Tyrannosaurus rex may actually be three separate species

After analyzing the teeth and femurs of 38 T. rex fossils, some researchers propose to reclassify them into three different species, but others are not convinced

The life

March 1, 2022

Sue, the Tyrannosaurus skeleton at the Field Museum in Chicago

EQRoy / Alamy

The “Tyrant Lizard King” – tyrannosaurus rex – could have belonged to a dynasty. A research team has proposed dividing the famous species into three, with Imperator Tyrannosaurus (tyrant lizard emperor) and tyrannosaurus regina (tyrant lizard queen) taking place next to T. rex. But the proposal is already proving unpopular with other paleontologists.

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

To explore this question, Persons and his colleague Jay Van Raalte worked with an independent researcher Gregory Paul. The trio examined the bones of 38 T. rex fossils, focusing in particular on two characteristics: the number of front teeth in the lower jaw and the robustness of the femurs.

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They discovered variations in both characteristics, which they believe justifies dividing the dinosaurs into three separate species. The oldest animals – which had four distinctly small incisors at the front of the lower jaw and the sturdy thighs of a heavily built dinosaur – are placed in the new species T.imperator.

The trio believe this later evolved into two younger species, both of which had only two small incisors at the front of the lower jaw. One of these younger species had thin thigh bones and was lightly built – it was named T. Regina. The second, younger species had sturdy thighs and was heavily built. This species retains the name T. rex.

“Some would say you just name them because it’s inherently fun and cool to name a new Tyrannosaurussays Persons, but he argues that the decision is justified. He says modern ecosystems show us that top predators evolve and diversify into separate species – lions and leopards, for example – and it’s very likely that Tyrannosaurus did it too.

The findings have implications for some of the most famous Tyrannosaurus fossils, he adds: Sue, a Tyrannosaurus skeleton at the Field Museum in Chicagois in fact T.imperatorwhile 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 argued that there are anatomical changes in the genus that seem [change] over time,” he says. “That’s pretty amazing on its own.”

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

Thomas Carr at Carthage College in Wisconsin also believes 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 discover patterns in the data, but he found nothing that made him think there was more than one species. “I just think they see what they want to see,” he says.

Not only does Carr see problems with the analysis itself, but he is troubled that the three researchers included data from fossils preserved in the Black Hills Institute (BHI) for Geological Researcha private company in South Dakota.

Carr argues that researchers should limit themselves to analyzing fossils in public collections, as these specimens will always be available for scientific study. Fossils in private collections have the potential to be sold to someone who will not allow scientific access. The BHI was Stan’s former home prior to the 2020 auction.

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

Paul adds that the results of the new analysis could actually help reduce the likelihood of fossils being sold in the future. He specifies that if a T. rex in a private institution is renowned T.imperator Where T. Reginait may not generate as much interest from potential buyers.

Journal reference: evolutionary biology, DOI: 10.1007/s11692-022-09561-5

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