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Tyrannosaurus Rex, or T. rex, has long been one of the most famous dinosaurs². Now, researchers are debating whether the giant meat-eating tyrannosaurus should be considered as one species or three different species.

Since 1905, T. rex has been recognized as the single species of the genus Tyrannosaurus. A genus is a larger grouping of related organisms than a species.

But three researchers, led by independent paleontologist Gregory Paul of Baltimore, Maryland, argued in March 2022 that three T. rex species should be recognized.

In addition to the species T. rex, meaning "tyrant³ lizard⁴ king," they proposed two additional species: T. imperator, meaning "tyrant lizard emperor," and T. regina, meaning "tyrant lizard queen."

The researchers said their study was based on differences in the thickness of the thighbones and the shape of the lower front teeth. They based their claims on over thirty Tyrannosaurus specimens⁶.

Other paleontologists disagreed

In July, another group of paleontologists argued that the March study did not offer enough evidence to show that there were three Tyrannosaurus species. They suggested the previous study did not use correct mathematical methods or measurements.

University of Edinburgh paleontologist Steve Brusatte was a co-leader of the new study published in Evolutionary⁷ Biology in July.

"The evidence needs to be convincing, and to suddenly divide such an ... animal like T. rex, which has been known for over a hundred years, into different species requires a high burden of proof. It is true that there is variation in the size and shape of T. rex bones, but in our new study we show that this variation is minimal," Brusatte said.

Tyrannosaurus was part of a group called theropods that included all the meat-eating dinosaurs. It had a large head and powerful bite strength. It walked on two strong legs and had small arms with just two fingers.

The new study looked at differences in thighbone thickness in four other meat-eating dinosaurs and 112 species of living birds. They found that the Tyrannosaurus differences were normal or usual.

"It is normal for any species to be variable in size and shape. Just look at the range of height and waistlines and toothy grins in people today, all of whom are members of a single species...," Brusatte said.

Agree to disagree

Gregory Paul, however, criticized the new study as "not a proper scientific study." Proper is a term that means correct.

He added that there is something about T. rex that causes people to become concerned or worried to a degree not seen with other ancient organisms.

"Had our paper been about the species of, say, the also gigantic Argentinian theropod Giganotosaurus there very likely would not have been so much fuss and bother," Paul added.

"We are open-minded that there may be multiple species of Tyrannosaurus," Brusatte noted⁸. "We just need more and better fossils. The number of fossils in their dataset is so small that it's hard to find any consistent way to divide Tyrannosaurus into multiple species based on clear, easy-to-define, consistent differences."

Tyrannosaurus lived in western North America during the Cretaceous Period – a time near the end of the dinosaur¹ age. Perhaps the largest-known Tyrannosaurus is one named Sue at the Field Museum in Chicago. Sue's skeleton is 12.3 meters long.

Words in This Story

species – n. biology : a group of animals or plants that are similar and can produce young animals or plants : a group of related animals or plants that is smaller than a genus

paleontologist – n. the science that deals with the fossils of animals and plants that lived very long ago especially in the time of dinosaurs

specimen⁵ -- n. something (such as an animal or plant) collected as an example of a particular kind of thing

burden – n. someone or something that is very difficult to accept, do, or deal with

variation – n. something that is similar to something else but different in some way

fuss – n. activity or excitement that is unusual and that often is not wanted or necessary

bother – n. trouble or difficulty

fossil – n. something (such as a leaf, skeleton, or footprint) that is from a plant or animal which lived in ancient times and that you can see in some rocks