Hello there. A few days ago, I read an interesting article stating that Tarbosaurus and/or Zhuchengtyrannus crossed a land-bridge from Asia to North America and evolved into the Tyrant King of the Dinosaurs, Tyrannosaurus rex (link: https://www.livescience.com/23868-tyrannosaurus-rex-facts.html). Today, I am going to share my thoughts on this theory, and evidence that both supports and opposes it. Let's start with the supporting evidence.
T.rex is strikingly similar to T.bataar and Z.magnus. All three are similar in size and shape, with Tarbosaurus originally being named Tyrannosaurus bataar. In addition, a dorsal spine from Zhuchengtyrannus was similar in size to one from Sue, the largest and most complete Tyrannosaurus specimen.
However, there is evidence that goes against this theory. For instance, T.bataar and T.rex have several difference in the structures of their skulls.
Notice the differences in the shapes of the skulls of T.bataar (left) and T.rex (right). Tarbosaurus has a more slender and narrow skull, while Tyrannosaurus has a wider and more robust skull. This, however, is not the only evidence that stands in the way of the theory.
Here's another example...
Daspletosaurus is often looked at as the evolutionary forerunner to Tyrannosaurus rex. Unlike its contemporary cousins Albertosaurus sarcophagus and Gorgosaurus libratus, which were more lightly built, for speed, Daspletosaurus was more bulkier and heavily built, similar to Tyrannosaurus. Here is a size comparison between Daspletosaurus and T.rex:
Note that D.torosus is closer to T.rex in size than G.libratus or A.sarcophagus. Daspletosaurus, however, is not the only Tyrannosaur that disproves the theory that Tarbosaurus/Zhuchengtyrannus evolved into Tyrannosaurus...
Enter Lythronax argestes.
Lythronax was a genus of coelurosaurian theropod dinosaur from Late Cretaceous Utah, living about 80 million years ago, alongside the ceratopsid Diabloceratops. Here's a size comparison between L.argestes and D.torosus:
Despite being smaller than Daspletosaurus, Lythronax is very similar in bone structure to its larger descendant. Yes, I said descendant. I think that L.argestes is the ancestor of D.torosus due to their similar builds. The two were separated by five-million years, which was likely enough time for Lythronax to evolve into Daspletosaurus. Now, let's compare Lythronax to Tyrannosaurus.
Despite being dwarfed by its much-larger relative, L.argestes and T.rex are similar in structure. This hints at a possible ancestry from Lythronax to Tyrannosaurus.
Personally, I disagree with the theory that T.bataar/Z.magnus crossed a land-bridge from Asia to North America and evolved into T.rex. I think that Lythronax argestes evolved into Daspletosaurus horneri/torosus, which in-turn gave rise to the Tyrant King of the Dinosaurs, Tyrannosaurus rex. But hey, those are just my thoughts on this theory. But feel free to leave your take on this theory in the comments below!
"Part of the journey is the end..."
^Yeah I was thinking the same.
"Part of the journey is the end..."
DRACONUS - What a compelling topic! I happen to believe you may be on to something interesting. However, could Tyrannosaurus rex not be the product of interspecies breeding? What if Daspletosaurus and Tarbosaurus met via land bridge and sired offspring together? That could potentially have been the root to the creation and evolution of Tyrannosaurus rex. This was a very interesting topic to ponder! Thank you ever so much for taking the time to share this with us! :)
@Draconus Tyrannus, I too disagree with the land-bridge anagenesis theory. While I don't disagree that creatures did migrate between both continents when the land bridge was not under water, the similarities and differences between all five tyrannosaurs you analyzed are too significant for me to overlook.
@Something Real, that's..uh...a creative hypothesis, which I admire and am intrigued, but I will poke holes through it.
Firstly, since Tarbosaurus and Daspletosaurus are two different genera, the chances that they would be able to fertilize the female's eggs, let alone produce VIABLE offspring, is remote, due to a significant difference in their genetics and chromosomes. Contemporary species of the same genus can produce offspring, but their offpsring are either entirely sterile (all mules), or only sterile in the male individuals (ligers and tigons). If they can barely produce viable offspring, what are the chances different genera could do the same?
Secondly, there would have to be several Daspletosaurus-Tarbosaurus couplings to occur for a viable hybrid to hatch, grow, reach sexual maturity, breed, then sire viable offspring of their own. I doubt most Tarbosaurus crossed the land bridge to sire offspring to produce a thriving population that could have driven other tyrannosaurs to extinction.
Thirdly, it is almost certain both genera had different courtship strategies. I hypothesize that each genus and species had distinct rituals to court mates, and they would not sway the other of a different genus. If one individual didn't want to breed with a member of the other genus, s/he would not have bred, since both males and females were equally capable of inflicting serious damage, regardless of genus. It wouldn't be worth mating with someone who you didn't consider a worthy mate. It would have been better to mate with your own kind as a fail-safe method to ensure your genes were definitely passed on.
ALPHADINO65 - That is extremely compelling information! I can certainly understand and respect your view on the matter. The fact that you are intrigued by the slim possibility of T.rex being the product of prehistoric hybridization is more than enough for me to be pleased! I have always enjoyed hypothesizing about how something can happen rather how it can not happen. It makes things more interesting - and fun! I look at it from the stand point of lions and tigers. They can breed - but their offspring are not viable (as you so very wisely observed in your original response). What if, through some fluke within Mother Nature's vast scheme, Tyrannosaurus rex is the product of two dinosaurs - completely alien to one-another - brought together by the unstoppable shift of continents? What if genetics as we know them were slightly more malleable before the vast extinctions that reset the evolutionary ladder? Imagine how amazing that would be! Think of how such a revelation would change our understanding of the greatest predators our world has yet known! Would that not make you smile with the delight of discovery and admiration for life's strange miracles? It would certainly have that effect on me! :)
^Well then that would mean Tyrannosaurus rex is the OG hybrid dinosaur, move over Indominus!
Jk, in all honesty, I do agree with Alphadino65.
"Part of the journey is the end..."
Godzilla Generations was a beautiful game! I loved the fact that there's a laser cannon inside of Dr. Serizawa's eyepatch.
Hi Draconus, It's really important that you understand the phylogeny of tyrannosaurs. The major tyrannosauroid phylogenies published over the past year and a half show that the closest relative of T. rex is T. bataar, Daspletosaurus is a distantly related lineage of T. rex (remote from the ancestry of T. rex), and Lythronax is a basal tyrannosaurine, also nowhere near the ancestry of T. rex. An argument based on overall similarity, which you have done here, no longer makes the grade in terms of producing a rigorous and defensible hypothesis of relationships. Include the science!
@Draconus Tyrannus, that very surprising, because why else would you be on this forum?
I should have made myself clearer in my previous comment. I disagree in the land-bridge anagenesis theory involving Tarbosaurus itself. It would have been THE COMMON ANCESTOR of both Tyrannosaurus and Tarbosaurus that crossed the land bridge. In that case, I fail to reject the land bridge CLADOGENESIS theory.
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