The Pipe Dream of Nanotyrannus Reaches New Heights: Comments on Longrich & Saitta 2024

Something of a New Year’s tradition has developed in paleontology. In 2023 and 2024, the start of the new year coincided with a study published regarding Tyrannosaurus rex garnering plenty of media attention. I guess T. rex can’t resist starting the year with a bang!

While last year’s study examined the intelligence of T. rex, this year’s paper focuses on the taxonomic status of Nanotyrannus lancensis and its relation to T. rex. For those blissfully unaware, the issue boils down to a handful of mid-sized Tyrannosaurid specimens from North America that date back to the end-Cretaceous. The small size and differing morphology of these individuals have led some to proclaim them to be a unique genus – Nanotyrannus – while other paleontologists cite the immature age of each specimen as evidence that they were juveniles of Tyrannosaurus rex. Both sides have valid arguments, but the prevailing view is that these individuals were immature T. rex. (For a more thorough examination of Nanotyrannus, follow the following link!)

Many studies have tried to settle the debate, but all have failed. It feels as though the latest study, authored by Nicholas Longrich and Evan Saitta, is doomed to follow the same path as all Nanotyrannus-related works published prior.

Released on January 3^rd, the study examines a wide assortment of evidence to prove that Nanotyrannus was a separate genus from Tyrannosaurus rex. First, the authors explained ecosystems in Cretaceous Laurasia (North America and Asia) typically featured two or more Tyrannosaurids, making the presence of two genera in Hell Creek a logical conclusion. Next, Longrich and Saitta listed 158 features that differ between specimens assigned to Tyrannosaurus and Nanotyrannus, with 44 of these traits being deemed unlikely to change between adults and juveniles using fossils of other Tyrannosaurids[i]. Combining these traits into a cluster analysis found two distinct morphotypes – one proposed to be Nanotyrannus and the other Tyrannosaurus rex.

That’s not all. The study also disputes previous notions that the known Nanotyrannus specimens were juveniles, instead using their bone histology and growth patterns to assert they were sub-adults. Next, Longrich and Saitta point towards a lack of any fossils showing an intermediate between Nanotyrannus and Tyrannosaurus as evidence of two different morphotypes, thus two genera. Lastly, a new specimen of a juvenile Tyrannosaurus, which the authors claim is younger than any Nanotyrannus fossil and has a structure more akin to an adult T. rex, is used to demonstrate juvenile T. rex and Nanotyrannus showed differences in their anatomy. Combining these factors led Longrich and Saitta to surmise that Nanotyrannus was a distinct genus.

If it sounds like a lot, it’s because it is. The paper is 65 pages, to be exact.

Look, the conclusions made by the study are ok. Even the staunchest supporter of Nanotyrannus being a juvenile Tyrannosaurus will admit that the proposed changes in appearance and structure from teenager to adult are extreme and don’t align perfectly with what we might expect. The comparison of bone histology and growth rate between older Tyrannosaurus individuals and those assigned to Nanotyrannus is the best evidence presented, but this alone is not enough to prove their hypothesis.

Other pieces of evidence are less intuitive. The notion that there must have been a second Tyrannosaurid in Hell Creek because we see this pattern elsewhere doesn’t consider that this trend may not be applicable in all ecosystems. It also doesn’t account for the fact that other dinosaur families, such as Ceratopsians and Hadrosaurids, were far less diverse in Hell Creek than in locations like the Dinosaur Park and Judith River Formations. For whatever reason, dinosaur life at the end of the Cretaceous in North America was less diverse than 10 million years prior, making any assumptions about dinosaur diversity at this time unfounded.

The reliance on inferring ontogenetic changes in Tyrannosaurus from contemporary Tyrannosaurids seems like a stretch. There may have been similarities between the growth patterns of the closely related Tyrannosaurus and Tarbosaurus, but it’s important to remember that these were still distinct animals and thus may have had very different life histories from one another. Basing conclusions of one species on the evolutionary history of another is simply insufficient.

