Sunday, September 15, 2013

NANOTYRANNUS ISN’T REAL, REALLY.

A cast of the Cleveland skull in the Hall of Horns and Teeth at the Museum of the Rockies (Bozeman, MT), where it is presented as a juvenile Tyrannosaurus rex, next to a cast of the subadult MOR 1125.
Introduction
The impending auction of the ‘Dueling Dinosaurs’ has brought Nanotyrannus back into the pop culture spotlight. I repeat here the evidence that I published in support of the hypothesis that the Cleveland skull is a juvenile Tyrannosaurus rex (Carr, 1999). A critical evaluation of the diagnosis for Nanotyrannus lancensis follows this, and I close with evidence documented by Bakker et al. (1988) that supports the specimen’s referral to T. rex.
What was Bakker et al.’s (1988) characterization of Nanotyrannus? They considered the skull to represent a fully adult individual (skull length: 572 cm) of the most primitive (basalmost) tyrannosaurid from the Late Maastrichtian Hell Creek Formation of Montana, which was sympatric with T. rex. They considered Nanotyrannus to be convergent with T. rex upon an identical and extensive suite of adaptations for stereoscopy and striking behavior (i.e., head-neck kinematics).
Isn’t the Cleveland skull from an adult? No – the basis for adulthood by Bakker et al. (1988) was based on the mistaken notion of suture closure in the skull (Carr, 1999). In fact, very few skull sutures actually close during the ontogeny of tyrannosaurids; regardless, every observable suture in the Cleveland skull is open, aside for the internasal and interparietal sutures, which are closed in other tyrannosaurid juveniles (1999).
Also, the surfaces of the bones in the Cleveland have a distinct striated texture that is typically seen in juvenile dinosaurs (Carr, 1999). A similar texture is seen in adults, but its distribution is patchy and the striae are less dense and distinct than the condition seen in small specimens.
Therefore, based on the absence of suture closure (beyond what is usually seen in tyrannosaurid juveniles), the presence of immature bone grain, and its small size, the Cleveland skull is unquestionably a juvenile tyrannosaurid.
Is the Cleveland skull similar in any way to other juvenile tyrannosaurids? Yes – in virtually every detail; this table summarizes the salient features (Carr, 1999):
Albertosaurus libratus juvenile condition
Is it seen in the Cleveland skull?
Premaxilla, narrow, rostral view
Yes!
Premaxilla, lateral margin, concave, rostral view
Yes!
Premaxilla, alveolar region, shallow, rostral and lateral views
Yes!
Maxilla, mediolaterally narrow, lateral and rostral views
Yes!
Maxilla, first tooth, incisiform, all views
Yes!
Maxilla, teeth except the first, labiolingually narrow
Yes!
Maxilla, ventral jugal process, not breached by neurovascular sulcus
Yes!
Maxilla, lateral surface along rostral end of the antorbital fossa, not strutlike, lateral view
