King of the dinosaurs, Tyrannosaurus rex might be one species, not three, after all

Is the world’s most famous dinosaur really one species? This question is becoming one of the most hotly debated topics in palaeontology.

Earlier this year, research suggested that Tyrannosaurus rex fossils might actually come from three different species. Now, a new study published by palaeontologists from the American Museum of Natural History and Carthage Collge in Evolutionary Biology refutes this claim.

It is not surprising that classifying animals that lived over 65 million years ago from just their fossilised bones is not that easy. Even animals today can throw up some skeletal doozies. It takes a real expert, to tell the skeleton of a lion apart from that of a tiger, for example (the distinguishing feature, by the way, is the slightly flatter, more upturned skull in lions).

“Tyrannosaurus rex remains the one true king of the dinosaurs,” says co-author Steve Brusatte, paleontologist at the University of Edinburgh. “Recently, a bold theory was announced to much fanfare: what we call T. rex was actually multiple species. 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.”

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Authors of the March paper suggesting T. rex is actually three separate species reclassified the animal based on the leg bones and teeth of 38 specimens. They believe it is more accurate to group these individuals into the species: the standard T. rex, the bulkier “T. imperator,” and the slimmer “T. regina.”

The new study’s authors revisited the data and added information gathered from 112 bird species (also known as living dinosaurs) and four non-avian extinct theropod (two-legged) dinosaur.

They argue that the multiple species argument breaks down due to limited comparative samples, non-comparative measurements and inadequate statistical techniques.

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

“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.”

According to the March paper, variation in the size of the second tooth in the lower jaw and femur robustness indicated multiple species. The new study, however, could not replicate these findings and revealed differing measurements, as well as noting issues in the statistical methods of the study published in March which made assumptions about the number of T. rex groups before running tests.

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“The boundaries of even living species are very hard to define: for instance, zoologists disagree over the number of living species of giraffe,” explains 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,” says co-author David Hone, from Queen Mary University of London. “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.”