The world’s most iconic dinosaur is going through an identity crisis.
In February, a team of scientists proposed that Tyrannosaurus rex was actually three different species. Instead of there being just one sovereign “tyrant lizard king,” his paper advocated a royal family of oversized predators. Joining the king of the genus Tyrannosaurus would be the bulkier and older emperor, T. imperator, and the slimmer queen, T. regina.
The proposed reclassification of T. rex hit the paleontology community like an asteroid, igniting passionate debate. On Monday, another team of paleontologists published the first peer-reviewed counterattack.
“The evidence was unconvincing and needed to be answered because T. rex research is moving far beyond science and into the public sphere,” said Thomas Carr, a paleontologist at Carthage College in Wisconsin and author of the new refutation. “It would have been unreasonable to leave the public thinking that the multiple species hypothesis was a fact.”
The above team of researchers has 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 claims in the rebuttal are outlandish.
“I don’t like the flat earth because the evidence is against it,” said Mr. Paul, who is an independent researcher and influential paleoartist. “It’s the same here: the evidence points very strongly to multiple species.”
This outsized taxonomic debate seems destined to rage for ages. Which is not surprising considering how difficult it is for researchers to differentiate between prehistoric species. Without dino DNA, the lines between one fossil species and another are messy. Thus, paleontologists measure different traits, such as the size and shape of a particular bone. However, fossils can be deceiving, as spending eons buried underground can distort bones. And that’s before considering how sex differences, injury, disease, and natural variation sculpt bones during the animal’s life.
In living populations, skewed traits are balanced with large datasets. 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 while the rough estimate of 100 known specimens of tyrannosaurus may seem like a lot, it’s not enough,” Dr Currie said.
With paleontologists forced to decipher these fragmented puzzles, the field is littered with mistaken identities and names of vanished species. And even legends aren’t 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 kept its nickname. But the recent study by Mr. Paul and his colleagues threatened to send shockwaves through museum halls by changing the names of their star attractions.
Several scientists immediately had their doubts. Initial study focused on the size of the tyrannosaurus’ femurs and the existence of two sets of incisor teeth protruding from the predator’s lower jaw.
In the rebuttal study, Dr. Carr claims that no trait is different from any of the supposed tyrannosaur species. “The features that were said to be different between the three species actually overlapped,” said Dr. Carr, who published a detailed study examining the traits of more than 40 T. rex specimens in 2020. “It doesn’t if there is to be any clean break between different species, we have to have a higher standard than that.” He adds that several well-preserved tyrannosaur specimens do not fall into any proposed species based on their teeth and the thickness of their femurs.
They also aim to puncture the statistical analyzes used in the original paper. According to James Napoli, a paleontologist at the American Museum of Natural History in New York and co-author of 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 people belong to which group and you know how many groups are in your data,” said Dr. Napoli. But using it to find different groups 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 among individual tyrannosaurus specimens with the variation found among several Allosaurus skeletons. However, the rebuttal states that comparing apex predators is misleading because Allosaurs come from a single bone bed in Utah, while Tyrannosaurus fossils come from a spread of sites over a longer period of time . Therefore, they say, greater amounts of regional and temporal variation should be expected in the tyrannosaurus dataset.
The rebuttal team also considered the variability of T. rex’s living relatives: birds. After examining the femurs of 112 living bird species, the team deduced that the differences between T. rex femurs were relatively unremarkable.
But Mr. Paul believes another feature might make this variation more apparent. In a recent study, he posits that the style of horns that adorn the tyrannosaurus skull are different for each species, like the contrasting ridges that differentiate species of cassowary. He says T. imperator’s horn-encrusted forehead consisted of spindle-shaped lumps while T. rex’s horns were more gnarled. “That should seal the deal,” Mr Paul said.
Dr. Napoli is not convinced. Like the armor of modern crocodiles, these bony droppings were likely encased in keratin, protecting the continuously growing bone underneath. He believes that the shape of a T. rex’s horns probably changed as the animal aged.
The only thing both groups of researchers agree on is the need for more tyrannosaurus specimens. “As more skeletons are found, they are added to the data set, and eventually, one way or another, the statistical support will be so strong that reasonable scientists cannot disagree,” said paleontologist W. Scott Persons from the College of Charleston. and a co-author with Mr. Paul in the previous document.
While neither side is willing to give up, Peter Makovicky, a paleontologist at the University of Minnesota who was not involved in either study, thinks the ongoing back-and-forth surrounding the identity of Tyrannosaurus rex is good for paleontology because it allows the public to experience the minutiae that define the discipline.
“This gives the layman an idea of why we care so much about differentiating new species in the fossil record,” said Dr Makovicky, who counts the single-species field. “It would be very difficult to convince someone of this if it was a brachiopod, but T. rex takes it to another level.”