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I was very pleasantly surprised (actually more thrilled to be honest) when I heard about the discovery of the northern tyrannosaurid dinosaur Nanuqusaurus hoglundi from the Cretaceous Prince Creek Formation dating 70.6-69.1 Million Years ago in what is now the U.S. State of Alaska. Image Credit and Source: https://www.researchgate.net/publication/260757717_A_Diminutive_New_Tyrannosaur_from_the_Top_of_the_World At first thought to be only 5-6 meters (16-20 feet) in length based on the currently catalogued specimens, it's now believed based on currently undescribed remains to be 8-9 meters (26-30 feet) in length fully grown (compared to the 12.3-12.4 meters (40.4-40.7 feet) in length it's cousin Tyrannosaurus rex could reach fully grown). Image Credit: Artist Nathan Rogers Image Source: https://www.artstation.com/artwork/Z5w900 Fiorillo, A. R., & Tykoski, R. S., 2014. A diminutive new tyrannosaur from the top of the world. PloS one, 9(3), e91287. https://doi.org/10.1371/journal.pone.0091287 Druckenmiller, P. S., Erickson, G. M., Brinkman, D., Brown, C. M., & Eberle, J. J. (2021). Nesting at extreme polar latitudes by non-avian dinosaurs. Current biology : CB, 31(16), 3469–3478.e5. https://doi.org/10.1016/j.cub.2021.05.041 https://www.cell.com/current-biology/fulltext/S0960-9822(21)00739-9?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982221007399%3Fshowall%3Dtrue The Tyrannosaurid Nanuqusaurus is also unique as the northern most tyrannosaur known from Western North America (at the time the continent of Laramidia). Adapted to the colder climates of the region, it shared its habitat with a vast diversity of plants, small mammals, and other non-avian dinosaurs including hadrosaurs such as Edmontosaurus, the pachycephalosaur Alaskacephale, the Ceratopsian Pachyrhinosaurus, and a large currently unnamed genus of troodontidae. But it's confirmed geologic range is only during the early Maastrichtian (70.6-69.1 Million Years ago) of the Cretaceous period. Images Credits and Sources: https://www.researchgate.net/publication/260757717_A_Diminutive_New_Tyrannosaur_from_the_Top_of_the_World Fiorillo, A. R., and Gangloff, R. A., 2001. Theropod teeth from the Prince Creek Formation (Cretaceous) of northern Alaska, with speculations on Arctic dinosaur paleoecology. Journal of Vertebrate Paleontology. 20(4):675-682 Image Credit: Artist Nathan Rogers Image Source: https://www.artstation.com/artwork/Z5w900 The question I have is why this is? Did anything major happen to the habitat of the Prince Creek Formation between the Early-Late Maastrichtian? Did Nanuqusaurus live up to 66 Million Years ago to the latest Maastrichtian of the Cretaceous period?

The two most prominent hypothesizes on the direct evolutionary origin of perhaps the most famous Theropod Dinosaur from the fossil record, Tyrannosaurus Rex (Tyrannosauridae, Late Cretaceous (68-66 Million Years ago)) are what I call the Laramidia and Asian Origins. The Laramidia origin (named after the region of the Western North America which was a separate Continent during most of the Late Cretaceous and home to a vast amount of dinosaurs including Tyrannosaurus rex) hypothesizes that Tyrannosaurus rex is the direct descendent of and evolved from slightly older North American Tyrannosaurids like Daspletosaurus (Tyrannosauridae, Late Cretaceous (79.5-74 Million Years ago)). Warshaw, Elías & Fowler, Denver. (2022). A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana. PeerJ. 10. e14461. 