T. rex or Tyrannosaurus rex stands alone as the single giant apex predator, not three separate species, claims a new study published today.

A controversial claim made earlier this year that fossils of the dinosaur Tyrannosaurus rex come from three different species has been debunked by a new study.

The rebuttal, released today in Evolutionary Biology, concludes the prior hypothesis lacks sufficient evidence to separate the iconic species.

Study co-author Steve Brusatte says that “Tyrannosaurus rex remains the one true king of the dinosaurs.” 

Study co-author Steve Brusatte says that “Tyrannosaurus rex remains the one true king of the dinosaurs.”  It was recently revealed, with much fanfare, that the dinosaur known as T. rex actually consisted of several different species. 

It was recently revealed, with much fanfare, that the dinosaur known as T. rex actually consisted of several different species. 

In March 2022, the authors of the controversial study, which was also published in Evolutionary Biology, argued that T. rex should be split into three species: the standard T. rex, the bigger “T. imperator,” and the smaller “T. regina.” The investigation was based on the examination of 38 T. rex specimens’ leg bones and teeth.

The authors of the new study went back and looked at the information in the first paper. They also added information from 112 species of living dinosaurs (birds) and four theropod dinosaurs that were not birds. It was discovered that the numerous species argument was based on a small comparative sample, non-comparable measurements, and inappropriate statistical procedures.

James Napoli, co-lead author of the rebuttal study says that “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.” 

“But this claim, according to the co-lead author, “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.”

“Pinning down variation in long-extinct animals is a major challenge for paleontologists,” adds co-lead author Thomas Carr from Carthage College. “Our 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. In practical terms, the three-species model is so poorly defined that many excellent specimens can’t be identified. That’s a clear warning sign of a hypothesis that doesn’t map onto the real world.”

The original study claimed that in addition to the femur’s toughness, variation in the size of the second tooth in the lower jaw also pointed to the existence of different species. However, the new study’s authors were unable to confirm the tooth findings and found different results from their own measurements of the identical specimens. The authors of the current study also questioned the statistical methodology used to establish the “breakpoints” for each species utilizing these features. According to the authors of the current study, the statistical analysis in the original study defined the number of groups before the test was conducted, so it cannot be used to test the hypothesis. The most recent study employed a different statistical method to count the number of clusters present in the data without making any additional assumptions and discovered that the T. rex species is the best classification for them.

“The boundaries of even living species are very hard to define: for instance, zoologists disagree over the number of living species of giraffe,” adds co-author Thomas Holtz. “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,” adds co-author David Hone. “There is still a good chance that there is more than one species of Tyrannosaurus out there, but we need strong evidence to make that kind of decision.”

Image Credit: Mark Witton 2022

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