The skeleton of Tyrannosaurus rex at the American Museum of Natural History in New York. not
Tyrannosaurus rex is the most iconic dinosaur. Its skeletons hold pride of place in museums around the world and fetch millions of dollars at auction – and an abundance of relatively complete specimens have made it the most studied dinosaur in the world.
But in a new article published late last month in evolutionary biologythree researchers argue that the animal we call Tyrannosaurus rex should be split into three species, with T rex being joined by two cousins they name Tyrannosaurus imperator, or the emperor, and Tyrannosaurus regina, the queen.
“This article is likely to shake up the paleo community and the public who are so used to good old T rex,” said Gregory Paul, freelance paleontologist and paleoartist and author of the article.
Tyrannosaur experts largely disagree. Thomas Carr, a paleontologist at Carthage College in Kenosha, Wis., calls the evidence for several species “extremely weak.” Another paleontologist stepped down as author of the paper before it went into publication. And curators at museums with Tyrannosaurus specimens that would be affected by these reclassifications say they won’t rename anything based on the proposal.
But even if children’s imaginations are never filled with the sharp teeth and tiny arms of three types of Tyrannosaurus, the premise advanced by Paul and his colleagues highlights an assortment of tensions in dinosaur paleontology.
The first is that naming dinosaur species is a subjective process, and each new species description is more of an argument than a statement. Some researchers believe the idea of multiple Tyrannosaurus species has merit, but say separating such a famous and well-studied species as Tyrannosaurus rex requires a high standard of evidence.
Whether Mr. Paul is ultimately right, he wouldn’t be the first researcher unaffiliated with formal institutions to upend a consensus in the field — or the first to potentially bite off more than he can chew.
An illustration shows T rex attacking with sharp jaws. www.123rf.com
The name of the game
Scientific names are divided into genus and species. Tyrannosaurus is a genus that is theoretically capable of containing multiple species. For now, there is only one: Tyrannosaurus rex.
It may sound simple, says David Hone, a paleontologist at Queen Mary University of London. But there is no universal agreement on what constitutes a species. To separate them in living animals, scientists typically rely on anatomy — the color of a bird’s feathers or specific bone traits — 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 greatly from each other.
In the absence of prehistoric DNA, dinosaur paleontologists must rely on the anatomical details of fossilized bones – which can be found at different stages of growth or levels of completeness. This can lead to headaches, such as scientists assigning multiple names to the same animal – as with Brontosaurus, which was thrown around for over 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 small tyrannosaurs that preceded it in the age of the dinosaurs. Its fossils have been found mostly in the Hell Creek Formation, a 66-million-year-old band of rock that records the last million years of dinosaur reign and is found in parts of Montana, Wyoming, Dakota South and North Dakota.
Long known from just two decent specimens, Tyrannosaurus rex, Paul said, has largely survived the decades without major taxonomic revisions. But as dinosaur fossil collecting exploded in the 1990s, newly discovered T rex skeletons found their way into museums.
A child looks at a tyrannosaur skeleton at the National Museum of Natural History in Paris. Reuters
Their proportions showed a remarkable amount of individual variation, and some researchers have argued that they belong to two groups: the bulky “robust” form and the relatively slender “slender”. Some scientists have argued that this is evidence of major differences between females and males of the species, said Thomas Holtz, a University of Maryland paleontologist who specializes in tyrannosaurs. Others said it was simply an unusual degree of individual variation. But some researchers have considered a third possibility: that Tyrannosaurus is a genus with more than one species.
One of these researchers was Mr. Paul. Although he does not have a formal degree in paleontology, he has a long history in the field and has authored or co-authored over 30 scientific papers. He also proposed new genera and species for other dinosaurs. An argument he made about Brachiosaurus has been widely adopted. Another one he did on Iguanodon was not.
His work influenced Michael Crichton, author of the novel jurassic parkas well as the creators of the film based on it.
In 2010, Mr. Paul and others began working on the question of T rex’s identity. They collected anatomical measurements from 38 specimens and rated them on a pair of anatomical traits: the relative proportions of the femur and the presence – or absence – of two sets of chisel-shaped front teeth in their lower jaws.
