Researchers say ‘Tyrannosaurus’ may have been three species instead of just one

A study based on femurs and incisor teeth posits that ‘T.rex’ had two more relatives, but some scientists are contesting the claim because of the small size of the sample

An illustration of ‘Tyrannosaurus imperator’ attacking a group of ‘Triceratops horridus.’
An illustration of ‘Tyrannosaurus imperator’ attacking a group of ‘Triceratops horridus.’Gregory S. Paul

Tyrannosaurus rex is known as one of the greatest predators in history. It lived for around a million and a half years, according to Luis Alcalá, a paleontologist and director of the science center Parque de las Ciencias in Granada, in southern Spain. This dinosaur is probably one of the world’s most recognizable extinct animals, and it has been the subject of much research, including the speed at which it walked, which was similar to humans, or the discovery of new “relatives.” The T.rex has featured in numerous blockbuster movies such as Jurassic Park, and it is even the name of a 1970s glam rock band. These animals lived during the Cretaceous period, which ended around 66 million years ago.

Until now, it was viewed as a single species within the genus Tyrannosaurus. But new research released on Tuesday in Evolutionary Biology, a publication of the Springer publishing group, points to the possibility that the fossils found over the years in fact belong to three different species instead of just one. The authors of the paper have even found names for these potential new species: Tyrannosaurus imperator and Tyrannosaurus regina. This classification is based on an analysis of physical variations in the thighbone and dental structures of several fossils.

In all, researchers analyzed the remains of 38 specimens, and in 24 of them they compared the robustness of the femurs, based on the length and circumference, which show the strength of the bone. They found that while some thighbones were more “robust,” others were more “gracile.” In some cases the differences were very significant, which led them to rule out the sex of the specimens as a cause. The variations were also not due to age, as robust femurs were found in some juvenile specimens and gracile ones in specimens that were adult size.

Researchers also analyzed the dental structures, focusing on the diameter at the base of the teeth and the space in the gums to determine whether the specimen had one or two incisor teeth. Specimens with one incisor tooth were correlated with higher femur gracility. However, both femur and dental remains were only available for 12 of the specimens.

The number of analyzed specimens is a fundamental factor, explained José Luis Sanz, a paleontologist, emeritus professor of Madrid’s Autonomous University and member of the Spanish Royal Academy of Exact, Physical and Natural Sciences. “This number is not enough to be statistically significant. For now I would not venture to give them a name and say they are different species, not until there’s a larger amount of specimens.” Alcalá agreed that it was premature to give the two potential new species a name, and said that even if the researchers turn out to be right, the evidence they have provided is “very weak.”

The Tyrannosaurus imperator has been linked to fossils that were found in lower and middle layers of sediment. These specimens had more robust femurs and, in general, two incisor teeth. The researchers said this could be something they inherited from their ancestors. Meanwhile, Tyrannosaurus regina has been linked to remains in the upper and middle layers of sediment, with finer and more gracile femurs and a single incisor tooth. And Tyrannosaurus rex remains were identified in the upper and possibly middle layers

Research co-author Gregory Paul, an independent paleontologist and paleoartist who helped establish the look of Mesozoic creatures for documentaries and movies such as Jurassic Park, said that only a fourth of all specimens were T. rex.

According to him, the investigation grew out of a 2010 talk at the Society of Vertebrate Paleontology, where John Scannella, a curator at the Museum of the Rockies in Bozeman (Montana), talked about triceratops, one of the favorite preys for Tyrannosaurus. Scannella wondered if, just like their prey, Tyrannosaurus had developed new species. He discussed the need for a taxonomic investigation into this dinosaur, because he suspected that a lot of specimens were being “thrown” into the T. Rex category without a proper anatomical and stratigraphic analysis. Paul said that the fossils they analyzed indicate that T. rex evolved, following Darwin’s theory of evolution.

While the authors did not rule out the possibility that the variations were caused by atypical sexual dimorphism or extreme individual differences, they defended that the differences in robustness were much greater than those observed in large species of dinosaurs. Sanz noted that while the authors did well to include arguments against their claim as well as those in favor, “I think that some of those problems, such as the small size of the sample, is really one of the main issues, as they themselves admit.”


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