The premise, put forth in a new paper, highlights an assortment of tensions in dinosaur paleontology, including how subjective the naming of species can be.
Tyrannosaurus rex is the most iconic dinosaur. Its skeletons hold pride of place in museums around the world, and sell for millions of dollars at auction — and a bounty of relatively complete specimens have made it the most thoroughly studied dinosaur in the world.
But in a new paper published Tuesday in Evolutionary Biology, three researchers argue that the animal we currently call Tyrannosaurus rex should actually be split into three separate species, with T. rex being joined by two cousins they name Tyrannosaurus imperator, or the emperor, and Tyrannosaurus regina, the queen.
“This paper is likely to rock the paleo community, and the public that is so used to good old T. rex,” said Gregory Paul, an independent paleontologist and paleoartist and author on the paper.
Tyrannosaur experts largely disagree. Thomas Carr, a paleontologist at Carthage College in Kenosha, Wis., calls the evidence for multiple species “vanishingly weak.” Another paleontologist removed himself as an author of the paper before it moved to publication. And curators at museums with Tyrannosaurus specimens that would be affected by these reclassifications say they aren’t going to rename anything based on the proposal.
But even if children’s imaginations never end up filled with the sharp teeth and tiny arms of three types of Tyrannosaurus, the premise put forward by Mr. Paul and his colleagues highlights an assortment of tensions in dinosaur paleontology. One is that naming dinosaur species is a subjective process, and each new species description is more of an argument than a declaration. Some researchers think that the idea of multiple Tyrannosaurus species has merit but say that splitting apart a species as famous and well-studied as Tyrannosaurus rex requires a high standard of evidence.
Whether Mr. Paul is ultimately proved right, he wouldn’t be the first researcher unaffiliated with formal institutions to shake up a consensus in the field — or the first to potentially bite off more than he can chew.
The Name Game
Scientific names are split between genus and species. Tyrannosaurus is a genus, which is theoretically capable of holding multiple species. As yet, it has only one: Tyrannosaurus rex.
This might sound simple, says David Hone, a paleontologist at Queen Mary University of London.But there’s actually no universally consistent agreement for what constitutes a species. To separate them in living animals, scientists generally rely on anatomy — the color of a bird’s feathers or the specific traits of bones — as well as genetic evidence. But separate species can and do interbreed, and animals of the same species at different ends of a very large geographic range can vary a startling amount from one another.
In the absence of prehistoric DNA, dinosaur paleontologists must rely on the anatomical details of fossilized bone — which can be found at different growth stages or varying levels of completeness. This can lead to headaches, like scientists assigning multiple names to the same animal — as with Brontosaurus, which was junked for more than a century before the name was revived in 2015.
But Tyrannosaurus rex has been a remarkably stable name. Discovered in 1902, the animal is the last and largest of the many, smaller tyrannosaurs that preceded it in the dinosaur era. Its fossils have been found primarily in the Hell Creek Formation, a band of 66-million-year-old rocks that records the last million years of the dinosaurs’ reign and is found in parts of Montana, Wyoming, South Dakota and North Dakota.
Long known from only two decent specimens, Mr. Paul said, Tyrannosaurus rex largely sailed through the decades without major taxonomic revisions. But as dinosaur fossil collecting boomed in the 1990s, newly found T. rex skeletons trickled into museums. Their proportions showed a remarkable amount of individual variation, and some researchers argued that they fell into two groups: the bulky “robust” form, and the relatively svelte “graciles.” Some scientists argued it was evidence of major differences between females and males of the species, said Thomas Holtz, a paleontologist at the University of Maryland who specializes in tyrannosaurs. Others said it was simply an unusual degree of individual variation. But some researchers toyed with a third possibility: that Tyrannosaurus was a genus with more than one species.
One of those researchers was Mr. Paul. While Mr. Paul lacks a formal degree in paleontology, he has a long history in the field, and has written or been a co-author of over 30 scientific papers. He has proposed new genera and species for other dinosaurs, too. An argument he made about Brachiosaurus was broadly embraced. Another one he made about Iguanodonwasn’t. His work also influenced Michael Crichton, the author of the novel “Jurassic Park,” as well as the makers of the movie based on it.
