Scientists have published a rebuttal to a claim made earlier this year in a study that fossils classified as the iconic dinosaurTyrannosaurus rex represent three separate species.
The latest research, published on Monday in the journal Evolutionary Biology, finds that the earlier provocative claim lacks sufficient evidence to split up the iconic species.
In the earlier findings, made in February this year, scientists said variations they spotted in an examination of about three dozen Tyrannosaurus fossils warranted the recognition of two additional species – T imperator, meaning “tyrant lizard emperor,” and T regina, meaning “tyrant lizard queen.”
However, scientists behind the new study, including those from the American Museum of Natural History, say while these fossils are somewhat variable in size and shape, the difference is “minor” and “cannot be used to neatly separate the fossils into easily defined clusters.”
“Tyrannosaurus rex remains the one true king of the dinosaurs …. 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,” paleontologist Steve Brusatte, a co-author of the new study from the University of Edinburgh in the UK said.
In the new research, scientists revisited data presented in the earlier study and also added data points from 112 species of living dinosaurs – birds – and from four non-avian theropod dinosaurs that are characterised by hollow bones and three-toes and claws on each limb.
The previous study also cited variation in the size of the second tooth in the lower jaw, in addition to robustness of the femur, as indication of the presence of multiple species.
However, the authors of the new study say they could not replicate the tooth findings, adding that they recovered different results from their own measurements of the same specimens.
They say the argument that T rex could be multiple species was based on a limited comparative sample, “non-comparable measurements, and improper 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,” James Napoli, co-lead author of the rebuttal study, explained.
“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,” he added.
Researchers say their rigorous statistical analysis technique, grounded in the knowledge of living animals, is the “best way to clarify the boundaries of extinct species.”
“The boundaries of even living species are very hard to define: for instance, zoologists disagree over the number of living species of giraffe,” Thomas Holtz, another co-author of the study from the University of Maryland said.
This becomes harder when the species involved are ancient and only known from a fairly small number of specimens, scientists say.
“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,” Dr Holtz said.
While there is still a “good chance” that there could be more than one species of Tyrannosaurus, researchers say “strong evidence” is needed to make this distinction.
“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,” David Hone, another co-author of the study from the Queen Mary University of London said.