Yes!
Maxilla, promaxillary fenestra, tall and not recessed, lateral view
Yes!
Antorbital fenestra, longer than tall, lateral view
Yes!
Maxilla, maxillary fenestra, size, small, lateral view
Yes!
Maxilla, maxillary fenestra, position, midway between the rostral margins of the antorbital fossa and fenestra, lateral view
Yes!
Lacrimal, rostroventral ala, rostral margin, concave, lateral view
Yes!
Lacrimal, rostroventral ala, contact with jugal, exceeds that of the ventral ramus, lateral view
Yes!
Jugal, maxillary ramus, dorsoventrally shallow and long, lateral view
Yes!
Jugal, pneumatic recess, rostrally restricted slit, lateral view
Yes!
Jugal, postorbital joint surface approaches the orbit floor, lateral view
Yes!
Jugal, base of the postorbital process, convex, lateral view
Yes!
Jugal, caudal margin of the postorbital process, convex, lateral view
Yes!
Jugal, caudal rim of the joint surface for the lacrimal, subvertical, lateral view
Yes!
Postorbital, laterodorsal margin, vertically oriented, dorsal view
Yes!
Postorbital, cornual process, absent, lateral view
Yes!
Postorbital, subocular prong, absent, lateral view
Yes!
Frontal, lacrimal notch, long and narrow, dorsal view
Yes!
Frontal, paired bones are longer than wide, dorsal view
Yes!
Frontal, dorsotemporal fossa, shallow, dorsal view
Yes!
Frontal, dorsotemporal fossa, indistinct rostral margin
Yes!
Parietal, nuchal crest, delicate in form
Yes!
Parietal, nuchal crest, low in height
Yes!
Ectopterygoid, jugal ramus, not inflated, ventral view
Yes!
Ectopterygoid, muscle scar on jugal ramus, caudolateral in position
Yes!
Supraoccipital, dorsal process, narrow, caudal view
Yes!
Basioccipital, occipital condyle, shape of an inverted triangle, caudal view
Yes!
Basioccipital, occipital condyle, form of caudoventral surface, flattened, caudal view
Yes!
Basioccipital, basituberal web, flat ventral surface, ventral view
Yes!
Basioccipital, basituberal web, ventral surface is dorsally arched, caudal view
Yes!
Basioccipital, subcondylar recess, deeply excavated, caudal view
Yes!
Basisphenoid, pneumatic foramina are small, caudoventral view
Yes!
Basisphenoid, pneumatic foramina are situated ventrally within the basisphenoid recess, caudoventral view
Yes!
Basisphenoid, oval scar, smooth and lateroventrally oriented, caudoventral view
Yes!
Dentary, dorsoventrally shallow, lateral and medial views
Yes!
Dentary, mediolaterally narrow, ventral view
Yes!
Surangular, dorsoventrally shallow, lateral view
Yes!
Surangular, surangular shelf, horizontally oriented, lateral view
Yes!
Prearticular, dorsal margin is restricted caudally, medial view
Yes!
Prearticular, caudal ramus is dorsoventrally shallow, medial view
Yes!
Prearticular, rostral ramus is dorsoventrally shallow and straplike, medial view
Yes!
Prearticular, rostral ramus, caudodorsal surface is smooth, medial view
Yes!

