Paper I’ve been working on for a while

[u/MeatElegant1968]

The past

Because the fossil record is necessarily incomplete, we do not have a complete evolutionary path for most dinosaurs past the Triassic period. 

Few extinct animals have captured the public imagination to the same extent as Tyrannosaurus rex.  And this lack of an evolutionary footprint is especially the case for Tyrannosaurus rex, despite more than a century of research. The evolutionary history of this iconic predator continues to be refined as new fossil discoveries reshape our understanding of Tyrannosaurid evolution. Recent discoveries, including Tyrannosaurus mcraeensis and Khankhuuluu mongoliensis, have prompted renewed discussion regarding the geographic origins and diversification of the genus Tyrannosaurus.

 

The following paper presents a speculative evolutionary hypothesis that combines currently accepted paleontological evidence with original interpretations of tyrannosaur biogeography. Established scientific findings are distinguished from proposed scenarios where direct fossil evidence remains incomplete.

The idea that basal Tyrannosauroids crossed over from the Laurasian region to Laramidia when the seas shrank is actually strongly supported by biogeographical modeling. Here is exactly how the shrinking seaways and land bridges created the timeline.

The First Influx from Asia (Around 80–75 Mya)

In the Late Cretaceous, the Western Interior Seaway was vast. However, global cooling events caused sea levels to drop periodically, exposing the Bering Land Bridge between Siberia and Alaska. Primitive, slender-skulled Tyrannosauroids crossed from Asia into North America. Once in Laramidia, they evolved into the "flesh-slicing" predators, such as Daspletosaurus and the newly discovered T. mcraeensis.

The "Carcharodontosaurid-like" Phase

These animals possessed "Carcharodontosaurid-like" teeth. Because their primary prey in Laramidia consisted of soft-skinned duck-billed dinosaurs and early ceratopsians, they did not require heavy bone-crushing mechanics. Instead, their jaws and laterally compressed teeth were optimized for slicing flesh, much like the Carcharodontosauridae that had dominated millions of years earlier.

The Trait Divergence (Around 70 Mya)

As the Western Interior Seaway continued to fluctuate and recede, the Bering Land Bridge was exposed once again. At this point, the tyrannosaur lineage may have diverged geographically.

Westward Migration (Asia): 

An offshoot of the early North American Tyrannosaurus lineage migrated back across the land bridge into Asia. There, it encountered massive, slim, long-necked sauropods such as Nemegtosaurus. This lineage may have become an animal turning out to eventually evolve into Tarbosaurus bataar. Because these predators specialized in feeding on giant sauropods, they retained a somewhat more streamlined skull and a specialized locking lower jaw. Again, similar to the Carcharadontosaurid, Acrocanthosaurus, the “High Spined Lizard.”

Home Evolution (Laramidia): 

The population that remained in North America experienced a different evolutionary arms race. Ceratopsians evolved into heavily armored forms such as Triceratops, while Ankylosaurus and other related saurischians appeared with extensive body armor. To overcome these defenses, the Laramidian lineage gradually abandoned flesh-slicing specialization in favor of tremendous bite force. Over time, they evolved a massive, heavily fused skull and thick, rounded teeth, even gaining extra lumps on the skull for muscle attachment points to contribute to its massive bite force. This, eventually culminating in Tyrannosaurus rex approximately 68 million years ago.

This model suggests that Tyrannosaur evolution involved a dynamic, back-and-forth pattern of migration driven by constantly changing sea levels.

 

This interpretation contrasts with both my earlier hypothesis and several previously proposed theories regarding the evolution of Tyrannosaurus rex and Tarbosaurus bataar. One long-standing hypothesis proposed that basal tyrannosauroids migrated into North America much earlier, during the Middle to Late Jurassic. Under this model, early genera such as Guanlong dispersed into North America and eventually evolved into animals such as Daspletosaurus, which later gave rise to Tyrannosaurus rex.

A later theory suggested that Tarbosaurus had migrated across the Bering Land Bridge into Laramidia, eventually evolving into Tyrannosaurus mcraeensis, which in turn gave rise to Tyrannosaurus rex. However, this hypothesis appears unlikely because the fossil record suggests that both Daspletosaurus and T. mcraeensis became extinct before T. rex began to emerge. There is also much evidence to suggest that Tarbosaurus lived along side Tyrannosaurus rex.

A more plausible alternative is that T. rex evolved from the recently described tyrannosauroid, Khankhuuluu mongoliensis. By the time this lineage arrived in North America, ecological niches for hunting more manageable prey may already have been occupied by smaller tyrannosaurids  and other therapods approximately the size of a large horse. As a result, Khankhuuluu may have gradually evolved a more robust body, bone-crushing teeth, and ocular osteoderms, possibly used for display. In this scenario, it would represent a smaller, more slender precursor to T. rex.

