The Scotty T. rex on display.
Image: Amanda Kelley

Updated measurements of a large fossil found in Saskatchewan nearly 30 years ago confirm it as the world’s largest known Tyrannosaurus rex skeleton. Remarkably, the new work suggests T. rex and other dinosaurs grew to a greater size than is typically appreciated.

New research published last week in The Anatomical Record describes “Scotty,” a T. rex skeleton otherwise known as specimen RSM P2523.8. Scotty is now officially the largest and most aged T. rex ever discovered, and the most gigantic of any of the known two-legged carnivorous dinosaurs known as theropods. At an estimated 19,555 pounds (8,870 kg), it’s also the biggest dinosaur ever discovered in Canada. The new study was led by paleontologist Scott Persons from the University of Alberta.

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Scotty’s skeleton was discovered near Eastend, Saskatchewan in 1991, but work to remove it from the ground didn’t fully start until 1994. It took paleontologists nearly a decade to excavate the fossil because it was encased in compact, cement-like sandstone. The extra effort to excavate the bones, plus the sheer size of the specimen, resulted in further delays. That said, the paleontologists were able to recover around 65 percent of the T. rex specimen, which terrorized Cretaceous Canada some 66 million years ago.

Early attempts to characterize the skeleton between 2008 and 2014 were marred by inaccuracies owing to the fact that the fossil hadn’t been fully prepared for analysis. Consequently, and as Persons pointed out in the new study, Scotty “has never been formally described and its skeletal proportions scientifically quantified.” The new study is now the first to offer detailed and accurate measurements of the skeleton, including a comparative analysis with other known T. rex fossils.

Measurements of Scotty’s legs, hips, and shoulder point to a T. rex of enormous size. Persons and his colleagues relied on an equation developed by other researchers to infer its body mass, using the circumference of the femur, or thigh bone, to glean the amount of weight the legs could reasonably support. In this case, Scotty’s femur suggested a weight of 19,555 pounds, which is nearly 10 tons. At 13 meters in length, Scotty may not have been the tallest or longest T. rex that ever lived—but it was certainly big in terms of mass.

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Analysis of the growth patterns in Scotty’s bones suggests the dinosaur died in its early 30s, making it the most elderly T. rex skeleton ever discovered. Extensive injuries were detected throughout the body, pointing to a particularly brutal life. Scotty had broken ribs, an infected jaw, and bite marks on its tail, the latter of which appeared to be inflicted by a rival T. rex, the new study suggests.

Persons and his colleagues compared Scotty’s skeleton to 11 other well-preserved T. rex skeletons, including Sue—a specimen otherwise known as FMNH PR 2081. Sue’s bones were found in 1990, and it was once considered the biggest T. rex skeleton, a title that now belongs to Scotty. Sue weighed in at 18,651 pounds (8,460 kg), which is around 5 percent lighter than Scotty.

“I’ve been waiting a long time for the description of this awesome T. rex fossil, as have many paleontologists,” Steven Brusatte, a University of Edinburgh paleontologist not involved with the study, told Gizmodo. “It is one of the largest and oldest T. rexes out there, and it gives us a glimpse at just how big T. rex got during those last years of its life.”

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Brusatte said the new study provides good evidence that a 13-meter-long, 7-to-8 ton body plan was “as big as the King got—but that’s pretty big,” adding that T. rex is still the “biggest pure meat-eating animal that has ever lived on land as far as we currently know.”

Indeed, other large theropod species, such as Spinosaurus aegyptiacus, Giganotosaurus carolinii, and Tyrannotitan chubutensis, may have been larger than T. rex, but more fossil evidence is needed for paleontologists to be sure. For now, T. rex remains king.

As an interesting aside, paleontologists have noticed substantial size variations among the T. rex fossils. Some scientists have proposed a classification scheme based on two forms: “gracile” T. rexes with long and slender skeletons and “robust” T. rexes featuring stockier builds and greater bulk. Other paleontologists say these size discrepancies can be explained by individual variation or age. Another possibility is sexual dimorphism, in which females are larger than males. The reason for the hypothesized larger female T. rexes could have something to do with the increased physical demands of having to lay and protect eggs.

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A fascinating conclusion reached in the new study is that the sizes of T. rex and other dinosaurs are being understated by paleontologists. The reason, according to the authors, is that very few dinosaurs managed to survive into full maturity. Consequently, there are more fossils of younger, smaller dinosaurs than there are of bigger, older dinosaurs, resulting in a kind of selection bias. Looking at Scotty’s skeleton, with its wide-ranging display of injuries, life as a dinosaur was clearly tough—even when you’re a T. rex.

John Hutchinson, professor of evolutionary biomechanics at the University of London’s Royal Veterinary College and an expert on T. rex physiology, was a bit blasé about the new study.

“If you read between the lines in past studies and this new one, the Scotty specimen is not appreciably larger than other known specimens—at best, maybe 5 percent larger, and that is with a wide margin of error,” Hutchinson told Gizmodo.

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Which is fair; the margin of error for the weight estimates presented in the study are at a whopping plus-minus 25 percent. What’s more, “some bones are smaller than known skeletons, while others are larger,” said Hutchinson, so “we do not learn something truly major, unambiguous, and new about the size of T. rex from this specimen.”

Paleontologist Thomas Carr from Carthage college, also not affiliated with the new study, said the absolute difference in size between Scotty, Sue, and others may not be great, but the Scotty specimen raises the ceiling for the maximum size of T. rex, which is now higher than previously thought.

“That changes our picture of what is within the range of possibility for these large animals and expands our understanding of the biology of large theropods at that extreme end of the size range,” Carr told Gizmodo. “Without Scotty, our estimate of the ceiling of the maximum size would be inaccurate—Scotty sharpens that picture.”

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[The Anatomical Record]

Correction: A previous version of this article incorrectly converted pounds to tons. We regret the error.