10.7717/peerj.14461. https://www.researchgate.net/publication/365746599_A_transitional_species_of_Daspletosaurus_Russell_1970_from_the_Judith_River_Formation_of_eastern_Montana The Asian origin hypothesizes that Tyrannosaurus’s direct ancestor was a Tyrannosaurid from Asia. This supported by how closely related the Asian Tyrannosaurid Tarbosaurus (Tyrannosauridae, Late Cretaceous (70 Million Years ago)). This hypothesis further elaborates that a that the Asian Tyrannosaurids arrived in Western North America via a land bridge between what is now Eastern Russia and Alaska around 73-72 Million Years ago. On arrival, theses Asian Tyrannosaurids outcompeted and caused the extinction of most of the Native Tyrannosaur species of Laramidia (including Albertosaurus (Tyrannosaurid, Late Cretaceous (71-68 Million Years ago)), creating conditions allowing for the emergence of the genus Tyrannosaurus. Brusatte, Stephen & Carr, Thomas. (2016). The phylogeny and evolutionary history of tyrannosauroid dinosaurs. Scientific Reports. 6. 20252. 10.1038/srep20252. https://www.nature.com/articles/srep20252 Takasaki R, Fiorillo AR, Tykoski RS, Kobayashi Y (2020) Re-examination of the cranial osteology of the Arctic Alaskan hadrosaurine with implications for its taxonomic status. PLoS ONE 15(5): e0232410. https://doi.org/10.1371/journal.pone.0232410 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0232410 Both hypotheses have points that are supported by the fossil record, but still don’t fill all the gaps in answering the question of the direct origin of the genus Tyrannosaurus. There is however another hypothesis I have pondered over for at least the past few months which could fill in some (if not all) the gaps to this question. It is the Hybrid Speciation Origin hypothesis. The Hybrid Speciation Origin hypothesis basically states that after a land bridge formed between Eurasia and Laramidia during the Late Campanian stage of the Cretaceous (73-72 Million Years ago) and the Asian Tyrannosaurids arrived in Laramidia, certain individuals of a Asian Tyrannosaurid genus breed with a species of a genus of Native Laramidia Tyrannosaurid (likely a direct descendent of Daspletosaurus). Enough of these inter-genus breeding events occurred that a new Tyrannosaurid genus distinct from its parent species (and genuses) emerged around 68 Million Years ago, Tyrannosaurus. I will admit this would be extremely difficult to prove, but I do believe it could be a valid hypothesis. It corroborates the many similarities in skeletal structure Tyrannosaurus shares (and how closely related it is phylogenetically) with the Asian Tyrannosaurid Tarbosaurus and the skeletal structure similarities and general body shape it shares with Daspletosaurus. Image Credit: https://www.nature.com/articles/srep20252 Brusatte, Stephen & Carr, Thomas. (2016). The phylogeny and evolutionary history of tyrannosauroid dinosaurs. Scientific Reports. 6. 20252. 10.1038/srep20252. https://www.nature.com/articles/srep20252 Warshaw, Elías & Fowler, Denver. (2022). A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana. PeerJ. 10. e14461. 10.7717/peerj.14461. https://www.researchgate.net/publication/365746599_A_transitional_species_of_Daspletosaurus_Russell_1970_from_the_Judith_River_Formation_of_eastern_Montana Stein, Walter W.; Triebold, Michael (2013). "Preliminary Analysis of a Sub-adult Tyrannosaurid Skeleton from the Judith River Formation of Petroleum County, Montana". In J. Michael Parrish; Ralph E. Molnar; Philip J. Currie; Eva B. Koppelhus (eds.). Tyrannosaurid Paleobiology. Bloomington: Indiana University Press. pp. 55–77. Currie, P.J. (2003). Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica 48 (2): pp. 191–226. https://www.app.pan.pl/archive/published/app48/app48-191.pdf I am interested in how the tyrannosaurid species Nanuqusaurus (Tyrannosaurid, Late Cretaceous (70-68 (likely also to 66) Million years ago), the Daspletosaur Tyrannosaurid specimen RMDRC 2002.MT-001 “Sir William”, and the Tyrannosaurid specimen CM 9401 could factor into the validity of the hybrid speciation hypothesis. I hold no illusions in thinking this hypothesis is not going to be controversial. But I do think it could be semi plausible. What do you guys think?

Hi, Since the group Daspletosaurini has just be established. Wouldn’t Nanuqsaurus be part of it, it is known as a Tyrannosaurinae but since the clade/group Daspletosaurini has been established wouldn’t it be a Daspletosaurini? Since it shares some features with the Daspletosaurini.