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Tyrannosaurus rex division into 3 species turns into a dinosaur battle royale

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The world’s most iconic dinosaur is experiencing an identity crisis.

In February, a team of scientists suggested that Tyrannosaurus Rex is actually made up of three different species. Instead of being just one sovereign “lizard tyrant king”, their paper argued for a royal family of huge predators. The king in the tyrannosaurus genus will be joined by a bulkier and older emperor, T. imperator, and a slenderer queen, T. regina.

The proposed reclassification of Tyrannosaurus hit the paleontological community like an asteroid, sparking a passionate debate. On Monday, another group of paleontologists published the first peer-reviewed counterattack.

“The evidence was not conclusive and needed to be responded to because T. rex research goes far beyond science and into the public realm,” said Thomas Carr, a paleontologist at Carthage College in Wisconsin and author of the new rebuttal. “It would be unwise to leave the public to think that the multiple species hypothesis was a fact.”

The team of researchers had previously anticipated a 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 the flat earth theory because the evidence is against it,” said Mr. Paul, an independent researcher and influential paleoartist. “It’s the same here: the data very strongly suggests that there are multiple species.”

This grandiose taxonomic debate seems destined to rage for centuries. Which is not surprising, given how difficult it is for researchers to differentiate between prehistoric species. Without dinosaur DNA, the boundaries between one fossil species and another are fuzzy. Thus, paleontologists measure various characteristics, such as the size and shape of a particular bone. However, fossils can be misleading, as burial underground for thousands of years can distort the bone. And that’s before looking at how sex differences, injury, disease, and natural variation shape bones over the course of an animal’s life.

In living populations, skewed traits are balanced by large datasets. But according to Philip Currie, a paleontologist at the University of Alberta, the sample size of even well-known dinosaurs like Tyrannosaurus Rex is tiny, not authoring any research. “The fundamental problem is that while a rough estimate of 100 known T. rex specimens may seem large, it’s not enough,” Dr. Curry said.

As paleontologists are forced to decipher these disparate puzzles, the field is littered with misidentifications and names of extinct species. And even the legends aren’t immune – Tyrannosaurus rex’s fossil enemy, the 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 retained its moniker. But a recent study by Mr. Paul and his colleagues threatened to send shockwaves through the museum halls over the rebranding of their star rides.

Immediately, several scientists had doubts. Initial research focused on the bulkiness of the Tyrannosaurus rex’s thighs and the presence of two sets of incisors protruding from the predator’s lower jaw.

In a rebuttal, Dr. Carr argues that none of the traits differ from any of the alleged tyrannosaur species. “Traits that were claimed to differ between the three species actually overlapped,” said Dr. Carr, who published in 2020 a thorough study of the traits of more than 40 T. rex specimens. “There was no clear divide between different species – we should have a higher standard.” He adds that several well-preserved Tyrannosaurus rex specimens cannot be assigned to any of the proposed species due to their teeth and the size of their thighs.

They also seek to pierce the statistical analysis used in the original article. 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 are misleading because the authors determined the expected number of species before testing. “This is a great test if you’re trying to predict which people belong to which group and know how many groups are in your data,” Dr. Napoli said. But using it to find individual clusters is less useful because “it will always group data into the number of groups you specify”.

In the original paper, the researchers compared the variation between individual Tyrannosaurus rex specimens with the variation found between several Allosaurus skeletons. However, the rebuttal argues that the apex predator comparison is misleading because allosaurs come from a single bone bed in Utah, while T. rex fossils come from scattered locations over a longer period of time. Therefore, they say, more regional and temporal variation in the tyrannosaur dataset should be expected.

The rebuttal group also took into account the variability of living relatives of Tyrannosaurus rex – birds. After examining the femurs of 112 species of live birds, the team concluded that the differences between T. rex femurs were relatively unremarkable.

But Mr. Paul thinks another feature could make the change more obvious. In a forthcoming study, he argues that the style of the horns that adorn the skull of a Tyrannosaurus Rex is different for each species, like the contrasting crests that distinguish cassowary species. He says that the horn-encrusted forehead of T. imperator consisted of spindle-shaped bumps, while the horns of T. rex were more knotty. “This should close the deal,” Mr. Paul said.

Dr. Napoli is not convinced. Like the armor of modern crocodilians, these bony outgrowths were likely coated in keratin to protect the ever-growing bone underneath. He believes that the shape of the tyrannosaurus rex’s horns probably changed as the animal aged.

The only thing both groups of researchers agree on is the need for more Tyrannosaurus rex 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 W. Scott Persons, a paleontologist at the College of Charleston. and co-author with Mr. Paul in an earlier article.

While neither side is ready to give up, Peter Makowitzki, a paleontologist at the University of Minnesota who was not involved in either study, believes that the ongoing debate over the identity of T. rex is good for paleontology because it allows the public to experience the little things that define the discipline.

“It gives non-specialists a sense of why we care so much about differentiating new species in the fossil record,” said Dr. Makowitzky, who considers himself a one-species proponent. “It would be very difficult to convince someone of this if it was a brachiopod, but T. rex takes it to another level.”

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