Thread by @JGN_Paleo

Is Tyrannosaurus rex the one true king of the Cretaceous? Our new paper reviews the evidence🧵
link.springer.com/article/10.100…

#paleontology #dinosaur #fossil #scicomm #museum #AcademicTwitter
(art by @MarkWitton, who graciously gave us permission to use his work for this project)

1/16

In March, a team of researchers published a provocative new idea: Tyrannosaurus rex, the most famous dinosaur, wasn't one dinosaur at all, but three separate species that were so similar all prior researchers had missed their differences.
link.springer.com/article/10.100…

2/16

The two new species were named Tyrannosaurus imperator (the tyrant lizard emperor*) and Tyrannosaurus regina (the tyrant lizard queen).

*Imperator is actually Latin for "commander", but was a title given to the Roman emperors, and became the origin of our word "emperor"

3/16

Species delimitation is challenging even in living animals. With only bones to study, it's even harder for paleontologists. So it's very possible (and in my opinion, very likely) that some dinosaurs need to be split into multiple species. This proposal is plausible.

4/16

But plausibility isn't enough - there needs to be evidence for multiple species. Paul et al. (2022) presented two key traits that they claimed indicated that T. rex is three species: femur robusticity, and the number of incisiform dentary teeth.

5/16

These lines of evidence seemed quite weak to many readers of the paper. @TyrannosaurCarr, @SteveBrusatte, @TomHoltzPaleo, @Dave_Hone, Thomas Williamson, @ExpeditionLive, and myself set out to test whether the data held up, and thus if a three-species model was supported.

6/16

First, we considered femur robusticity - the "thickness" of the thighbone. Paul et al. (2022) claimed that T. rex had an unusually high variation in this trait, but only compared it to a handful of other theropod dinosaurs - a small comparative sample!

7/16

Using a huge published dataset of measurements of modern birds , we show that T. rex (vertical line) actually has a typical range of variation for a theropod dinosaur. It is more variable than the other extinct theropods, but that's because there are more specimens.

8/16

Paul et al. (2022) used two statistical tests, K-means and fuzzy C-means clustering, to show that femur robusticity divided specimens into two discrete groups. But both tests were inappropriate for the question at hand, because both *assume* a number of clusters in advance!

9/16

These tests are useful to classify observations into a known number of groups. But here, they provide artificial support for the hypothesis being tested. We used hierarchical clustering to determine how many clusters *really* existed in the data...

10/16

And unequivocally, the answer was *one* (c). Furthermore, our hierarchical clustering analysis didn't even find 'T. imperator' and 'T. regina' as sub-clusters. Specimens of each were mixed throughout the resulting dendrogram (a) - evidence against them being real species.

11/16

There were also complications in considering the number of incisiform dentary teeth. Our own measurements didn't match those of Paul et al. (2022), and clustering again failed to recover the three "species" proposed (b and d) or more than one cluster within their own data.

12/16

To summarize - the evidence for 'T. imperator' and 'T. regina' falls apart under scrutiny, and most of it was based upon the use of improper statistical tests to answer the question at hand. This proposal remains possible, but the evidence in favor of it is insufficient.

13/16

What's the verdict?

There is no evidence supporting the division of Tyrannosaurus rex into three separate species.

In the late Cretaceous of North America, Tyrannosaurus rex was the one and only king.

14/16

Deepest thanks to my excellent coauthors - working on this paper was an absolute pleasure. Special thanks to @TyrannosaurCarr for graciously inviting me on board, and welcoming me to co-lead the paper with him!

15/16

Coincidentally, this paper dropped during my travel into the field; I am joining @ExpeditionLive's crew in the Mussentuchit Member of the Cedar Mountain Formation! I will be unable to respond to questions about this paper until I return on August 10th.

16/16