|The Cleveland Skull|
Nanotyrannus never bothered me that much and as the years progressed it seemed more and more likely that it was going to end up being a juvenile Tyrannosaurus and more and more palaeontologists were convinced that this was so, although I was still unsure since nobody could explain the fused sutures in the skull.
Jane (BMRP 2002.4.1) was found by the Burpee Museum in 2001 as the first more or less complete Nanotyrannus ever found. Here, at last, was the specimen that would apparently confirm the taxon’s existence but even then, I thought, since we already had a complete skull and still couldn’t confirm its validity, the chances of validation were slim. Jane was ultimately described as a juvenile Tyrannosaurus – no surprises there then.
Larry Witmer’s recent paper (2009) which looked at the braincase region of tyrannosaurs was excellent. The “type” skull (CMNH 7541) of Nanotyrannus appears a conundrum. The skull demonstrates basal characteristics found in albertosaurines but also shared characteristics with Tyrannosaurus. Indeed, Witmer emphasises the fact that this skull is uniquely different from all other tyrannosaurs and not just T.rex.
As to why the skull is so divergent remains unclear. The skull has suffered a little minor distortion and displacement but not that much. Pathology also appears unlikely since there are no obvious signs of disease or tumours. Ontogeneric change appears to be the natural choice but even here there are dichotomies. The amount of difference in the skull cannot be put down to ontogeny alone. The skull does indeed display signs of maturity and yet fundamental differences remain and it is strange that the braincase of a Gorgosaurus juvenile (ROM 1247) is actually closer to Tyrannosaurus than the Cleveland skull is.
Some fascinating insights by the Witmer Lab and yet even here, the Nanotyrannus issue came out unresolved. In Witmer’s own words:
“Our data on CMNH 7541 may be taken as evidence for the validity of N. lancensis on the grounds that it is ‘‘too different’’ from T. rex. However, we are hesitant to argue that the debate over its status is settled for the simple reason of sample size. CMNH 7541 presents one specimen—one highly divergent specimen. Although we see no clear signs of distortion or pathology in the braincase, its divergent nature concerns us, and we maintain that the possibility remains that future discoveries will show CMNH 7541 to be aberrant. For that reason, we urge caution and continue to regard the specimen’s status as open.”
This brings us to a couple of weeks ago when the Nanotyrannus question raised its head yet again. For me the biggest single problem with the taxon is that there is only the one specimen – the Cleveland skull. Jane has been more or less defined as a juvenile Tyrannosaurus and this in itself is exceptional since juveniles of the species are unheard of. Even if Nanotyrannus was valid then where were the fossils? Granted that some tyrannosaurid post-crania could easily be misinterpreted as Tyrannosaurus even if it were Nanotyrannus, but the fact remains there is no other skull material.
And although Nanotyrannus is a “pygmy tyrant”, at maybe 16 to 20 feet in length, it is still a pretty big animal and you would have thought that more material related to the taxon would have been uncovered. And when you consider the rich diversity of fossils in the Hell Creek Formation (HCF) and the fact that Hell Creek is certainly the most sampled formation in the world, it is surprising that nothing else of note has ever turned up or been recovered.
“Nano” teeth are, however, fairly commonplace and are often labelled as simply tyrannosaurid teeth of uncertain affinities although they are markedly different from what I would consider conventional tyrannosaur teeth. But they are just teeth. It is possible, of course, that there was a form of niche partitioning during the late Maastrichtian and that Nanotyrannus avoided the territory of T.rex, thus avoiding the deltas and river channels and greatly reducing chances of fossilisation.. Maybe.
So I was a little amazed to hear that a lower jaw had turned up that would settle the debate once and for all! The dentary was recovered in 2006 from the HCF in South Dakota and I have to admit is a pretty impressive jaw except that it isn’t from Nanotyrannus. No – it appears to be clearly from a juvenile Tyrannosaurus. I would love to share a couple of images of this jaw but have been requested not to do so just yet and that’s fair enough as far as I am concerned.
The dentary is about 0.4 metres in length and has all of its teeth of which some are huge. The bone is very robust and strong and the big Tyrannosaurus-like teeth are up to about 60mm in length. I believe that the jaw has been prepared at the Field Museum of Natural History in Chicago but feel free to correct me if you know different. The jaw is currently being described by Pete Larson of the Black Hills Institute who, I am led to believe, regards it, and I quote, as the “smoking gun” which will confirm that Nanotyrannus is a valid taxon.
On the face of it, I can see why this would seem to be the case. This dentary is markedly different from both the Cleveland skull and Jane and is very Tyrannosaurus-like. Very powerful with very big conical robust teeth. So if this is a juvenile Tyrannosaurus (and it really does look like it is) then it appears that Jane actually is the most complete Nanotyrannus ever found. Right? Well maybe.
The problem I have with tyrannosaurid phylogeny is that they are extremely similar morphologically. I don’t like keep going over old ground but there are obvious problems defining taxa. As I keep alluding to, the debate over ceratopsian lumping and splitting goes on and the same problems exist in defining tyrannosaurids. The fact that Jane has been declared a Tyrannosaurus suggests that, if it indeed exists, Nanotyrannus is so similar to Tyrannosaurus as to make no difference. And I’m still not convinced because of the lack of detailed fossils.
So for me, the debate goes on and that, for now, I remain to be convinced about the validity of Nanotyrannus. The old adage “wait for the paper” was never better realised than in this particular case. Not only the Larson paper but whatever happened to the new Bakker, Currie and Larson research as well? We will see.
In any event, as I commented earlier, I’m not too bothered either way. If Nanotyrannus exists – fine, if not then Tyrannosaurus remains as the last its kind but it will be a great day when the Nanotyrannus saga is finally put to bed forever. Until the next one that is!
Bakker, R. T., M. Williams, and P. J. Currie. 1988. Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana. Hunteria 1:1-30.
Witmer, L.M. and Ridgely, R. C. 2009. New insights into the brain, braincase, and ear region of tyrannosaurs (Dinosauria, Theropoda), with implications for sensory organization and behavior. The Anatomical Record 292:1266-1296.