Splitting T. Rex into 3 species becomes a dinosaur royal rumble
By Jack Tamisiea
The world’s most iconic dinosaur is undergoing an identity crisis.
In February, a team of scientists posited that Tyrannosaurus rex was actually three distinct species. Instead of there being only one sovereign “tyrant lizard king,” their paper made the case for a royal family of supersized predators. Joining the king in the genus Tyrannosaurus would be the bulkier and older emperor, T. imperator, and the slimmer queen, T. regina.
The proposed T. rex reclassification struck the paleontology community like an asteroid, igniting passionate debates. On July 25, another team of paleontologists published the first peer-reviewed counterattack.
“The evidence was not convincing and had to be responded to because T. rex research goes well beyond science and into the public sphere,” said Thomas Carr, a paleontologist at Carthage College in Wisconsin and an author of the new rebuttal. “It would have been unreasonable to leave the public thinking that the multiple species hypothesis was fact.”
The earlier team of researchers anticipated the rebuttal, which was published in the journal Evolutionary Biology. Gregory Paul, one of the authors of the original study, is working on another paper and says many of the rebuttal’s claims are outlandish.
“I don’t like flat-Earthism because the evidence is against it,” said Paul, who is an independent researcher and influential paleoartist. “It’s the same here: the evidence indicates very strongly that there are multiple species.”
This king-size taxonomic debate seems destined to rage for epochs. Which is unsurprising considering how difficult it is for researchers to differentiate prehistoric species. Without dino DNA, the lines between one fossil species and another are messy. So paleontologists measure different traits, like the size and shape of a particular bone. However, the fossils can be misleading, as spending eons entombed underground can distort bone. And this is before considering how sexual differences, injuries, illness and natural variation sculpt bones during the animal’s lifetime.
In living populations, warped traits are balanced by large data sets. But the sample sizes of even well-known dinosaurs like T. rex are tiny, according to Philip Currie, a paleontologist at the University of Alberta who was not an author on either study. “The fundamental problem is that although the rough estimate of 100 known specimens of Tyrannosaurus may sound like a lot, it is not nearly enough,” Currie said.
With paleontologists forced to decipher these fragmented puzzles, the field is littered with mistaken identities and defunct species names. And even the legends are not immune — T. rex’s fossil foe, Triceratops, experienced its own naming drama in 1996 when scientists split the three-horned herbivore into two species.
But perhaps no scientific name is as sacred as Tyrannosaurus rex. Since it was named in 1905, the world’s most-studied dinosaur has maintained its moniker. But Paul and colleagues’ recent study threatened to send shock waves through museum halls by rebranding their star attractions.
Several scientists immediately had their doubts. The initial study focused on the bulkiness of Tyrannosaurus femurs and the existence of two sets of incisor teeth poking out of the predator’s lower jaw.
In the rebuttal study, Carr claims that neither trait is distinct to any of the purported Tyrannosaurus species. “The features that were claimed to be different between the three species were actually overlapping,” said Carr, who published a meticulous study examining traits in more than 40 T. rex specimens in 2020. “There wasn’t any clean break between the different species — we have to have a higher standard than that.” He adds that several well-preserved Tyrannosaurus specimens fail to fall into any proposed species based on their teeth and the heftiness of their femurs.
They also aim to puncture the statistical analyses used in the original paper. According to James Napoli, a paleontologist at the American Museum of Natural History in New York and a co-author on the rebuttal, the statistics used were misleading because the authors defined the number of species they expected before running the tests. “It’s a great test if you’re trying to predict which individuals belong to which group and you know how many groups are in your data,” Napoli said. But using it to find distinct clusters is less useful because “it will always group the data into the number of groups you tell it to.”
In the original paper, the researchers compared the variation between individual Tyrannosaurus specimens with the variation found between several Allosaurus skeletons. However, the rebuttal claims that comparing the apex predators is misleading because the Allosauruses hail from a single bone bed in Utah while the Tyrannosaurus fossils came from a scattering of sites over a longer period of time. Therefore, they say, higher amounts of regional and temporal variation in the Tyrannosaurus data set should be expected.
The rebuttal team also considered the variability of T. rex’s living relatives — birds. After examining the femurs of 112 species of living birds, the team deduced that the differences between T. rex femurs were relatively unremarkable.
But Paul believes another feature could make this variation more apparent. In an upcoming study, he posits that the style of horns adorning Tyrannosaurus’s skull are distinct to each species, like the contrasting crests differentiating cassowary species. He says that the horn-encrusted brow of T. imperator consisted of spindle-shaped lumps while T. rex’s horns were knobbier. “This should seal the deal,” Paul said.
Napoli is not convinced. Like the armor of modern crocodiles, these bony outgrowths were likely encased in keratin, protecting continually growing bone underneath. He thinks the shape of a T. rex’s horns probably changed as the animal aged.
The one thing both sets of researchers agree on is the need for more Tyrannosaurus specimens. “When more skeletons are found, they are added into the data set and eventually one way or another, the statistical support is going to be so strong that reasonable scientists cannot disagree,” said W. Scott Persons, a paleontologist at the College of Charleston and a co-author with Paul on the earlier paper.
While neither side is ready to surrender, Peter Makovicky, a paleontologist at the University of Minnesota who was not involved with either study, believes the continued back-and-forth surrounding Tyrannosaurus rex’s identity is good for paleontology because it allows the public to experience the minutiae that defines the discipline.
“This gives the layperson an insight into why we care so much about differentiating new species in the fossil record,” said Makovicky, who counts himself in the single-species camp. “It would be very difficult to convince someone of that if it’s a brachiopod, but T. rex takes it to another level.”