Study Finds T. rex Should Remain One Species

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Mounted Tyrannosaurus rex fossil skeleton. Paleontologists originally thought that T. rex had three claws, but later determined that it only had two.
R. Mickens/©AMNH

Earlier this year, a group of researchers made a provocative claim: Tyrannosaurus rex should be split into three different species. Now, a group of paleontologists led by the Museum and Carthage College in Wisconsin have published a rebuttal to the “multiple species” study, finding that the proposal does not have enough evidence to hold.

 “Tyrannosaurus rex remains the one true king of the dinosaurs,” said study co-author Steve Brusatte, a paleontologist at the University of Edinburgh who conducted his Ph.D. work at the Museum.

In March 2022, a study published in Evolutionary Biology made the case that the iconic T. rex should be reclassified as three species: the standard T. rex, the bulkier “T. imperator,” and the slimmer “T. regina.” The study was based on analysis of the leg bones and teeth of 38 T. rex specimens. 

The authors of the new study revisited the original data and also added measurements from 112 species of birds, which are living dinosaurs, and from four non-avian theropod dinosaurs. They found that the multiple species argument was based on a limited comparative sample, non-comparable measurements, and improper statistical techniques. They also published their findings in the journal Evolutionary Biology.

“Their 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,” said James Napoli, co-lead author of the rebuttal study and a graduating doctoral student in the Museum’s Richard Gilder Graduate School. “But this claim was based on a very small comparative sample. When compared to data from hundreds of living birds, we actually found that T. rex is less variable than most living theropod dinosaurs. This line of evidence for proposed multiple species doesn’t hold up.”

The original paper declared that variation in the size of the second tooth in the lower jaw, in addition to robustness of the femur, indicated the presence of multiple species.

But the authors of the new study could not replicate the tooth findings, and they recovered different results from their own measurements of the same specimens. In addition, the authors of the new study took issue with how the “breakpoints” for each species using these traits were statistically determined.

“The boundaries of even living species are very hard to define: for instance, zoologists disagree over the number of living species of giraffe,” said co-author Thomas Holtz, from the University of Maryland and the National Museum of Natural History. “It becomes much more difficult when the species involved are ancient and only known from a fairly small number of specimens. Other sources of variation—changes with growth, with region, with sex, and with good old-fashioned individual differences—have to be rejected before one accepts the hypothesis that two sets of specimens are in fact separate species. In our view, that hypothesis is not yet the best explanation.”