In 2010, Mr. Paul and others began working on the question of T. rex’s identity. They gathered anatomical measurements from 38 existing specimens, and assessed them according to a pair of anatomical traits: the relative proportions of the femur and the presence — or absence — of two sets of chisel-like front teeth in their lower jaws.
They also compared the tyrannosaurus specimens — which were collected from across North America — to specimens of dinosaurs that most likely belonged to one species, such as the 14 Allosaurus specimens collected from the University of Utah’s Cleveland-Lloyd Dinosaur Quarry. The T. rexes, they said, showed significantly wider variation in body proportions, suggesting they might belong to multiple species.
They found some of the specimens difficult to classify. But 26 seemed to group into three types, Mr. Paul said: one robust form from early in the Hell Creek with two sets of incisors in its lower jaws, and a robust and gracile form from later with only one incisor set.
These three forms, Mr. Paul and his colleagues concluded, were probably different enough from one another — and appeared over a sufficient length of time — to warrant separate names. Therefore, the earliest, bulky Tyrannosaurus to show up in the Hell Creek received the name Tyrannosaurus imperator (“emperor tyrant lizard”), which then — over the course of one million to two million years — split into the robust Tyrannosaurus rex and the newly named, comparatively slender Tyrannosaurus regina (tyrant lizard queen).
That proposed evolutionary trajectory — from one tank-like population to another relatively lithe one — matches ecosystems earlier in the Cretaceous period that were dominated by Tyrannosaurus relatives, Mr. Paul said. In that period, bruisers like Daspletosaurus coexisted with long-legged hunters such as Gorgosaurus.
According to Dr. Holtz, such an idea is entirely plausible — but proving it will require support from future finds.
“It’s a testable hypothesis, as any statement of species identity should be,” Dr. Holtz said. “With additional new specimens, we can see if specimens they didn’t include or we haven’t found yet are consistent with this suggestion, or if they reject it.”
While Dr. Holtz felt the authors’ case would be more convincing if the different species they describe were organized more chronologically in specific rock layers, he noted that the Hell Creek Formation contains other examples of species divergence. Other fossils found there led to a general agreement that there was more than one species of triceratops, Dr. Holtz said. And there’s a possibility that the hadrosaurs and dome-headed Pachycephalosaurus found at the top of the formation differ significantly from those at the bottom.
“At the same time, in the same place, the same size classes of organisms are going through a succession of species,” Dr. Holtz said. “It’s not inconceivable that Tyrannosaurus did so as well.”
Bones to Pick
Others took a less positive stance.
A major problem with the study is that the femur proportions of all three proposed species overlap rather than showing clear separations, said Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum in Chicago. The same is true of the proposed periods of time in which they existed. To her, that suggests a continuous spectrum of change, arbitrarily split into three groups — not the distinct differences you’d expect to see in three species.
“The diagnoses provided for each species are incredibly vague, using words like ‘usually’ and ‘generally,’” Dr. O’Connor said. “Even in well-preserved specimens, the authors are unable to refer them to a specific species.”
The proposed species groupings also don’t necessarily match existing anatomical data, Dr. Carr of Carthage College said. After reading the study, Dr. Carr compared its conclusions to a massive analysis of T. rex he performed in 2020 — which included every existing Tyrannosaurus specimen. He found no significant correlation between the 2020 data and the types Mr. Paul’s team described.
“Four of those specimens have perfect skulls!” Dr. Carr said. “They should be able to tell species apart if they have perfect skulls, and they can’t do it.”
Similar concerns led Philip Currie, a paleontologist at the University of Alberta who had been an author on Mr. Paul’s paper, to take his name off the project.
“I think the paper is great in that it shows that morphological changes were taking place in Tyrannosaurus over the years that it existed,” Dr. Currie wrote in an email. “I am pretty conservative, so my preference was not to be directly involved in setting up even more species.”