Therefore, every bone in the Cleveland skull is identical to the condition seen in small juvenile tyrannosaurids, not adults (Carr, 1999). Therefore, we can add the details of the bone structure to the stack of evidence regarding its immaturity.
What about the diagnosis of Nanotyrannus lancensis? Doesn’t it adequately define the new genus and species? This is the heart of the matter because the validity of a taxon rests on its diagnosis. A diagnosis is a list of the anatomical features that set a new taxon apart from its closest relatives; presumably the diagnosis of Nanotyrannus includes indisputably unique features. For Nanotyrannus, let’s consider each character (Bakker et al., 1988) one at a time:
1) “muzzle width greatly constricted to only one fourth the width of the temporal region” (Bakker et al., 1988:3)—This feature is not unique; a narrow snout and wide temporal region is also seen in T. rex. Also, the snout is not as narrow in the Cleveland skull as the authors claim, a condition that has  been exaggerated by mediolateral crushing. This can be seen in the plate of the specimen in dorsal view (1988:28), where  fractures extend across the right jugal and maxilla, and the base of the snout extends rostrally at an unnaturally sharp angle.
2) “extraordinarily wide basicranium between basitubera and basipterygoid processes” (Bakker et al., 1988:3)—This is also seen in adult T. rex (Bakker et al., 1988).
3) “two large pneumatic foramina, placed one behind the other, near the midline of the basisphenoid” (Bakker et al., 1988:3)—This condition is the result of damage (Carr, 1999).
4) “Differs from Daspletosaurus, Gorgosaurus and Alioramus and agrees with Tyrannosaurus, Albertosaurus and Tarbosaurus spp. in having the derived features of a marked expansion of the width across the temple, relative to the skull length…”(Bakker et al., 1988:3)These are not a diagnostic characters since they are seen in other taxa; there is no reason to think that it didn’t evolve once in the common ancestor of Tyrannosauridae and later became lost in several species.
5) “…and basitubera that are displaced forward towards the basipterygoid processes” (Bakker et al., 1988:3)—The same issue applies here as in (4).
6) “Differs from all other tyrannosaurids, except Alioramus, in retaining the primitive characters of a long, low snout…”(Bakker et al., 1988:3)—A long, low snout is typical of juvenile tyrannosaurids, and it has recently been shown (Brusatte et al., 2012) that the extremely long and shallow depth of the snout seen in Alioramus is derived beyond the norm in tyrannosaurids; i.e., the Cleveland skull does not have the condition seen in Alioramus, its snout depth is normal for a tyrannosaurid of its size.
7) “…and maxillary teeth that are strongly compressed side-to-side” (Bakker et al., 1988:3)—Narrow teeth are typical of juvenile tyrannosaurids, and the Cleveland skull does not have unusually narrow teeth in comparison with them (Carr, 1999).
8) “Differs from all other tyrannosaurids in lacking strong ridges and striae along the dorsal surface of the nasal” (Bakker et al., 1988:3)—It turns out that this is not true; a smaller and less mature juvenile T. rex (LACM 28471) has smoother nasals than are seen in the Cleveland skull, indicating that the nasals coarsened with maturity in T. rex (Carr and Williamson, 2004). In other tyrannosaurids, such as Albertosaurus, the nasals are coarse relatively earlier in growth (Carr, 1999).
9) “Differs from Tyrannosaurus in retaining the primitive character of a smooth depressed area for muscle origin on the anterior-ventral corner of the lachrimal” (Bakker et al., 1988:3)—This is the antobital fossa, a pneumatic surface and not an origin for muscle, which is seen in adult T. rex, except that this region is swollen by the internal inflation of the bone by the antorbital air sac system (Carr, 1999). This is a simple growth change that is seen in all tyrannosaurids (Carr, 1999).
Why then is the Cleveland skull referable to T. rex?
To answer this we don’t need to go any further than Bakker et al. (1988), who provided a long list of characters that are only seen in T. rex and the Cleveland skull:
1) “In Tyrannosaurus the basituberal width is enormously expanded, reaching 19% of the skull length…In Nanotyrannus the basituberal web is fully 20% of skull length…”
In other tyrannosaurids the ratio is much lower,  12%-15% (Bakker et al., 1988: 9).
2) “in Albertosaurus, Tyrannosaurus and Nanotyrannus [the main tuberous process of the basal tuber] is reduced in bulk” In other tyrannosaurids, the process is a “swollen development” (Bakker et al., 1988: 9).
Given the modern understanding of tyrannosaurid phylogeny, the reduced condition would have almost certainly have evolved twice, once in Albertosaurus, and again in Tyrannosaurus.
3) “In Nanotyrannus the ventral floor of the basisphenoid is nearly flat between the basituberal and basisphenoid webs, a condition seen in Tyrannosaurus” (Bakker et al., 1988: 12).
