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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?

So last year my friend Jared Voris named both Thanatotheristes degrootorum and Daspletosaurini (as you all probably know). For the past year a few others and I been studying this clade (you probably all know as well) and have been able to put up a good argument for two unnamed and controversial Daspletosaurus species, one from the Dinosaur Park Formation of Alberta and the other from the Judith River Formation of Montana. This presents that there’s most likely 4 Daspletosaurus species, there’s also been evidence of a possible (note possible, just kind of a guess based of age and location) Thanatotheristes from the early Judith River Formation. Along with Thanatotheristes degrootorum that’s 6 species of Daspletosaurini. Though there has been some recent news of a new Tyrannosaur that seems to fit perfectly in this lineage that my friend discovered, I can’t say much on it though until the paper is out. Careful radiometric and geological dating has shown the species don’t overlap. In order that unnamed species my friend discovered is 83.4-80 million years old, Thanatotheristes degrootorum is 80-79.5 million years old, the Judith Rivers possible Thanatotheristes sp. is 79.5-78 million years old, the Judith River Daspletosaurus sp. is 78-77 million years old, Daspletosaurus torosus is 77-76.5 million years old, the Dinosaur Park Formation Daspletosaurus sp. is 76.5-74.8 million years old, and Daspletosaurus horneri is 74.6-74 million years old (note that through time the features on these species change and flow perfectly together) (there’s also a gap there of 100,000 years I’ll get to soon) (and there might be another species in the Horseshoe Canyon Formation of Alberta but for now we suspect they went extinct 74 million years ago due to absence of diagnostic fossils and the Elkhorn range volcanoes in Montana experiencing huge volcanic events and the Western Interior Seaway rising causing an extinction and faunal turnover in North Western North America) anyways that gap of 100,000 years between the Dinosaur Park Formation Daspletosaurus sp. and Daspletosaurus horneri is pretty special as it contains three mysterious specimens from the upper Two Medicine Formation (Currie, P.J., Trexler, D., Koppelhus, E.B., Wicks, K., Murphy, N., 2005 An Unusual Multi-Individual Tyrannosaurid Bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA). pp. 313-324 In: The Carnivorous Dinosaurs. Carpenter, K. (ed.) Indiana University Press, 371 pp.) Interestingly, now that Daspletosaurus horneri has been described, TA.1997.002 (which is one of the specimens) instead seems to share characters with D. torosus plus the Dinosaur Park Formation species. In particular, the maxillary fenestra is longer than tall, the upper half of the lacrimal orbital margin is convex, the lacrimal horn is tall, the dorsal margin of the posterior postorbital process is convex, and the surangular shelf overhangs the posterior foramen. And this adding on that the sediments it was found in were between the Dinosaur Park Formation Daspletosaurus sp. and Daspletosaurus horneri in age, along with some Daspletosaurus horneri features (more research has to be done on these features) shows it’s most likely a transitional form! Especially since it’s the right age to be and shares features of both its predecessor and ancestor! Along with a transitional form this shows possibly our first complete Anagenetic lineage! I’m quite interested to hear everyone’s thoughts on this subject.