We all knew Tyrannosaurus rex as one of the most formidable dinosaurs to ever walk the Earth — until a study published in the spring suggested a radical idea: could the T-rex we know actually be three different species?
Fast forward to the summer and a new scientific rebuttal has an answer: No.
It claims the controversial study has “insufficient evidence” to dethrone the T-rex.
According to a new assessment of the data, published in Evolutionary Biology on Monday — the same journal that published the first paper in March — the morphological differences between some T-rex bones don’t point to three different species, but show only minor variations of each individual T-rex.
“Tyrannosaurus rex remains the one true king of the dinosaurs,” Steve Brusatte, co-author and a paleontologist at the University of Edinburgh who conducted his Ph.D. work at the American Museum of Natural History, said in a press release.
“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.”
The study published in March presented an idea that could’ve changed how we perceive the world of dinosaurs. After all, out of thousands of dinosaur species, the T-rex — with its massive size, tiny arms, terrifying jaws and commanding presence on a movie screen — has been a standout in our collective imagination.
What researchers proposed was that if you inspected enough fossils purported to belong to T-rexs, there were patterns in the bone structures that pointed to the T-rex actually needing to be reclassified as three species. They argued that there was a T-rex with shorter and thinner femur bones and a single incisor tooth, a T-rex with thicker femurs and two incisor teeth, and a T-rex with thick femurs and one incisor tooth.
They claimed that neither age of the T-rex at death nor sex could account for these bone and teeth differences they were observing across the fossil record.
However, not all scientists were convinced by the data presented.
While the authors of this new study agree that there very well could be multiple species of T-rex, or fossils miscategorized as T-rex that belong to an unrecognized species that existed simultaneously, they wanted to inspect the data presented to see if that theory had actually been proven well.
They also added data from 112 species of birds, known as “living dinosaurs” for their close evolutionary connection, and from four other dinosaurs of a different, non-avian lineage in order to get a better idea of fossil variation.
The original study, they concluded, decided on the multiple species theory out of a limited sample, a lack of proper comparative measurements and improper statistics, and had not proven their case.
“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 and a graduating doctoral student in the Museum’s Richard Gilder Graduate School, said in the release. “But this claim was based on a very small comparative sample.”
He added that when they compared the data to their data on birds, they saw much more variation in the bone structure of well-known bird species across one species itself than had been found in the T-rex fossils.
“This line of evidence for proposed multiple species doesn’t hold up,” he said.
Comparing the data to that of living creatures allowed them to hold a real-world lens up to the idea, scientists said, which can be key when dealing with creatures that no one has ever seen.
“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, co-author of the new study with University of Maryland and the National Museum of Natural History, said in the release. “It becomes much more difficult when the species involved are ancient and only known from a fairly small number of specimens.”
Thomas Carr, another co-author with Carthage College, added in the release that “pinning down variation in long-extinct animals is a major challenge for paleontologists.”
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.”
Another nail in the coffin for the initial study, according to this new piece, is that researchers could not replicate the teeth measurements that the first researchers had recorded.
In fact, when they performed their own measurements on the exact same specimens, they got different numbers which did not support the initial study’s findings.
They also stated that the previous study had defined the end result of three potential species before they ran their statistics. When researchers in the new study ran the statistics on the fossils to determine how best to sort them without determining the parameters beforehand, the stats told them that the fossils belonged to one group: the T-rex.
“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, co-author with Queen Mary University of London, said in the release.
“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.”