In other tyrannosaurids the “ceiling of the central basisphenoidal cavity is very tall and the cavity is very deep top-to-bottom” (1988:12). Although the authors are inconsistent in using the terms “ceiling” and “floor”, they are discussing the same feature, namely, the basisphenoid recess.
4) “The occipital condyle…in Nanotyrannus…faces…strongly downward…agreeing with the condition in Tyrannosaurus” (Bakker et al., 1988: 12).
In other tyrannosaurids the condyle presumably faces more caudally than downward.
5) “Moreover, the articular surface on the condyle in Nanotyrannus has been displaced downwards and around onto the ventral surface of the [occipital condyle] neck…again agreeing with [the condition seen in] Tyrannosaurus. Clearly the head was flexed far more sharply downwards in Nanotyrannus and Tyrannosaurus than in more primitive [i.e., all other] tyrannosaurids” (Bakker et al., 1988: 12).
In other tyrannosaurids the articular surface presumably does not extend ventrally onto the neck of the occipital condyle.
6) “…in Tyrannosaurus and Nanotyrannus, the basitubera are displaced forward and project less strongly below the floor of the basicranium” (Bakker et al., 1988:13).
In other tyrannosaurids “the basitubera are located far aft, beneath the occipital condyle, and project strongly downward” (1988:13).
7) “In Nanotyrannus the width of the…entire temporal region is greatly expanded relative to the snout…the breadth between the jaw joints…is 57% of the skull length…[In Tyrannosaurus] the quadratojugal width is 62% of skull length” (Bakker et al., 1988:14).
In other tyrannosaurids this ratio is much lower, ranging from ~33%-42% (1988:14).
8) “As a consequence of the temporal breadth/muzzle breadth ratio in Nanotyrannus and Tyrannosaurus, right and left eyes have a wide overlap of visual fields. Stereoscopy was probably present over a range of 30 degrees or more” (Bakker et al., 1988:15).
In other tyrannosaurids this overlap is not seen.
9) “the orbits face upwards as well as forwards in Tyrannosaurus…In Nanotyrannus, the upward orientation of the orbits is less marked” (Bakker et al., 1988:16).
Although the authors are making a contrast here, it is worth noting that both taxa have orbital fenestrae with an upward component, a condition that is not seen in other tyrannosaurids.
10) “The paroccipital processes of Nanotyrannus agree with those of Tyrannosaurus in being oriented nearly directly outwards and having much less of a backward orientation than that seen in gorgosaurs and daspletosaurs” (Bakker et al., 1988:16).
Arguably, the authors made a better case for referring the Cleveland skull to T. rex than I did (cf. Currie, 2003).
What did Bakker et al. (1988) make of these striking similarities between the Cleveland skull and T. rex? How could they think they had a new genus and species?
Here it is, in their own words:
“Whatever its evolutionary origins, Nanotyrannus seems to have evolved its stereoscopy independently of Tyrannosaurus. The low, long snout and primitive teeth of Nanotyrannus seem to preclude an ancestry from an animal as advanced as Gorgosaurus. The two stereoscopic genera, Nanotyrannus and Tyrannosaurus, are the only well known large predators from the Lancian Age, and it is noteworthy that both have achieved the highest degree of potential stereoscopy known among large theropods…Clearly natural selection was favoring greater precision of depth perception…Thus Nanotyrannus seems to have been shaped by an inexorable law of the evolution between predator and prey, a law that operated to produce ever greater finesse in the choreography of attack behavior” (Bakker et al., 1988:26).
The authors considered this suite of characters to have evolved by convergent evolution in Nanotyrannus and Tyrannosaurus, evidently at the same point in geological time and in the same geographic location. Clearly, that is not a parsimonious hypothesis.
Summary
Here’s my train of thought on the matter:
1) The Cleveland skull is of a juvenile tyrannosaurid.
2) The diagnosis of Nanotyrannus lancensis fails under scrutiny: the characters reflect the specimen’s relative maturity, damage to the skull, and characters shared with other tyrannosaurids. Consequently, N. lancensis is an invalid taxon; it entirely lacks defining features.
3) The Cleveland skull shares many specializations that are seen nowhere else except in adult T. rex, the species with which it happens to be sympatric.
4) Therefore, the Cleveland skull is a juvenile T. rex.
5) Ergo, the ‘dueling tyrannosaurid’ cannot belong to a taxon that does not exist.
So what species is the dueling tyrannosaurid?
It isn’t in a legitimate repository, so we can't know this at present. We’ll almost certainly never know if the auction goes ahead and the specimen is sold to a private individual – unless, as a colleague recently suggested to me, the people of the US want to intervene and rescue it for science under a successful argument for eminent domain.