They also compared Tyrannosaurus specimens – which have been collected from across North America – with dinosaur specimens that most likely belonged to one species, such as the 14 Allosaurus specimens collected from the Cleveland Dinosaur Quarry. Lloyd from the University of Utah. T rex, they said, showed significantly wider variation in body proportions, suggesting they could belong to more than one species.
They found some of the specimens difficult to classify. But 26 seemed to cluster into three types, Paul said: a sturdy form from early Hell Creek with two sets of incisors in its lower jaws, and a robust, slender form from later with a single set of incisors. .
These three forms, Paul and his colleagues concluded, were probably sufficiently different from each other – and appeared over a long enough period of time – to warrant separate names. Therefore, the first and bulky Tyrannosaurus to appear in Hell Creek was given the name Tyrannosaurus imperator (“tyrant emperor lizard”), which then – over the course of 1 to 2 million years – split into the sturdy Tyrannosaurus rex and the new Tyrannosaurus regina (tyrant lizard queen).
The head of an approximately 67 million year old Tyrannosaurus skeleton, one of the largest and most complete ever discovered and named “STAN” after paleontologist Stan Sacrison who first found it Times, on display ahead of its public auction at Christie’s in New York City on September 15, 2020. MIKE SEGAR/Reuters
This proposed evolutionary trajectory — from a reservoir-like population to a relatively flexible one — matches ecosystems earlier in the Cretaceous period that were dominated by relatives of Tyrannosaurus, Paul said. At that time, murderers like Daspletosaurus coexisted with long-legged hunters like Gorgosaurus.
According to Mr Holtz, such an idea is entirely plausible – but proving it will require the support of future findings.
“It’s a testable hypothesis, as any statement of species identity should be,” Holtz said. “With additional new specimens, we can see if the specimens they haven’t included or that we haven’t yet found are consistent with this suggestion, or if they reject it.”
Although Dr. Holtz said the authors’ case would be more compelling if the different species they described were more chronologically organized in specific rock layers, he noted that the Hell Creek Formation contains other examples of species divergence.
Other fossils found there have led to general agreement that there was more than one species of triceratops, Holtz said. And it’s possible that the domed-headed hadrosaurs and 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 pass through a succession of species,” Holtz said. “It’s not inconceivable that Tyrannosaurus did it too.”
bone to pick
Others took a less positive position.
A major problem with the study is that the femur proportions of the three proposed species overlap rather than show clear separations, said Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum in Chicago. The same goes for the proposed time periods during which they existed.
To her, this suggests a continuous spectrum of change, arbitrarily divided into three groups – not the distinct differences you would expect to see in three species.
“The diagnoses provided for each species are incredibly vague, using words like ‘usually’ and ‘usually’,” Ms O’Connor said. “Even in well-preserved specimens, authors are unable to refer them to a specific species.”
Part of the proposed impact of Mr. Paul’s article would accrue to 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 skeleton in the Smithsonian National Museum of Natural History, dubbed “the Nation’s T rex”, was crowned a holotype of Tyrannosaurus regina.
Paul hopes museums will use the debate around his findings to educate the public about how science works. But he says they can’t just ignore it – and he promises to make a fuss if they try.
The key is that his team takes advantage of the fact that the concepts of species in dinosaur paleontology are best summed up with a shrug, said Leonard Finkelman, who studies the philosophy of paleontology at Linfield University in Oregon.
Until dinosaur paleontologists establish a consistent standard for the minimum anatomical differences needed to name a new species, he says, controversies will continue to arise.
Still, provocative proposals can have value in themselves, Finkelman said. Sometimes an idea that seems poorly supported can be confirmed by future findings. And when they don’t, such insights always inspire researchers to revisit old work in light of new insights – which is one way science advances.
“Whether it makes a visible contribution now, forcing scientists to go back and look at their old data will always be a worthwhile exercise,” Finkelman said.
And that’s basically what Mr. Paul wants.
“I’m aware there might be a lot of people who won’t be happy about this,” he said. “And my answer is: publish a rebuttal.”