Another issue I had was the lack of a Tyrannosaurus-Nanotyrannus intermediate being used as evidence for the two being separate genera. Could this support the author’s hypothesis? Possibly. But it’s accepted by most that in any scientific field, the absence of evidence is not evidence of absence, and in paleontology, this is even more true. The fossil record is a cold and unforgiving lover who conceals more secrets than paleontologists could ever imagine, which means imperfections in what it reveals and preserves cannot be used to base an argument. The fact that an intermediate is not currently known to paleontologists does not mean it didn’t exist, which makes this point null.

My final issue is the description of the new juvenile material. The specimen UCMP V84133 consists of a tiny proportion of the skull (specifically the frontal bone) – and that’s it. I’m not saying this specimen is unimportant, but I think it’s overly confident to say that it was a Tyrannosaurus based on a sliver of bone alone. We can revise this theory if a more complete Tyrannosaurus juvenile is discovered and exhibits similar anatomy to UCMP V84133.

I am not very knowledgeable in the discourse surrounding Longrich. I will say that his last major study was publishing Vectidromeus, a juvenile ornithopod described from mere fragments of bone that lived alongside another, larger genus of ornithopod known from adult specimens. Sound familiar? 

If you would like a more intuitive review of the paper courtesy of an acclaimed paleontologist, I recommend reading the following analysis by Thomas Holtz. It’s safe to say he has far more experience with Tyrannosaurids and other theropods than I do!

It’s inaccurate to say that this study settles the Nanotyrannus debate. Longrich and Saitta’s study is an excellent review of all evidence regarding Nanotyrannus being a distinct genus but offers little new information. Don’t get me wrong, reviews like this are undoubtedly important, helping to provide crucial communication of known research to the public and fellow paleontologists alike. However, I suspect the paper does not present any information that could persuade people in paleontology to change their opinion.

Therein lies the issue with Nanotyrannus in the present. It doesn’t matter what study is released; members of the paleo community seem to have picked their side and are unlikely to change their opinion from anecdotal evidence alone. For things to change, a new specimen must show a fully mature Nanotyrannus or a distinct juvenile Tyrannosaurus. Since neither fossil exists, analyzing studies like this in meticulous detail should be reserved until new evidence emerges.

It doesn’t help that it feels like the status of Nanotyrannus is irrelevant. While I understand that the status has implications for Tyrannosaur diversity at the end-Cretaceous and Tyrannosaurid growth patterns, at the end of the day, paleontologists are arguing about the same dinosaur. It doesn’t matter if we call it Tyrannosaurus or Nanotyrannus; in life, it was a mid-sized animal that occupied the role of mid-sized predator in its ecosystems. It was faster than an adult T. rex and likely hunted smaller prey, like Acheroraptor, Thescelosaurus, and Pachycephalosaurus. Whether you call it Nanotyrannus or Tyrannosaurus rex is unimportant in my opinion because the bottom line is that it was an immature Tyrannosaurid and would have been an awesome sight in the Cretaceous of North America regardless!

After all, most people don’t pick up on the nuance if the duck in front of them is a mallard or a mandarin; all that matters is that it’s a duck!

Thank you for reading this article! I am not going to lie; this study almost drove me crazy. The length of the paper alone is absurd, but at the end of the day, it isn’t a subject I’m too invested in one way or the other (though I do side on the juvenile rex side of things). If you want to hear about another ongoing and controversial paleontological saga (that I follow religiously), I suggest you read my various articles about Spinosaurus here at Max’s Blogosaurus!

I do not take credit for any images found in this article. All images come courtesy of the authors noted below each image.

Header image courtesy of Max’s blogosaurus (for once!), taken at the Quinte West Museum of Natural History.

Works Cited:

---

[i] Longrich, Nicholas R., and Evan T. Saitta. “Taxonomic Status of Nanotyrannus Lancensis (Dinosauria: Tyrannosauroidea)—A Distinct Taxon of Small-Bodied Tyrannosaur.” Fossil Studies, vol. 2, no. 1, MDPI AG, Jan. 2024, pp. 1–65. Crossref, https://doi.org/10.3390/fossils2010001.