T. rex could have had 2 equally terrifying sister species, new research suggests

A new analysis of the bones and teeth of 37 T. rex specimens suggests the dinosaur may have to be grouped into three separate species – with the fearsome predator that lived 90 million at 66 million years old, potentially obtaining two sister species: the Tyrant Lizard Queen and the Tyrant Lizard Emperor.

The study, published in the journal Evolutionary Biology, said it had long been recognized that the “thickness” of adult tyrannosaur skeletons varied widely. This was explained by sex differences – female T. rex may be smaller than their male counterparts. Alternatively, it could be explained by developmental stages or simply by individual variation in traits.

Other differences include its banana-sized teeth – some T. rex jaws have a single D-shaped incisor that is noticeably smaller than the next tooth, while others have two such small teeth D-shaped.

The researchers compared the length and circumference of the femur, or thigh bone, of 24 T. rex specimens. They also measured the base of the teeth or the gap in the jaw to figure out if 12 of the dinosaurs had one or two slender incisors. The conclusion of the study team was that T. rex was not a single, immutable dinosaur, but that it may have had two equally terrifying sister species.

“All three species weighed 6 to 7 tons with similar skulls and bodies. It would be like the difference between being attacked by a lion or a tiger. Not much,” said study author Gregory Paul, author of “The Princeton Field Guide to Dinosaurs”. .”

The differences were “subtle”, similar to how “lion (Panthera leo) and tiger (Panthera tigris) skeletons are difficult to tell apart, even among experts”, he said.

Fossil variation vs distinct species

However, other paleontologists disagreed with the findings.

“I understand the temptation to split T. rex into different species, as there is some variation in the fossil bones we have. But ultimately, to me, this variation is very minor and does not indicate significant biological separation. of distinct species that can be defined on the basis of clear, explicit and consistent differences,” said Steve Brusatte, Professor and Personal Chair of Paleontology and Evolution at the School of Geosciences at the University of Edinburgh. , by email.

“Dividing T. rex into three species based on the measurements of 38 bones just isn’t a strong enough case for me.”

Thomas Carr, associate professor of biology and director of the Carthage Institute of Paleontology at Carthage College in Kenosha, Wisconsin, said the characteristics identified by the researchers “represent meaningless variation, not a biological signal.” A study he is conductingd on the variation of T. rex skeletons published in 2020 did not reveal different species.

‘T. rex for me

The study the authors said that the sturdiest tyrannosaurs in their sample outnumbered the “gracile” or slender ones 2 to 1, and such a large disparity could not be explained by the small sample size, nor by a difference based on gender, which would result in a more equal distribution.

The variation in leg bones was also unrelated to developmental stages, they added, as the sturdier femurs were found in some juvenile dinosaurs at two-thirds the size of an adult, while the bones slender legs have also been found in some adult-sized specimens. .

As for tooth structure, specimens with an incisor tooth were correlated with often thinner leg bones – although they only had leg bone measurements for 12 dinosaurs.

While the authors admitted the data “does not meet ideal evidence” of three separate species, they said their study indicated there were three recognizable “morphotypes” in the Tyrannosaurs they studied.

It was the already famous Tyrannosaurus rex (tyrant lizard king) with robust features and a smaller incisor; another stout dinosaur with two incisors, which the researchers called Tyrannosaurus imperator (tyrant lizard emperor), and a third, more slender tyrannosaurus they named Tyrannosaurus regina (tyrant lizard queen).

“It is expected that these new taxa will be tested and, if necessary, revised accordingly as additional specimens and analyzes are revealed,” the study states.

Placing animals that died out tens of millions of years ago into strictly defined categories is inevitably a challenge, but the authors said they hope their work will frame and guide future research. But Edinburgh University’s Brusatte said he was unconvinced.

“Basically, it goes back to the age-old debate about grouping or dividing when classifying species. Defining a species is difficult, even for animals today, and these fossils have no genetic evidence that can test s there really were separate populations.

“Until I see much stronger evidence, they’re still T. rexes to me, and that’s what I’ll call them.”

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