 

One branch of this population could subsequently have dispersed back into Asia after evolving a more robust body plan, eventually resembling Zhuchengtyrannus, albeit with several important biological differences. (It should be noted that this portion of the hypothesis is highly speculative.) Meanwhile, selective pressures created by long-necked sauropods such as Nemegtosaurus may have favored a return to a more streamlined skull in the Asian lineage. In contrast, the population that remained in Laramidia continued adapting to increasingly armored Late Cretaceous herbivores, gradually evolving stronger teeth, a more powerful bite, and a heavier overall build. If this hypothesis is correct, Tyrannosaurus rex would share an evolutionary history with Asian Tyrannosaurids while coexisting with larger relatives such as Tarbosaurus bataar.

The End-Cretaceous Extinction and Tyrannosaur Legacy

The end-Cretaceous mass extinction eliminated nearly all non-avian dinosaurs, pterosaurs, and numerous marine reptiles. Today, the surviving descendants of that world include birds, which evolved from Maniraptoran theropods, and crocodilians, which represent the last surviving members of Crocodylomorpha.

Although Tyrannosaurus rex itself vanished during this event, its broader evolutionary lineage remains an important part of dinosaur history. Tyrannosaurs and Megaraptorans both belong within Therapoda, sharing a common ancestor that lived during the Middle to Late Jurassic, approximately 170 million years ago, called Coelosaur. Likewise, birds also descend from coelurosaurian ancestors, making them very distant relatives of tyrannosaurs.

This relationship explains why certain skeletal features of modern chickens, particularly the pelvis and pubis, bear a surprising resemblance to those of Tyrannosaurus rex. While separated by tens of millions of years of evolution, these shared anatomical characteristics illustrate the remarkable continuity of theropod evolution.

Conclusion

Tyrannosaurus rex remains one of the most extraordinary terrestrial predators ever to have lived. Its immense popularity reflects not only its size and power but also the wealth of scientific knowledge it has generated. Few prehistoric animals have inspired as much research or public fascination.

However, appreciating T. rex requires more than admiration for the animal itself. Understanding its evolutionary origins, the ecosystems it inhabited, the plants and animals with which it coexisted, and the environmental pressures that shaped its anatomy provides a far richer picture of its significance. Continued study of its ancestry and its changing world will undoubtedly improve our understanding of tyrannosaur evolution and of Late Cretaceous ecosystems as a whole.

Although many aspects of the evolutionary scenario presented here remain speculative, they demonstrate how new fossil discoveries and biogeographical evidence can inspire fresh hypotheses. Future discoveries may support, modify, or refute these ideas, but each new specimen brings paleontologists one step closer to reconstructing the remarkable evolutionary history of the "Tyrant Lizard King."

this is my first post btw so I hope you guys like it and make sure to correct me for any errors

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[u/magcargoman]

Look up how citations work to give your writing support.

[u/Brontozaurus → u/magcargoman]

Seconded. This needs sources, especially since so much of it is speculative. What is informing the ideas here?

[u/Ex_Snagem_Wes → u/Brontozaurus]

Especially since much of it is ignoring phylogeny of Tyrannosauria

[u/SquiffyRae → u/Brontozaurus]

And also the fact in a reply to another comment they admit there's a chance they didn't see a paper that likely destroys their hypothesis

That's always a sign that someone hasn't read widely enough to begin making hypotheses because it's likely based on a limited knowledge of existing research. As you say though, without sources there's no way of knowing what they have or haven't read.

It's the same reason why review papers are usually offered to senior researchers because they'll know what the current state of knowledge in their domain is and where future research is needed or could be directed. It's often harder to write a good review paper than it is to write up original research

[u/Powerful_Gas_7833]

All depends on the age of the nemegt formation 

I remember an abstract 25 years ago that essentially said it was magnetostrat dated to 75 to 70 million years ago. Obviously it was an abstract not peer-reviewed 25 years ago and nothing has come out of it since. 

But a year or so ago they released a paper where they found eggshells can be used as geochronometers. In that paper a site in eastern Mongolia was dated to 75 million years ago. More importantly the locality was regarded as coeval or temporally equivalent to the baruun goyot formation. This formation is interfingered with and thus temporally equivalent to the nemegt formation. This might lend Credence to the idea that the nemegt formation is actually late Campanian in age. Which means your whole speculative hypothesis falls apart in a factual sense because it would mean turbo is a lot older 

But then I've talked to other researchers of the Gobi desert and they have disparaged the section in the paper that made the connection which means the nemegt formation might be late Maastrichtian in age?

So....I don't even know what the fuck to think the nemegt formation is a pain in the ass

[u/MeatElegant1968 → u/Powerful_Gas_7833]

It’s funny how many things could be totally dismantled by a million or so date change. Also keep in mind I wrote this a year or so ago so it might have been before this was published or maybe it was around the same time and I just didn’t see it😅 thanks for letting me know though I’ll look into it

[u/Waste_Translator_975]

Slop