Earlier this year, a paper suggesting the iconic T. rex was actually three separate species was met with criticism from the paleontology community. The study, led by independent researcher and paleoartist Gregory Paul, proposed adding two new T. rex relatives: the T. regina and the T. imperator.
“I’m aware that there could be a lot of people who aren’t going to be happy about this,” Paul told the New York Times’ Asher Elbein in March. “And my response to them is: Publish a refutation.”
Paleontologists took Paul’s advice and published a rebuttal this week in the journal Evolutionary Biology.
“The evidence was not convincing and had to be responded to because T. rex research goes well beyond science and into the public sphere,” Thomas Carr, a paleontologist at Carthage College in Wisconsin and an author of the new study, tells the New York Times’ Jack Tamisiea. “It would have been unreasonable to leave the public thinking that the multiple species hypothesis was fact.”
The original study examined about three dozen T. rex specimens, and concluded that the differences in femur robustness and dentistry meant the species should be split.
The authors of the new paper re-examined this data, and found several issues with the methods, including “a limited comparative sample, non-comparable measurements and improper statistical techniques,” per a statement from the American Museum of Natural History.
“Their measurements for the size of each tooth don’t match what our measurements for them are,” James Napoli, co-lead author from the American Museum of Natural History in New York, tells New Scientist’s Carissa Wong. “And in fact, it looks like in some cases, if that tooth was missing, they measured the size of the tooth socket, which can be misleading,”
The new paper also added measurements from the femurs of 613 living bird specimens from 112 species and 52 specimens from four non-avian theropod dinosaur species.
“When compared to data from hundreds of living birds, we actually found that T. rex is less variable than most living theropod dinosaurs,” Napoli says in the statement. “This line of evidence for proposed multiple species doesn’t hold up.”
Organizing dinosaurs into separate species is somewhat of a subjective process, and without DNA, the lines between species become especially hazy, per the Times. Physical features like bone size and shape, which are often used to split up species, can become distorted over time, and other factors like injury, illness and sexual differences can cause further uncertainty, per the newspaper. Larger sample sizes can reduce doubt, but adult T. rex specimens are relatively few in number.
Charles Marshall, a paleontologist at the University of California, Berkeley tells New Scientist “I think the debate is, in fact, resolved – the specimens assigned to T.. rex constitute just one species. Now it appears that [Napoli] and his colleagues have shown this rigorously.”
But Paul criticizes the new research as "not a proper scientific study,” per Reuters’ Will Dunham, and says the T. rex’s fame has contributed to the lack of acceptance for dividing the species.
“It comes across as paleopropaganda that appears to be structured to defend T. rex, rather than seriously explore the possibilities that fossil specimens of the genus Tyrannosaurus contained the more than one species that the genus certainly did,” he tells the publication. “There is something about beloved T. rex that causes people to become agitated to a degree not seen with other paleotaxa.”