Tyrannosaurus rex ancestry

[Vladimír Socha]

Good day to all listmembers! I wonder, what is the current most probable opinion on the evolutionary ancestry of Tyrannosaurus rex? Which of these possibilities is the most plausible given the current status of our knowledge?

1. It evolved directly in Laramidia, from T. mcraeensis or some advanced Daspletosaurines?

2. It's ancestors migrated to Laramidia from Eastern Asia and Tarbosaurus and/or Zhuchengtyrannus are perhaps direct evolutionary ancestors of the T. rex.

3. It evolved in Laramidia from Asian immigrant species that transformed into T. rex about 68 mya.

4. Some other possibility?

Thank you in advance, VS.

References:

Brusatte, Stephen L.; Norell, Mark A.; Carr, Thomas D.; Erickson, Gregory M.; Hutchinson, John R.; Balanoff, Amy M.; Bever, Gabe S.; Choiniere, Jonah N.; Makovicky, Peter J.; Xu, Xing (September 17, 2010). "Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms". Science. 329

Loewen, M. A.; Irmis, R. B.; Sertich, J. J. W.; Currie, P. J.; Sampson, S. D. (2013). Evans, D. C (ed.). "Tyrant Dinosaur Evolution Tracks the Rise and Fall of Late Cretaceous Oceans". PLoS ONE. 8 (11): e79420.

Dalman, Sebastian G.; Loewen, Mark A.; Pyron, R. Alexander; Jasinski, Steven E.; Malinzak, D. Edward; Lucas, Spencer G.; Fiorillo, Anthony R.; Currie, Philip J.; Longrich, Nicholas R. (January 11, 2024). "A giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism". Scientific Reports. 13 (1): 22124.

Scherer, Charlie Roger; Voiculescu-Holvad, Christian (2024). "Re-analysis of a dataset refutes claims of anagenesis within Tyrannosaurus-line tyrannosaurines (Theropoda, Tyrannosauridae)". Cretaceous Research. 155 105780.

[Gregory Paul]

Chucking all TT-zone Tyrannosaurus into T. rex is obsolete with the growing data otherwise. The first Tyrannosaurus in the TT-zone was T. imperator retaining the tyrannosaurid standard of two incisors on the dentary and those novel wild and wacky postorbital display spindles, which probably was the ancestor of either T. rex or T. regina or both. I tend to assume the writer has not read the new paper on the subject at https://www.mapress.com/mz/article/view/mesozoic.2.2.1. Enjoy.

As for the ancestry of TT-zone Tyrannosaurus, that was probably via something like T.? mcraeensis. That probably was earlier than the TT-zone, it's more basal morphology indicates that. As I note in the Mesozoic paper it probably was not classic Tyrannosaurus (unless one includes Tarbosaurus in a larger Tyrannosaurus genus). 

As for where those came form good question. Daspletosaurus remains a potential regional ancestor in which case Tarbo and Tyranno converged and cannot be the same genus. Or maybe Tyranno is a Tarbo descendent. But the latter requires a really big tyrannosaurid make it across the chilly Bering land bridge, but there is not evidence of such there. I discuss this also in the new paper as well as Paul et al. (2022). 

GSPaul

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[Franco Sancarlo]

You're right, we don't have any evidence that large Tyrannosaurids moved from Asia to North America, but it seems to me that we have evidence that this migration happened with the Saurolophines (Ford 2024), so it's likely that a tyrannosaurid followed them.

Ford 2024:MONGOLIA TO THE WEST COAST OF CALIFORNIA. A POSSIBLE SAUROLOPHINE LARAMIDIA CONNECTION, Conference: 14th Symposium on Mesozoic Terrestrial Ecosystems and Biota. DOI:10.13140/RG.2.2.28490.79043

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[Tim Williams]

Franco Sancarlo <franco.sa...@gmail.com> wrote:

> You're right, we don't have any evidence that large Tyrannosaurids moved from Asia to North America,

The recent _Khankhuuluu_ paper had some views on this (Voris et al. 2025 doi.org/10.1038/s41586-025-08964-6), which you (and GSP) might not agree with.

