A group of researchers proposed that Tyrannosaurus, the best-known dinosaur and highest apex predator, actually included three species and not just a single T. rex, based on variations in its femur and teeth among its dozens of fossils.
T. rex, meaning "tyrant lizard king" has been the only recognized species of the genus Tyrannosaurus since dinosaurs were first described in 1905. A genus is a broader grouping of related organisms than species.
A team of three researchers led by Baltimore-based independent paleontologist and paleontologist Gregory Paul said the variation they found in examining some thirty Tyrannosaurus fossils warranted recognition of two additional species: T. imperator, meaning "tyrant lizard emperor." (tyrant lizard emperor) and T. regina, which means "tyrant lizard queen".
"After more than a century of carefully examining all specimens placed into a single species, the first and only analysis found that Tyrannosaurus variation fell outside the dinosaur norm, and was distributed over time in a way that suggests that Darwinian speciation from one ( species) into two new species had occurred before the last dinosaur extinction cut further evolution," Paul said.
Tyrannosaurs roamed western North America during the Cretaceous Period in the twilight of the dinosaur age before an asteroid hit Mexico's Yucatan peninsula 66 million years ago, destroying the dinosaurs.
Paul and his colleagues cited differences in robustness, some larger and others lighter, than the femur, or femur, and differences in the number of small teeth at the tip of the lower jaw between the fossils examined.
"There were concerns that this would be controversial because of the charismatic status of T. rex, but on the other hand this study would not have received much attention otherwise," said Paul, whose research was published in the journal Evolutionary Biology.
Paul's fears that his research would lead to controversy turned out to be true. Several paleontologists who were not part of the study disagree with the conclusions.
"Ultimately, to me, these variations are very small and do not represent any meaningful biological separation of the different species that can be defined on the basis of clear, explicit and consistent differences," said University of Edinburgh paleontologist Steve Brusatte.
"It's hard to define a species, even for today's animals, and these fossils have no genetic evidence that can test whether there were truly separate populations. Until I see stronger evidence, this is all still T. rex to me. ," added Brusette.
Paul does not rule out that differences between individuals or differences between male and female Tyrannosaurus played a role. But he called this impossible.
Tyrannosaurus had a large head and incredible bite force, walked on two strong legs, and had small arms with only two fingers. Perhaps the largest known Tyrannosaurus is a specimen named Sue at the Field Museum in Chicago, which measures 12.3 meters long and weighs an estimated 9 tons. The new study concludes that Sue was not T. rex but T. imperator.
The scale of the difference between the three proposed Tyrannosaurus species, Paul said, is similar to that between a lion (Panthera leo) and a tiger (Panthera tigris). Lions and tigers are members of the same genus. The Panthera, however, have enough differences that they are recognized as separate species.
Paleontologist Thomas Carr of Carthage College in Wisconsin, conducted a study in 2020 on variations of T. rex. They didn't find evidence of multiple species, so that's different from Paul's new study.
"Perhaps most damning is the fact that the authors were unable to refer some of the very fine skulls to any of the three species," Carr said. "If their species is valid, then more than two traits must identify them: almost every detail, especially in the head, must be distinct," he concluded.