Study Claiming T. rex Was Three Separate Species Receives Rebuttal

Earlier this year, researchers claimed that Tyrannosaurus rex (T. rex) may have been three different dinosaur species, not one. However, the "multiple species" study has now received rebuttal.

Earlier this year, researchers claimed that Tyrannosaurus rex (T. rex), one of the most ferocious predators to ever walk the Earth, may have been three different dinosaur species, not one. The researchers analysed Tyrannosaurus skeletal remains as part of a study, and found physical differences in the femur, other bones, and dental structure across specimens. The differences suggested that T. rex specimens need to be re-categorised into three distinct groups or species. 

However, the study claiming T. rex was three species recently received rebuttal. 

The new study refuting the claim was recently published in the journal Evolutionary Biology. Led by palaeontologists at the American Museum of Natural History (AMNH) and Carthage College, the study finds that the earlier proposal lacks sufficient evidence to split up the iconic species, a statement released by AMNH said. 

What The “Multiple Species” Study Claimed

The study claiming that T. rex was three different species was also published in Evolutionary Biology. The researchers had nominated two potential new species of Tyrannosaurus based on their analysis. According to the study, the first species, Tyrannosaurus imperator (tyrant lizard emperor), is characterised by more robust femurs and usually two incisor teeth compared to the other species.

The second species, Tyrannosaurus regina (tyrant lizard queen), is characterised by slender femurs and one incisor tooth. 

The study said that the recognised Tyrannosaurus rex (tyrant lizard king) had more robust femurs but only one incisor tooth. The researchers concluded that the physical variation found in Tyrannosaurus specimens combined with their stratigraphy (scientific discipline concerned with the study of rock layers) are indicative of three potential groups that could be nominated as two new species, the bulkier "T. imperator" and the slimmer "T. regina", alongside the only recognised species to date, the standard T. rex. 

However, the rebuttal to the "multiple species" study finds that the proposal does not have enough evidence to hold. 

What Does The Rebuttal Study Say?

In the AMNH statement, Steve Brusatte, a co-author of the new study, said: "Tyrannosaurus rex remains the one true king of the dinosaurs". Brusatte, a palaeontologist at the University of Edinburgh, also said that it is true that the fossils of T. rex are somewhat variable in size and shape. But researchers have shown in the new study that the variation is minor and cannot be used to neatly separate the fossils into easily defined clusters, he added. 

Brusatte said that based on all the fossil evidence researchers currently have, T. rex stands alone as the single giant apex predator from the end of the Age of Dinosaurs in North America. 

The researchers involved in the new study revisited the data presented in the earlier paper and added data points from 112 species of living dinosaurs — birds — and from four non-avian theropod dinosaurs. Theropods included all the flesh-eating dinosaurs, and were characterised by hollow bones and three-toes and claws on each limb.

The researchers found that the multiple species argument was based on a limited comparative sample, non-comparable measurements, and improper statistical techniques.

The "multiple species" study was based on an analysis of the leg bones and teeth of 38 T. rex specimens. 

James Napoli, co-lead author of the rebuttal study, said the "multiple species" study claimed that the variation in T. rex specimens was so high that they were probably from multiple closely related species of giant meat-eating dinosaur. He added that the claim was based on a very small comparative sample. When compared to data from hundreds of living birds, the authors of the new study found that T. rex is less variable than most living theropod dinosaurs, Napoli added. 

The "multiple species" study stated that the robustness of the femur, and the variation in the size of the second tooth of the lower jaw indicated the presence of multiple species.

The researchers involved in the new study could not replicate the tooth for findings. Also, the authors obtained different results from their own measurements of the same specimens. 

Thomas Holtz, a co-author on the new paper, said the boundaries of even living species are very hard to define, and that it becomes much more difficult when the species involved are ancient and only known from a fairly small number of specimens. He added that other sources of variation including changes with growth, with region, and good old-fashioned individual differences have to be rejected before one accepts the hypothesis that two sets of specimens are separate species.  

Thomas Carr, co-lead author on the paper, said the new study shows that rigorous statistical analyses that are grounded in our knowledge of living animals is the best way to clarify the boundaries of extinct species. He said that in practical terms, the three-species model is so poorly defined that many excellent specimens cannot be identified. 

David Hone, a co-author on the paper, said T. rex is an iconic special and an incredibly important one for both palaeontological research and communicating to the public about science. He stated that there is still a good chance that there is more than one species of Tyrannosaurus out there, but strong evidence is needed to make that kind of decision.