"A final dispersal event from Asia to North America occurred at the end

of the Cretaceous (around 73–67 Ma) among tyrannosaurins, which
resulted in the establishment of _Tyrannosaurus_ in North America, a

dispersal event consistent with most other studies"

And more context later in the paper:

"An eventual dispersal of
a eutyrannosaurian lineage back to Asia led to the origin of Alioramini
and Tyrannosaurini where divergent heterochronic processes led to
stark morphological differences between these two clades, allowing
them to co-exist in Maastrichtian Asia and probably occupy different
ecological niches as gracile mesopredators and robust apex predators,
respectively. With a later tyrannosaurin dispersal from Asia to North
America (where alioramins are absent based on current evidence),
juvenile tyrannosaurins (_Tyrannosaurus_) in North America were able to
fill the mesopredator niche, while their adult conspecifics dominated
the ecosystems as apex predators for the last 2 million years until the
end-Cretaceous mass extinction event."

[Franco Sancarlo]

Hi Tim, maybe I expressed myself badly. What I meant was that although we have no proof that this happened, it is likely that it happened since a similar thing happened with Saurolophine, so yes I agree with Voris et al (2025), but not on the last part (i.e. the one where it says that the young of the species filled the mesopredator niche) which has already been answered by Paul (2025)

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[Milo Gaillard]

To Franco,

I agree with you, except I think the Khankhuuluu paper provides a greater case for tyrannosaur Asian ancestry than before.

-Milo Gaillard

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On Jul 15, 2025, at 04:15, Franco Sancarlo <franco.sa...@gmail.com> wrote:



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[Tyler Holmes]

Part of the problem here is that we don't have a great Laramidian fossil record in the latest Campanian and early Maastrichtian, which is a fairly critical window for determining what species where going back and forth. We know plenty of groups were making trips to Asia or from Asia, but what were tyrannosaurs doing? 

Take CM 9401, a tyrannosaurine right lacrimal that was originally associated with the type specimen of Deinosuchus hatcheri, CM 963. Urban and Lamanna (2006) noted it was extremely similar to the lacrimal of Tyrannosaurus, but the initial inclusion with Deinosuchus and the similar preservation suggested it was from the Judith River Formation. Dalman et al. (2018) claimed the preservation was the most like fossils collected from the Willow Creek Anticline of the Two Medicine Formation and they also noted that teeth that closely resembled those of Tyrannosaurus had been recovered from the same site. So apparently there was a tyrannosaurin more similar to Tyrannosaurus than Tarbosaurus running around Campanian North America, which complicates the picture of Tyrannosaurus origins. 

On a side note, I'm curious what this means for the holotype of Deinosuchus. If the preservation of CM 9401 is close enough to that of CM 963 that it can be argued they came from the same location, but also CM 9401 is probably from the Two Medicine Formation and not the Judith River Formation, does that mean that CM 963 is also probably from the Two Medicine Formation?

References

Dalman, S.G.; Lucas, S.G.; Malinzak, D.E. (2018). "Tyrannosaurid teeth from the upper Cretaceous (Campanian) Two Medicine Formation of Montana" New Mexico Museum of Natural History and Science Bulletin. 79: 125-139.

Urban, M.A. and Lamanna, M.C., 2006, Evidence of a giant tyrannosaurid (Dinosauria: Theropoda) from the Upper Cretaceous (?Campanian) of Montana: Annals of Carnegie Museum, v. 75, p. 231-235.

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[Jura]

On Tuesday, July 15, 2025 at 10:29:46 AM UTC-4 Tyler Holmes wrote:

On a side note, I'm curious what this means for the holotype of Deinosuchus. If the preservation of CM 9401 is close enough to that of CM 963 that it can be argued they came from the same location, but also CM 9401 is probably from the Two Medicine Formation and not the Judith River Formation, does that mean that CM 963 is also probably from the Two Medicine Formation?

 

Adam Cossette will have an answer for this in (hopefully) a few months. The relevant paper has gone through a lot of rigamarole with the ICZN over the past three years.

[Tyler Holmes]

Is this related to the statements in the 2020 review of Deinosuchus that the ICZN would be petitioned to change the type species from D. hatcheri to D. riograndensis (despite the former being diagnostic and the latter arguably being its junior synonym)?

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[Jura]

On Tuesday, July 15, 2025 at 2:54:15 PM UTC-4 Tyler Holmes wrote:

Is this related to the statements in the 2020 review of Deinosuchus that the ICZN would be petitioned to change the type species from D. hatcheri to D. riograndensis (despite the former being diagnostic and the latter arguably being its junior synonym)?

 Yes, it's the long overdue follow-up to this. I'll also clarify they D. hatcheri is comprised of scant, non-diagnostic material, whereas D. riograndensis is known from several, largely complete specimens.