References Cited
Bakker RT, Williams M, Currie PJ. 1988. Nanotyrannus, a new genus of pygmy tyrannosaur from the latest Cretaceous of Montana. Hunteria 1: 1-30.

Brusatte, S. L., T. D. Carr, and M. A. 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 366:1-197.


Carr, T. D. and T. E. Williamson. 2004. Diversity of Late Maastrichtian Tyrannosauridae from western North America. Zoological Journal of the Linnean Society 142:479-523.


Carr, T. D.  1999.  Craniofacial Ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology 19:497-520.

Currie, P. J. 2003. Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica 48: 191-226.

18 comments:

  1. Isn't there a referred Nanotyrannus specimen with a dramatically different postcranium (long neck, big arms, etc.)?

    (Yes, this is a half-remembered fragment of conference talk -- what can I tell you, I am sauropod guy.)

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    1. Well though I'm not an expert on dinosaurs, I heard that young tyrannosaurids had longer arms than that of adults..

      Williams, S., S. Brusatte, J. Matthews and P. Currie 2010. A new juvenile Tyrannosaurus and a reassessment of ontogenetic and phylogenetic changes in tyrannosauroid forelimb proportions. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 187A.

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  2. Given the state of tyrannosaurid knowledge in 1988, would it be reasonable for Bakker et al to conclude otherwise? I'm just curious, since back in 1988 I was more interested in the finer details of Teenage Mutant Ninja Turtles....

    -Caleb Lewis

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  3. Caleb: No it isn't reasonable, as explained.

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    1. Is that really fair to paleontologists in 1988? Almost no theropod specimens were even recognized as juveniles, besides some Gallimimus and Allosaurus individuals. The few known juvenile tyrannosaurids (e.g. the then unnamed Jordan theropod and Shanshanosaurus) were thought to be small basal tyrannosauroids or even dromaeosaurids. Similarly, there was no consensus the smaller Nemegt tyrannosaurids were young bataar. AFAIK, no one had thought to look for immature bone grain in any Mesozoic dinosaur yet, and there was no consensus of (Alberto(Daspleto(Tarbo,Tyranno))) like we have now to use to find nested traits that suggest synonymy. The sample size of Tyrannosaurus was low enough to make the number of specimens a notable media figure, when now we have more than most species of theropod known from more than just their holotype. The argument Nanotyrannus is a juvenile Tyrannosaurus thus depends on a lot of data which didn't exist in 1988 or wasn't organized yet. Rozhdestvensky (1965) was remarkably prescient decades earlier regarding some of these issues, though his same methodology incorrectly found Lufengosaurus to be young Yunnanosaurus, so we might question whether his seemingly correct tyrannosaurid ideas were validly justified or at least partly luck.

      It's that old standing on the shoulders of giants cliche, and while you are a major giant here, suggesting Currie, Russell, Molnar, Paul, Carpenter, Bakker, etc., basically every theropod worker of the 80s, was unreasonable seems unfair. A valid criticism is that Bakker et al. (1988) never cite or mention Molnar's (1980) description of what would become the type of Dinotyrannus, that was referred to lancensis at the time. It was published 8 years earlier in a well read journal, so should have gotten some comment.

      The supposed unreasonableness of Bakker et al. (1988) also affects your main argument, as some of it depends upon the truth of our current consensus. Accepting Nanotyrannus as a juvenile Tyrannosaurus seems more plausible in a world where Maleevosaurus is just a young Tarbosaurus, Stygivenator is a juvenile Tyrannosaurus, etc.. But if the well-studied Cleveland skull is valid based on newly recognized characters, what does this mean for Maleevosaurus, which has never been studied firsthand since its description to my knowledge? Larson considers Stygivenator a younger Nanotyrannus, for instance, so the former can no longer be used as an example of what juvenile Tyrannosaurus are like.

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  4. https://www.facebook.com/photo.php?fbid=567678526601057&set=a.241256149243298.52034.100000767435138&type=1&theater&notif_t=photo_comment
    I believe I'm holding the key to the answer we all seek.

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    1. Very interesting, Bob. Hopefully this will end up in a public institution where it will be available for study, so that it can add to our scientific knowledge instead of being locked away from it.

