With the good news that the Dueling Dinos are now in an accredited museum that got me wondering if it is yet possible to verify the claim by Peter Larson that the arms and hands of the gracile specimen are so over sized that they prove it cannot be a juvenile Tyrannosaurus. I don't care one way or the other what the situation is, but would like to see the issue finally put to rest one way or another. And there are the necessary images of the specimen that clearly show the forelimbs as per https://www.theguardian.com/science/2019/jul/17/montana-fossilized-dueling-dinosaurs-skeletons-dino-cowboy (best photos are further down in article).
The arms and hands are amazingly HUGE. Far larger relative to the rest of the animal than those of any known tyrannosaurid. The fingers broadly match the toes in robustness -- at first looking at one of the photos I thought they were toes until I realized both actual feet are near the pelvis and my mind boggled. And in absolute terms they are larger than those of adult Tyrannosaurus as my spiffy skeletals show (at gspauldino.com/tyrannoskels.jpg the adult Tyrannosaurus is Sue, the gracile form is a combo of Jane and Dueling, the smaller former being scaled up ~15% to the possible adult latter which is said to be 22 ft long, humerus/femur ratio as per Dueling). Of course it is not possible for a major appendage to actually shrink with growth so the Dueling beast cannot be a juvie Tyrannosaurus. There is no more reason to continue to assert that graciles are juv Tyrannosaurus just because the first are small and lived in the same habitat, than there is to say gracile Alioramus is a juv Tarbo when the first is small and lived in the same habitat. That issue is totally settled.
Note that in comparison a juvenile Gorgosaurus is nearly identical to the adult form except for some modest allometric shifts (the specimens are the ones at the AMNH). Much the same is true with the Tarbosaurus growth series in my Princeton Field Guide. Dueling and Jane are so much more different from Tyrannosaurus that ontogeny is clearly not involved.
There being nothing else like them in known tyrannosaurids, the super sized arms of Dueling were an apparently late appearing radical departure from the tyrannosaurid norm, and leaves it anatomically and very possibly phylogenetically further from Tyrannosaurus than is Tarbosaurus, so unlike the latter Dueling cannot possibly be in the same genus, much less a juvenile of the giant. One possibility is that small armed tyrannosaurids were a feature of Asia-Laramidia, the long arms of Appalachia as per Dryptosaurus (which may have had only two long fingers), and they meet as the seaway regressed. Maybe the Dueling specimen is not in Tyrannosauridae.
It has been bothering me that papers have been published supposedly proving that Nano is a juvie Tyrannosaurus when Larson has been warning that the Dueling specimen's forelimbs are too large for it to be a juv Tyranno. Remember that patience is a virtue.
All that said, it is not certain that the Dueling Nano is N. lancensis. As far as I know the Dueling specimens are from low in the Hell Creek, the N. lancensis may be from the middle of the Lance and hundreds of thousands of years younger, and could easily be a different species and maybe genus. Very hard to tell because the Nano type is just a seriously crushed skull with no arms. Nor am I sure what level of the Hell Creek that Jane is from, and its arms although they seem bigger than the tyrannosaurid norm do not appear to match those of the otherwise only marginally bigger Dueling specimen. (If anyone has solid info the the levels of the Hell Creek and Lance these specimens come from please let me know). There could have been a whole lot of Nano evolution going on during the 0.7 to 1.5 million years that the Hell Creek/Lance et al. existed (it is not yet possible to accurately date the time span of the two formations) -- I am not even sure the graciles made it to the end of the Cretaceous. Likewise Tyrannosaurus probably consists of two or three species, T. rex being confined to the top and probably including Scotty, and not including Sue or poor Stan. To put it another way, T. rex is very unlikely to be the sole late Maastrichtian upper plains tyrannosaurid in multiple ways.