[Tyler Holmes]

Deinosuchus hatcheri is certainly scant, but we shouldn't be throwing out perfectly good holotypes because there are more complete specimens known. To that point, the type specimen of D. riograndensis, AMNH 3073, isn't that much more complete than CM 963, and critically it can't be differentiated from CM 963. There are much more complete specimens referred to D. riograndensis, but there's nothing that keeps them from being referred to D. hatcheri. 

As far as whether or not D. hatcheri is diagnostic, Colbert and Bird (1954) don't list any features separating D. riograndensis from D. hatcheri and where there is overlapping material they note the two specimens are extremely similar. That's a problem for the validity of D. riograndensis, not D. hatcheri. This has been noticed by others. For example, Schwimmer (2002) considers Deinosuchus to be monotypic but he states that even if eastern and western Deinosuchus are separate species, "it is highly likely" that CM 963 and AMNH 3073 are the same species. Lucas et al. (2006) follows Schwimmer (2002) in recognizing only one species of Deinosuchus, but they also mention that there are no morphological differences between CM 963 and AMNH 3073. So the holotypes of D. hatcheri and D. riograndensis can't be differentiated, which seems more like a reason to consider D. riograndensis to be a junior synonym of D. hatcheri, not a reason to change the type species. 

Cossette and Brochu (2020) don't argue D. hatcheri is not diagnostic (they state it is "clearly distinct" from the other species), and they actually unintentionally made a pretty solid argument for synonymizing lumping D. riograndensis into D. hatcheri. They point out that the pelvic and vertebrae features Holland used to diagnose D. hatcheri can be found in specimens referable to both D. riograndensis (from Texas) and D. schwimmeri (from the southeastern US), but they also noted that "massive, inflated osteoderms" are found in Deinosuchus specimens from "Texas and the Western Interior". So we have very similar osteoderms from D. riograndensis and other Laramidian specimens, but none from Appalachian Deinosuchus. Later in the same paper, the osteoderms of D. hatcheri are described as "very lumpy, bear deeply pitted surfaces, and have inflated keels". The osteoderms of D. riograndensis are described as "irregular, lumpy, and have inflated keels". They note the keels of D. riograndensis osteoderms aren't as inflated as those of CM 963, but CM 963 is also about 10-20% larger than any specimens of D. riograndensis with comparable osteoderms. When it comes to the osteoderms of D. schwimmeri they note "In opposition to this condition [referring to the descriptions above], the osteoderms of D. schwimmeri preserve inflated keels, but are often thinner and more regular in shape". 

All of this to make the point that the osteoderms of Laramidian D. hatcheri and D. riograndensis are extremely similar, but those of the Appalachian D. schwimmeri are clearly different. This is also noted by Schwimmer (2010), where he argues for two species based on differences between the teeth and osteoderms. Massive, inflated osteoderms with deep pitting might feel like a thin character to base a synonymy on, but those same osteoderms are regularly considered one of the most distinctive features of the genus and we have a whole lot of them. Multiple specimens from Wyoming, Utah, and New Mexico are referred to Deinosuchus purely because of those osteoderms. If we keep D. hatcheri and D. riograndensis separate those specimens are in limbo as Deinosuchus sp. If there's only one Deinosuchus species in Laramidia (which most workers seem to consider to be the case), than D. hatcheri is the oldest name used for the taxon and is clearly diagnostic. It also ends up being pretty complete when all is said and done. 

I'll wait for the paper to see what arguments get made, but the 2020 paper's arguments for change the type species were pretty thin.

References 

Colbert, E.H., Bird, R.T. 1954. "A gigantic crocodile from the Upper Cretaceous beds of Texas" American Museum Novitates. 1688: 1–22. 

Cossette A.P., Brochu C. 2020. "A systematic review of the giant alligatoroid Deinosuchus from the Campanian of North America and its implications for the relationships at the root of Crocodylia", Journal of Vertebrate Paleontology, DOI: 10.1080/02724634.2020.1767638

Lucas, S.G., Sullivan, R.M., Spielman, J.A. 2006. "The Giant Crocodylian Deinosuchus from the Upper Cretaceous of the San Juan Basin, New Mexico." Lucas, S.G., Sullivan, R.M. eds. New Mexico Museum of Natural History and Science Bulletin. 35: 245-248

Schwimmer, D.R. 2002. King of the Crocodylians: The Paleobiology of Deinosuchus. Indiana University Press ISBN 978-0-253-34087-0.

Schwimmer, D.R. 2010. "One or two species of the giant crocodylian Deinosuchus". Journal of Vertebrate Paleontology. 30(Supplement 2): 1A–198A.