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New study refutes the claim that T. rex was multiple species

The first fossilized skeleton of a Tyrannosaurus rex was discovered in 1902, in Montana’s Hell Creek Formation. Subsequently, similar finds were few and far between, with only another three specimens being excavated by 1980. However, since then, more than two dozen additional specimens have been uncovered. As the sample has grown, scientists have identified some variation between specimens that has become the subject of discussion and further investigation. 

Some scientists have asserted that the variations in tooth and femur measurements of 38 Tyrannosaurus specimens are distinct enough to represent three separate species. These experts proposed naming the two new species T. regina and T. imperator. However, in a rebuttal of this provocative hypothesis, paleontologists from the American Museum of Natural History and Carthage College reanalyze the fossil specimens and find that the variations fall well within what can be expected from different individuals in a single species, that of T. rex. 

“Recently, a bold theory was announced, to much fanfare: what we call T. rex was actually multiple species,” explained Steve Brusatte, co-author of the paper refuting these claims. “It is true that the fossils we have are somewhat variable in size and shape, but as we show in our new study, that variation is minor and cannot be used to neatly separate the fossils into easily defined clusters.” 

“Based on all the fossil evidence we currently have, T. rex stands alone as the single giant apex predator from the end of the Age of Dinosaurs in North America,” said Brusatte, who is currently a paleontologist at the University of Edinburgh but conducted his Ph.D. work at the Museum. “Tyrannosaurus rex remains the one true king of the dinosaurs,” he said.

The original paper, published in the journal Evolutionary Biology in March 2022, asserted that variation in the size of the second tooth in the lower jaw, and in the robustness of the femur in T. rex specimens, indicated the presence of multiple species. The authors of the rebuttal paper, published today in the same journal, took their own measurements of the same specimens and found different results. They also analyzed data on variation from 112 species of birds – which are all descended from theropod dinosaurs – in addition to data from four non-avian theropod dinosaurs.

“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 authors of the new study also took issue with how the different groupings for the three putative species were determined statistically, using the tooth and femur measurements. In the original study, the statistical analysis defined the number of putative groups before the test was run, which is not an acceptable method for testing the hypothesis, according to the authors of the new study. In their new analysis, they used a different statistical technique to determine how many clusters exist within the data, and did not start out with any advanced assumptions. They found that the measurement data are best considered as a single group, representing only one species – T. rex.

“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.”

T. rex is an iconic species and an incredibly important one for both paleontological research and communicating to the public about science, so it’s important that we get this right,” said co-author David Hone from Queen Mary University of London. “There is still a good chance that there is more than one species of Tyranosaurus out there, but we need strong evidence to make that kind of decision.”

According to Brusatte, the results of the new study confirm that “Tyrannosaurus rex remains the one true king of the dinosaurs.” 

By Alison Bosman, Staff Writer

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