Part of the paper’s proposed impact would fall on museum collections. According to the new proposal, Sue, the Field Museum’s nearly complete T. rex, is now the holotype — the specimen that anchors a species name — of Tyrannosaurus imperator. The Smithsonian National Museum of Natural History’s skeleton, nicknamed “the Nation’s T. rex,” has been anointed the holotype for Tyrannosaurus regina.
Mr. Paul said he hoped museums would use the debate around his conclusions as an opportunity to educate the public about how science works. But he says they can’t just ignore it — and he promises to raise a fuss if they try.
The Field Museum won’t be changing the labels on Sue anytime soon, however. “Modifications to exhibit texts are only made after new hypotheses have been vetted and widely accepted by the scientific community,” Dr. O’Connor said. “The new hypothesis regarding Tyrannosaurus diversity is very poorly justified and is unlikely to make it through this rigorous process.”
Ironically, Dr. Holtz said, even if everyone agreed with the team’s proposal, at least one of Mr. Paul’s proposed names might prove to be dead in the water because it crashes into another long-running battle in Tyrannosaurus taxonomy: the identity of Nanotyrannus lancensis, an animal named in 1946 from relatively early in the Hell Creek. Some researchers have argued it represents a separate genus of pygmy tyrannosaur. But most consider it a juvenile Tyrannosaurus.
Dr. Holtz says that because the “lancensis” species name has been around longer, it would take precedence over Tyrannosaurus imperator. “It’s going to be Tyrannosaurus lancensis,” he said.
Mr. Paul argues, however, that the new species names are entirely reasonable, and that many dinosaur species are named based on significantly less evidence. “If it had been a paper about putting another dinosaur into different species, nobody would really care,” he said. “This is T. rex, so people are getting uptight about it.”
He also said that if his team had simply published an argument for splitting Tyrannosaurus up without naming the newly created animals, someone else might have been able to swoop in and do so, “which would have been very irritating.”
He added, “I wasn’t going to let that happen, so we went ahead and named them.”
Under normal circumstances, Dr. Hone said, pointing out a few different anatomical traits and correlating them with different rock layers could make a decent case for naming new species. But splitting up T. rex — “the most intensively studied and best-known dinosaur on earth” — requires a higher standard.
That’s partly because debates over dinosaur taxonomy can also linger a surprisingly long time in popular culture: In Mr. Paul’s 1998 book, “Predatory Dinosaurs of the World,” he proposed uniting the large raptor Deinonychus with its smaller Mongolian cousin velociraptor. The suggestion didn’t stick — except in the novel “Jurassic Park” and its subsequent films, which feature supersized velociraptors to this day.
“If you’ve got it wrong, it’ll be decades of public confusion and argument,” Dr. Hone said, which often leads to suggestions that all taxonomic divisions are arbitrary and beloved species don’t exist. Worse, it’ll be centered on T. rex, the most famous dinosaur around.
“That’s not the kind of thing you should be doing based on femur robusticity and the presence or absence of a tooth,” Dr. Hone added. “If you’re going to shoot for the king, don’t miss.”
The bottom line is that Mr. Paul and his team are taking advantage of the fact that species concepts in dinosaur paleontology are best summarized by a shrug, said Leonard Finkelman, who studies the philosophy of paleontology at Linfield University in Oregon. Until dinosaur paleontologists work out a consistent standard for the minimum anatomical differences it takes to name a new species, he says, controversies like this one will keep happening.
On the other hand, provocative proposals can have value in and of themselves, Dr. Finkelman said. Sometimes an idea that at first seems poorly supported can be upheld by future finds. And when it isn’t, such ideas still prompt researchers to revisit old work in light of new ideas — which is one way science can progress.
“Whether it makes a visible contribution now, forcing scientists to go back and look at their old data is always going to be a useful exercise,” Dr. Finkelman said.
And that is basically what Mr. Paul wants.
“I’m aware that there could be a lot of people who aren’t going to be happy about this,” he said. “And my response to them is: Publish a refutation.”
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