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Tyrannosaurus rex may actually be three separate species

After analysing the teeth and thigh bones of 38 T. rex fossils, some researchers propose reclassifying them as three different species, but others are unconvinced

Life 1 March 2022

T0KTWC CHICAGO, IL -7 FEB 2019- View of the Field Museum of Natural History (FMNH), a large natural history museum containing the skeleton of the Tyrannosaur

Sue, the Tyrannosaurus skeleton at the Field Museum in Chicago

EQRoy / Alamy

The “tyrant lizard king” – Tyrannosaurus rex – might have belonged to a dynasty. A research team has proposed splitting the famous species into three, with Tyrannosaurus imperator (tyrant lizard emperor) and Tyrannosaurus regina (tyrant lizard queen) taking their places next to T. rex. But the proposal is already proving unpopular with other palaeontologists.

T. rex was an apex predator that lived in North America between about 68 and 66 million years ago. The first T. rex fossils were discovered more than a century ago, but for decades very few skeletons were known. More have come to light since the 1990s, says Scott Persons at the College of Charleston, South Carolina, which means it is now possible to assess whether or not the animals fall into a single species.

To explore this question, Persons and his colleague Jay Van Raalte worked with independent researcher Gregory Paul. The trio looked at bones from 38 T. rex fossils, focusing in particular on two features: the number of front teeth in the lower jaw and the stoutness of the thigh bones.

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They discovered variation in both features, which they think justifies splitting the dinosaurs into three distinct species. The oldest animals – which had four distinctly small incisors at the front of the lower jaw and the stout thighs of a heavily built dinosaur – are placed in the new species T. imperator.

The trio believe this then evolved into two younger species, both of which had just two small incisors at the front of the lower jaw. One of these younger species had slender thigh bones and was lightly built – it has been named T. regina. The second younger species had stout thighs and was heavily built. This species retains the name T. rex.

“There will be those who say you’re naming them just because it’s intrinsically fun and cool to name a new Tyrannosaurus,” says Persons, but he argues the decision is justified. He says modern ecosystems show us that apex predators evolve and diversify into distinct species – lions and leopards, for example – and it is very likely that Tyrannosaurus did so too.

The conclusions have implications for some of the most famous Tyrannosaurus fossils, he adds: Sue, a Tyrannosaurus skeleton at the Field Museum in Chicago, is actually T. imperator, while Stan – which was sold to a private buyer for $32 million in 2020 – should be reclassified as T. regina.

Philip Currie at the University of Alberta, Canada, expects the proposal to stimulate debate. “I think the authors have made a case that there are anatomical changes in the genus that seem to [change] with time,” he says. “That is pretty amazing in itself.”

But others will take more persuading. “I understand the temptation to divide T. rex into different species, because there is some variation in the fossil bones that we have,” says Stephen Brusatte at the University of Edinburgh, UK. “But ultimately, to me, this variation is very minor. Until I see much stronger evidence, these are all still T. rex to me.”

Thomas Carr at Carthage College in Wisconsin also thinks the evidence in the new paper is weak. In 2020 he published an in-depth analysis of T. rex. He says his analysis was structured to uncover patterns in the data, but he found nothing that made him think there was more than a single species. “I just think they’re seeing what they want to see,” he says.

Not only does Carr see problems with the analysis itself, he is troubled that the three researchers included data from fossils housed at the Black Hills Institute (BHI) of Geological Research, a private corporation in South Dakota.

Carr argues researchers should limit themselves to analysing fossils in public collections, because these specimens will always be available for scientific study. Fossils in private collections have the potential to be sold to someone who won’t allow scientific access. The BHI was the former home of Stan before the 2020 auction.

It is a significant problem, says Paul. “On the other hand, eliminate all those [BHI] specimens from the sample and it is not possible to assess the taxonomy of Tyrannosaurus.”

Paul adds that the results of the new analysis might actually help reduce the likelihood of fossils being sold in future. He points out that if a T. rex in a private institution is renamed T. imperator or T. regina, it may not attract as much interest from potential buyers.

Journal reference: Evolutionary Biology, DOI: 10.1007/s11692-022-09561-5

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