      Minor quibble - social media can be an excellent tool for science outreach but I feel that referring to the specimen as "T-rex" instead of the more proper T. rex works as a small disservice with regard to educating the general public about general palaeo/bio-science matters. I found it even more jarring because you identified the bone as a femur rather than using the non-technical "thigh".

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    2. Hi Robert,

      I second Mark's concern - will that specimen go to a valid museum? Otherwise, it is out of the reach of science, which is what happened to Samson.

      -Thomas

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    3. Hi Thomas,
      In 2004 Samson was sent to the Carnegie Museum of Natural History, which allowed any and all researchers access to that remarkable skeleton. The skull was also sent to NASA and was allowed to be cat-scanned. Although its in private hands now, I've not heard of the owner refusing any legitimate assess to Samson.
      I understand the importance of this newly discovered juvenile tyrannosaurus and I'm not looking to make it available to private collectors. At this point it's only being offered to institutions, museums and academia, I might also add that 98% of all fossil specimens I've collected are in institutions or museums worldwide.
      I've always had an open door policy with anyone interested and always will.
      Thank you for your concern,
      -Robert
      "Science knows no country, because knowledge belongs to humanity, and is the torch which illuminates the world."
      - Louis Pasteur

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    4. Robert,

      The private owner of Samson could roll out a red carpet and slap a set of calipers in my hand, but I wouldn't be able to collect any data from the specimen because it is not in a legitimate repository. To obtain data from such a specimen, I would be in violation of the Society of Vertebrate Paleontology's Ethics guidelines that state,

      "Section 4. Deposition of fossil specimens
      Scientifically significant fossil vertebrate specimens, along with ancillary data, should be curated and accessioned in the collections of repositories charged in perpetuity with conserving fossil vertebrates for scientific study and education (e.g., accredited museums, universities, colleges and other educational institutions)."

      Unless I'm mistaken, Samson is not in such a repository. Therefore, when you sell a scientifically significant fossil such as Samson to a private individual it's lost to science, period. I'm glad you're looking to do the right thing with the small skeleton.

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  5. A nice post, but if I were reading it in 2013 and thought Nanotyrannus were valid, it would be quite unconvincing. Note I do provisionally agree with you that Nanotyrannus are juvenile Tyrannosaurus, so here I'm arguing as Devil's Advocate.

    Notably it's only really engaging with the first and worst arguments for Nanotyrannus' validity. Sure Bakker's 25 year old ideas are flawed, but any modern defense of the genus includes a swath of newly recognized characters, and of course Jane. So yes, Bakker was wrong to think the Nanotyrannus type had extensive cranial fusion. But Larson (2013) reports Jane has presacral neurocentral, scapulocoracoid and pelvic fusion. By not addressing these characters (also including glenoid position, subnarial foramen position, quadratojugal pneumaticity, maxillary tooth count, etc.), your analysis loses most of its force.


    There's also the problem of using similarity to juvenile stages of other species as an argument. In the 4 stage ontogeny of your 1999 paper, Albertosaurus simply never gets to stage 4. It stops at what is stage 3 in Daspletosaurus. Indeed, in 1996, you used this to argue Daspletosaurus are just adult Albertosaurus. You later changed your mind on this, but couldn't Nanotyrannus be a tyrannosaurine that never grows past stage 1? This makes your huge table moot, just as a huge table of stage 3 characters listed as "Yes!" in Albertosaurus would not mean it was a subadult Daspletosaurus.


    A related point is that Larson agrees Nanotyrannus could be the sister taxon of Tyrannosaurus, so your long list of characters they share works well for both hypotheses. We don't need to assume Bakker's odd phylogeny with a basal Nanotyrannus convergent with Tyrannosaurus for the genus to be valid.


