Not Just 'Rex', Scientists Propose Tyranosaurus Has Three Species

The Tyranosaurus Rex. (Wikimedia Commons/ScottRobertAnselmo)

JAKARTA - A group of researchers have 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 dozens of its fossils.

T. rex, meaning 'king lizard tyrant', 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 on Monday said the variation they found in examining some three dozen Tyrannosaurus fossils warranted recognition of two additional species: T. imperator, meaning T. imperator. 'tyrant lizard emperor,' and T. regina, meaning 'tyrant lizard queen'.

"After more than a century of carefully vetted and problem-free specimens of all specimens placed into a single species, the first and only analysis finding Tyrannosaurus variation falls outside the dinosaur norm, and is distributed over time in a way that suggests, Darwinian speciation. from one (species) to two new species has occurred before the last dinosaur extinction cut off 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 among the fossils examined.

"It is a concern that this will 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'.

Tyranosaurus Rex illustration. (Wikimedia Commons/Zissoudisctrucker)

Paul is right about the controversy. 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 pinpoint 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, and that's what I'm going to call them," Brusatte added.

Paul doesn't rule out differences between individuals or differences between male and female Tyrannosaurus at play, but calls this unlikely.

It is known, Tyrannosaurus had a large head and extraordinary 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 was 40-1/2 feet (12.3 meters) long and an estimated 9 tons. The new study concludes that Sue is not a T. rex but an imperator T.

The scale of the differences between the three proposed Tyrannosaurus species, Paul said, is similar to the differences between the lion, scientific name Panthera leo, and tiger, scientific name Panthera tigris. Lions and tigers are members of the same genus, Panthera, but have enough differences that they are recognized as separate species.

Meanwhile, paleontologist Thomas Carr of Carthage College in Wisconsin, whose 2020 study of variations in T. rex found no evidence of multiple species, is also different from the new study.

"Perhaps most damning is the fact 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.