Plausibility/Validity of a Hybrid Speciation hypothesis for the direct evolutionary origin of the Tyrannosaurid Tyrannosaurus rex

[Joseph Fossil]

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?

[Joseph Fossil]

@Troodon @Kane @dinosaur man What do you guys think?

[Kane]

I’m not focused on dinosaurs.

[Troodon]

I don't study the evolution of tyrannosaurids so it's difficult to comment on your question with an argument that is based on science.  If Paleontologists who do study tyrannosaurids cannot agree whether T rex orgins are Asian or Laramidian anything said is pure speculation.  I like the Warshaw, Fowler paper but they made no conclusive statement on Trex's origin.  We just dont know.

[FossilDAWG]

Hybridization has been shown to play a role in speciation in Darwin's Finches and some other modern taxa.  In the Darwin's Finch example the evidence consists of decades of detailed records establishing mating pairs, progeny, genealogies, detailed records of morphology of all the animals in the hybrid lineages, and DNA genotyping.  I recommend reading "The beak of the Finch" for a very approachable description of that research.  Given the extreme granularity of the fossil record this level of evidence could never be achieved.  Think of the data you'd get from watching and measuring every single individual in a population for 40 years (which is what Peter and Rosemary Grant and their students did) in comparison to a situation where you are comparing a few bones, or rarely an articulated skeleton, from individuals spaced hundreds of generations apart at best.  For the fossil species it would be impossible to distinguish hybridization from evolution within a species. 

 

Don

[jpc]

and just for future reference, the r in rex is not capitalized.  Only the genus name is capitalized, not the species name.  Big T, little r.

 

 

[FossilDAWG]

While we're on the subject, the plural of genus is genera, not genuses.

 

Don

[jpc]

3 hours ago, FossilDAWG said:

While we're on the subject, the plural of genus is genera, not genuses.

 

Don

Oooh, I missed that one.  

[DPS Ammonite]

4 hours ago, jpc said:

and just for future reference, the r in rex is not capitalized.  Only the genus name is capitalized, not the species name.  Big T, little r.

 

 

I also wish everyone would spell out or abbreviate correctly in two parts: T. rex. Trex sounds like a composite decking product to most Hsapiens.

[Francesco1994]

I imagine that it is practically impossible to establish the genealogy with precision and to study at an evolutionary level species that have been extinct for millions of years without the aid of DNA, therefore relying mainly on morphological aspects.

[Joseph Fossil]

On 2/3/2023 at 11:44 AM, Troodon said:

I don't study the evolution of tyrannosaurids so it's difficult to comment on your question with an argument that is based on science.  If Paleontologists who do study tyrannosaurids cannot agree whether T rex orgins are Asian or Laramidian anything said is pure speculation.  I like the Warshaw, Fowler paper but they made no conclusive statement on Trex's origin.  We just dont know.

 

@Troodon I agree and I appreciate the input. I'll also admit this is a hypothesis that is difficult to prove, but definitely possible and worthy of debate.

[Kane]

26 minutes ago, Joseph Fossil said:

 

@Troodon I agree and I appreciate the input. I'll also admit this is a hypothesis that is difficult to prove, but definitely possible and worthy of debate.

Every hypothesis would be difficult to prove because hypotheses by their nature cannot be proven.