is apparent posteriorly, and appears to be less extensive
anteriorly. Thus, in agreement with these authors, there is such a
groove in "Nanotyrannus lancensis."

Most tyrannosaurine specimens seem to lack this feature, except in
portions of the rear of the skull, where a short, but broad
(dorsoventrally tall) sulcus may contain the last few nutrient
foramina. The mandibles of the basal tyrannosaurines *Bistahieversor
sealeyi* (NMMNH P-27469 & P-25049) and *Teratophoneus curriei* (BYU
8120 & 9398) contain dentaries, partial or nearly complete,
respectively. The more basal *Lythronax argestes* is also a
tyrannosaurine, and preserves a dentary (UMNH VP 20200). A cursory
examination of these dentaries demonstrates they all possess at least
a slight, but sometimes a more pronounced, sulcus, extending almost
half the length of the dentigerous portion of the dentary.

In the case of NMMNH P-25049, the dentary sulcus is more pronounced,
and the foramina lie clearly within; in the holoty NMMNH P-27469, the
sulcus is less pronounced, apparently relatively shallower, and the
foramina lie incised into the dorsal margin where they form definite
channels ascending towards the dentigerous margin (likely preserved as
housing vascular channels during incrassation of the mandible during

Second, the question of ontogeny.

Because of *Bistahieversor sealey*, I went to examine definite
juvenile tyrannosaurids, of which several are known. It served the
function of this limited rebuttal that I could confirm that
albertosaurine juveniles possess the groove; the question properly
concerns what tyrannosaurines have this feature.

LH PV18 is a juvenile tyrannosaurid which includes a nearly complete
skull and mandible, designated *Raptorex kriegsteini.* While initially
posed as a near adult taxon, a revision published by Loewen et al.
(https://dx.doi.org/10.1371%2Fjournal.pone.0079420) determined that
the question of its juvenility (enforced by Fowler et al.,
https://dx.doi.org/10.1371%2Fjournal.pone.0021376) did not bear as
much weight on the taxon's nature: it was distinct, but also a
juvenile. The dentary possesses a slight sulcus with distinct margins.

Juvenile *Tarbosaurus* sp. specimens are known. In their assessment of
one, Tsuihiji et al. (http://dx.doi.org/10.1080/02724634.2011.557116)
described MPC-D 107/7 and found it to be a juvenile *Tarbosaurus
bataar*. The dentary possesses a distinct sulcus with distinct
margins. This structure is more prominent further anteriorly than in
LH PV18. The formal description does not consider the question of the
sulcus, but it is apparent in work presented therein.

Concluding, the authors seem to correctly posit that adult (or
apparent adult) tyrannosaurine dentaries lack the sulcus. In this
manner, these resemble more closely the adult morphology in
*Bistahieversor* discussed above. However, the apparent juvenile state
differs from the adult state. In reference to Carr's 1999 work
detailing craniofacial ontogeny in tyrannosaurids, it appears to me
that some revision is needed, such that the lateral dentary sulcus
form, extent, and presence in ontogeny should be reviewed, and
considered an ontogenetically relevant feature. That this feature does
not necessarily disappear in even large tyrannosaurids, but has the
form as described in *Bistahieversor*, as seen in such tyrannosaurids
as Stan (BHIGR 3033) but not Sue (FMNH PV 2801). Jane (BMNH 2002.4.1)
preserves an intermediate morphology, as in *Bistahieversor*, in which
the sulcus is pronounced, but foramina present channels ascending
towards the dentigerous margin of the dentary.

In consideration, then, the arguments that the mere presence of the
feature is 1) absent in tyrannosaurines, and 2) diagnostic of
albertosaurines are incorrect. Without considering the possibility
that all heretofore "tyrannosaurine" juvenile specimens are not, in
fact, tyrannosaurine, but albertosaurine, it must be concluded that
this thesis is in error. The presence of this feature in CMNH 7541
merely confirms the relative ontogenetic age of that specimen, and not
its taxic relation.

On Thu, Jan 14, 2016 at 12:05 PM, Ben Creisler <bcreisler@gmail.com> wrote:
> Copy-paste problem (blame Windows). Here's the citation with the doi:
> 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
> doi:10.1016/j.cretres.2015.12.016
> http://www.sciencedirect.com/science/article/pii/S0195667115301415
> On Thu, Jan 14, 2016 at 12:03 PM, Ben Creisler <bcreisler@gmail.com> wrote:
>> 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.

Jaime A. Headden
The Bite Stuff: http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth" - P. B. Medawar (1969)