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“Tyrannosaurus rex remains the one true king of the dinosaurs,” said paleontologist Steve Brusatte, study coauthor of the latest analysis and professor at The University of Edinburgh’s School of Geosciences in Scotland, in a news release.

“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.”

The earlier controversial paper, which was published in March, had analyzed the bones and teeth of 37 T. rex specimens. This research suggested that T. rex should have two sibling species – a more slender Tyrannosaurus regina (tyrant lizard queen) and the stouter and toothier Tyrannosaurus imperator (tyrant lizard emperor).

Comparison with ‘living dinosaurs’
In the latest study, the researchers looked at the same fossil T. rex specimens and data from 112 bird species, which the scientists described as “living dinosaurs” because they are the direct descendants of therapods – the group of dinosaurs that T. rex belongs to.

The analysis that argued in favor of three Tyrannosaur species had used a limited number of samples, noncomparable measurements and improper statistical techniques, the authors of the new report said.

“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,” said James Napoli, co-lead author of the new study and a doctoral student at the American Museum of Natural History in New York, in the release.

Gregory Paul, one of the authors of the original study and the author of “The Princeton Field Guide to Dinosaurs” said the critique “went too far in being critical of what was the first serious effort to examine the species of Tyrannosaurus.

“There is something about beloved T. rex that causes people to become agitated to a degree not seen with other paleotaxa. 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, ” he said via email.

He added that the latest research comes across as “paleopropaganda that appears to be structured to defend T. rex, rather than seriously explore the possibilities that fossil specimens of the genus Tyrannosaurus contained the more than one species.”

The researchers involved in the latest paper said it’s still possible that there was more than one species of Tyrannosaur that terrorized Cretaceous North America, but there is not enough available evidence to make that kind of decision based on the current fossil record.

“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 coauthor David Hone, a paleontologist and senior lecturer at Queen Mary University of London.

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Determining what makes one species different from another is a fraught, complicated process even among living animals, the researchers emphasized.

“The boundaries of even living species are very hard to define: For instance, zoologists disagree over the number of living species of giraffe,” said coauthor Thomas Holtz, principal lecturer in vertebrate paleontology at the University of Maryland, College Park, and the Smithsonian Institution National Museum of Natural History in Washington, DC, in the statement.

“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