Tyrannosaurus rex suffers from an identity crisis

Illustration of a T. rex feeding. / Mark Whitton

Science | Paleontology

The paleontological community does not agree on whether the remains of the extinct animal belong to a single species or not

Elena Martin Lopez

Is it one species or is it three? Earlier this year, a team of scientists analyzed the leg bones and teeth of 38 Tyrannosaurus Rex (T. rex) specimens and published
A study in the journal Evolutionary Biology in which he argued that the classified fossils associated with that species should be reclassified, since they actually belong to three different species. His suggestion was that, in addition to the standard T. rex, the larger specimens should be attributed to a new species called T. imperator, and the smaller, slimmer ones to another they named T. queen. Currently, the T. Rex is the only recognized species in the genus Tyrannosaurus, so the statement caused a stir among the scientific community and opened the debate. A
new studioled by paleontologists from the American Museum of Natural History (New York) and Carthage College (Wisconsin), and published in the same journal, has refuted the provocative proposal, considering that it lacks sufficient evidence.

“Recently, a bold theory was announced with great fanfare: what we call T. rex was actually divided into three species. It is true that the fossils we do have vary in size and shape but, as we show in our new study, that variation is minor and cannot be used to clearly separate fossils into well-defined groups. Based on all the fossil evidence we currently have, T. rex stands out as the only giant apex predator from the end of the Age of Dinosaurs in North America," said study co-author Steve Brusatte, a palaeontologist at the University of Edinburgh. , who did his doctoral work at the New York museum.

The original article stated that the variation in the size of the second tooth in the lower jaw and the robustness of the femur indicated the presence of multiple species. To argue their refutation, the researchers of the new study reviewed the data presented in the original study and also analyzed the differences between the femurs of 112 species of living birds, the closest living relatives of T. rex. The authors were unable to replicate the tooth findings and obtained different results from measurements of the same samples, so they are of the opinion that the argument for three species of T.rex is based on a limited comparative sample, non-comparable measurements, and inadequate statistical techniques.

It's easy to fall into error

Determining the differences between long-extinct animals is a huge challenge for paleontologists, especially given the lack of DNA samples to analyze. The only thing they have are fossils, and not many, so detecting the variables between one species and another is complex and can easily lead to error. "Even living species boundaries are very difficult to define: for example, zoologists don't fully agree on how many living species of giraffe there are," said co-author Thomas Holtz of the University of Maryland and the University of Maryland. National Museum of Natural History. "This becomes much more complex when the species involved are ancient and only known from a fairly small number of specimens." And that without taking into account other possible sources of variation between individuals of the same species, such as changes in their growth, the influence of where they live or sex.

Specimen from the Museum's temporary exhibit T. rex: The Ultimate Predator, which ran from 2019-2021 at the American Museum of Natural History. /

D. Finnin/AMNH

In fact, one of the things the authors of the new study take issue with from the original paper is that the researchers compared the differences between individual Tyrannosaurus specimens to those of various skeletons of Allosaurus, a theropod dinosaur that lived in the Jurassic period, millions of years before the T-rex. In his opinion, this comparison is "misleading" because the Allosaurus fossils come from a single site in Utah, while the Tyrannosaurus fossils are scattered in several places and these animals lived longer, so it is to be expected that there are more regional and temporal differences in the available dataset for T. rex.

In addition, they denounce that in the original article the number of species that was expected to be found was determined before carrying out the tests, which "is not useful for testing the hypothesis," they say. "Our study shows that rigorous statistical analyzes that build on 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 not many specimens can be identified. That is a clear warning that this is a hypothesis that does not correspond to the real world, "says co-senior author Thomas Carr, of Carthage College.

"There's a good chance there is more than one species of Tyrannosaurus, but we need stronger evidence."

David Hone

Researcher at the University of Mary

For this reason, they consider that it is not the best explanation for this extinct animal. «T. rex is an iconic and incredibly important species, both for paleontological research and for bringing science to the public, so it's important that we get it right. 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," said co-author David Hone, from Queen Mary University of London.

The authors of the first article knew that their statement would generate controversy in the paleontology community, so they are already preparing a new article that expands the evidence on the three species of T. rex, which will lengthen the debate over time.

Tyrannosaurus rex from 1905

Since being named in 1905, the world's most studied dinosaur has kept its nickname. This is a species that lived about 66 or 68 million years ago, in what is now the western side of North America. It could reach up to 12 meters long and 4 meters high. It was carnivorous (it had one of the strongest bites in the animal kingdom) and shared characteristics similar to those of modern birds. There are currently more than 50 fossilized specimens of T. rex collected.

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