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Dinosaurs The Mesozoic Mailbag

The Tyrannosaurus Trifecta: Questions Regarding T-rex’s Species Split

A controversial new study has proposed that Tyrannosaurus comprised three different species. As always, paleontologists weren’t too excited.

If the thought of one Tyrannosaurus species is frightening, just imagine three living together.

Almost too terrifying to be true.

On March 1st, paleontologist Gregory S. Paul published a controversial article proposing the split of Tyrannosaurus into three separate species. For those familiar with his work, this came as no surprise. My (2018) edition of Paul’s “The Princeton Field Guide to Dinosaurs” alludes to two “unnamed Tyrannosaurus” species, clearly hinting at the subject of his future work. Four years later, Paul has followed through on his promise.

Courtesy of Julius Csotonyi

The first of Paul’s species, Tyrannosaurus rex, is the one I am sure we are all familiar with. In the ever-evolving field of paleontology, the status of Tyrannosaurus rex is likely to never be questioned…I hope. Paul’s other two species have been dubbed Tyrannosaurus imperator and T. regina, Latin for “emperor” and “queen.” King, queen, and emperor; not bad, but wouldn’t it have made more sense to be king, queen and jack? Or perhaps prince?

In his study, Paul justifies his speciation by comparing tooth structure and physical robustness (bulkiness) between Tyrannosaurus specimens.

While some specimens are stout and have dual incisor teeth common to primitive tyrannosaurids, others are lithe and only have one of these incisors. Paul proposes that over the approximate 3-million-year reign of Tyrannosaurus, the initial two-incisor species (T. imperator) diversified into the two subsequent species (T. rex and T. regina). Some overlap may have occurred between the species, potentially resulting in multiple species of Tyrannosaurus living together. No wonder Triceratops got such big horns…

It should be no surprise that the study wasn’t received well within the paleontological community. Renowned paleontologists like Thomas Holtz at the University of Maryland and tyrannosaurid specialist Thomas Carr were highly skeptical. Many pointed out that individual differences and changes in Tyrannosaurus’ physiology over its lifespan could lead to variations between specimens. While Paul’s study stirred up controversy, most paleontologists have had trouble accepting its conclusion.

Courtesy of Corbin Rainbolt

While I believe that Tyrannosaurus may have undergone speciation, Paul’s reasoning isn’t convincing. The variation in physical stoutness can easily be chalked up to individual differences. Different incisor patterns may demonstrate speciation, but this argument alone cannot be considered conclusive. When Allosaurus jimmadseni, a new species of Allosaurus, was proposed in early 2020, the researchers cited numerous differences in skull anatomy to justify their research. Compare this to Paul’s single difference in dental structure, and the argument is thoroughly incomplete.

The study also ignores the topic of Tyrannosaurus’ other potential species: Nanotyrannus and Tarbosaurus. Nanotyrannus, the mid-sized tyrannosaurid that may or may not have been a juvenile Tyrannosaurus, is never mentioned under any of Paul’s species. Does it represent a juvenile T. rex? A juvenile imperator? Or does Paul consider it to be its own species? On the other hand, some paleontologists have theorized that the Asian tyrannosaurid Tarbosaurus may also represent another Tyrannosaurus species. Should it be lumped in with the primitive T. imperator? Is it still its own genus? Without thorough examination, these questions remain open for discussion.

Part of the issue is that naming species within paleontology is inconsistent. Some paleontologists, like Jack Horner, believe there are too many species and that some should be eliminated and grouped together. Paleontologists like Paul lie on the opposite end of the spectrum, making consensus on how to approach species within the field challenging.

© N. Tamura

While some genera of dinosaurs have multiple species, most have only one. The Asian ceratopsian Psittacosaurus has over 10 species based on minute differences, yet the suggestion of two Tyrannosaurus species is enough to drive paleontologists into a frenzy. Not exactly fair…

At the end of the day, dinosaur speciation is messy. Splitting species or genera is never an easy task, especially when it pertains to one of the most iconic dinosaurs of all. While Paul’s conclusion that multiple Tyrannosaurus species may have existed is probably fair, his evidence isn’t enough to support it. Future studies may be more in-depth, but for now, Tyrannosaurus regina and Tyrannosaurus imperator are merely unlikely propositions.

But jeez, those are cool names. Maybe we could make a commemorative plaque or something?

“R.I.P T. regina and T. imperator, the tyrant dinosaurs that maybe never were”

Courtesy of John Conway

Works Cited:

I do not take credit for any images found in this article.

Header image of Tyrannosaurus found here

Multiple Tyrannosaurus courtesy of Julius Csotonyi, found here

Tyrannosaurus family vs an intruder courtesy of Corbin Rainbolt, found at their twitter here

Psittacosaurus courtest of Nobu Tamura, found at their website here

Tyrannosaurus rex at sundown courtesy of John Conway, found here

Chure, Daniel J., and Mark A. Loewen. “Cranial Anatomy of Allosaurus Jimmadseni, a New Species from the Lower Part of the Morrison Formation (Upper Jurassic) of Western North America.” PeerJ, vol. 8, 2020, https://doi.org/10.7717/peerj.7803.

Paul, Gregory S., et al. “The Tyrant Lizard King, Queen and Emperor: Multiple Lines of Morphological and Stratigraphic Evidence Support Subtle Evolution and Probable Speciation within the North American Genus Tyrannosaurus.” Evolutionary Biology, 2022, https://doi.org/10.1007/s11692-022-09561-5

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