    Further, it implies a taxon can be invalidated merely by finding the original diagnosis wanting. Let's see how that works for Tyrannosaurus, based on Osborn's (1905) original diagnosis.
    1. "Carnivorous Dinosaurs attaining very large size." Tarbosaurus got as large as the average Tyrannosaurus, so this is invalid even within Tyrannosauridae.
    2. "Humerus believed to be of large size and elongate (Brown)." This was based on an incorrectly identified bone.
    3. "No evidence of bony dermal plates (Brown)." This was to distinguish it from Dynamosaurus, whose holotype had incorrectly referred plates.
    Well, guess Tyrannosaurus is an invalid taxon we can't refer the Cleveland skull to...


    Finally, couldn't the Cleveland skull be a juvenile and a valid genus, just without known adult specimens? That's what you argue for in Alioramus, which is (1) based on two specimens of juvenile tyrannosaurid, (2) has an original diagnosis which relies entirely on characters claimed to be ontogenetic in the case of Nanotyrannus (long snout, high tooth count, compressed teeth, etc.), (3) has derived characters only known in the sympatric Tarbosaurus, so is by your reasoning (4) therefore juvenile Tarbosaurus. Yes, Alioramus has additional suggested apomorphies, but so does Nanotyrannus and they seem to be of equal import (e.g. additional pneumatic features in each). So I'd like to know why you came to divergent conclusions in these similar cases.

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  6. Hi Mickey,

    Thank you for your comments! I have decided that my response to them will be a post entry in the near future.

    Sincerely,

    Thomas

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    1. I'm looking forward to Thomas' reply. But I can give a brief one of my own: Alioramus was a potential juvenile Tarbosaurus, but the subsequent discovery of a specimen which was more clearly a juvenile Tarbosaurus but distinct from Alioramus causes us to reject this hypothesis.

      As I have stated for a decade and a half or so, that's all I'd need to be convinced of the validity of Nanotyrannus: either a juvenile Tyrannosaurus of the same ontogenetic status as Nano but is distinct from it, OR an adult Nanotyrannus which is distinctly not Tyrannosaurus.

      Until such a situation, parsimony suggests we accept the simpler solution: the taxon known only from juveniles and the sympatric taxon known only from subadults and adults which share apomorphies with each other are most likely the same taxon.

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    2. I'd actually go the other way with this and view Alioramus as a juvenile Tarbosaurus. The differences between Alioramus' two known specimens and the few Tarbosaurus of similar size are mostly those known to vary ontogenetically (so could be explained by size not exactly correlating to skeletal maturity) or individually (especially pneumatic and ornamentation characters). There's enough variety in adult Tyrannosaurus rex to cause that whole rex vs. X debate, so seeing similar variety in juvenile Tarbosaurus seems expected. It's also not as if we have multiple Tarbosaurus subadults which all have one state and multiple Alioramus that all have another- due to incompleteness it's actually two Alioramus maxillae vs. a larger Tarbosaurus maxilla, the jugal, postorbital and tibia of "altai" vs. one similar-sized Tarbosaurus example, two Alioramus surangulars vs. a different Tarbosaurus than the last two, etc.. The very fact we have five subadult Mongolian tyrannosaurines but only two Hell Creek subadults also makes it more likely the former will show more variation.

      Finally, it should be noted Brusatte et al.'s (2009) description of Alioramus "altai" coded Alioramus as if it were adult, so it seemed more different from Tarbosaurus in the cladogram and matrix.

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  7. This is an excellent article,

    Could you comment on Currie's (2003a) claim that the maxillary tooth counts vary between Nanotyrannus (14) and Tyrannosaurus (11-12) and that there's little evidence to suggest that the number of maxillary teeth becomes reduced during ontogeny (Currie 2003b). I'm not convinced by this argument myself but I'd just like to play devil's advocate a little and get your thoughts on it.

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    1. Hi Kyle,

      If the Cleveland skull is a juvenile T. rex, and all of the available evidence indicates this is so, then we have to accept the occurrence of ontogenetic tooth loss in that species. In 1999, I also hypothesized that the loss happened at the front of the tooth row. The variation seen in adults is consistent with the tooth loss hypothesis.

      -t.

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