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



Damn, Thomas beat me to the comment.

On Thu, Jan 14, 2016 at 3:22 PM, Jaime Headden <jaimeheadden@gmail.com> wrote:
> 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
> ontogeny.
>
> 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)



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


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