In March 2019, a Tyrannosaurus rex made headlines 66 million years after it had died. Was its skeleton the largest of its kind ever discovered?
Dubbed "Scotty", the skeleton had been discovered in Saskatchewan, Canada. In fact, these bones had been unearthed decades before. They were found in 1991, by then-school teacher Robert Gebhardt, but were so deeply encased in sandstone that it has taken decades to painstakingly remove them.
Until now, the largest T. rex skeleton known to science was that of "Sue". It was uncovered in South Dakota, USA, on 12 August 1990, by explorer and fossil collector Sue Hendrickson, after whom it was named.
On 4 October 1997, Sue's skeleton sold at auction for $8.3 million (£5.1 million) to The Field Museum in Chicago, Illinois, USA, becoming the most expensive dinosaur bones.
So how do Scotty and Sue size up?
Unfortunately, it's practically impossible to make a direct comparison, as the two specimens are not equally whole. Sue is approximately 90% complete, compared to 65% for Scotty – indeed, Sue is the most complete T. rex skeleton – comprising 250 of the 380 bones that the body would have featured. That also makes it possible to calculate this dinosaur's original size – 12.5 m (41 ft) long and 4 m (13 ft) tall at the hip – fairly precisely.
From what we can tell, Scotty was probably slightly longer – perhaps up to 13 m (42 ft 7 in), according to the University of Alberta's Dr W Scott Persons, who led a study into calculating Scotty's dimensions.
But scientists always allow a margin of error when comparing such ancient skeletons, and it's likely that it may not have been demonstrably longer or taller than Sue. The research estimates that Scotty weighed in the region of 8,870 kg (19,555 lb), around 410 kg (900 lb) heavier than Sue, which would make it the most massive T. rex ever discovered. (That's around two-and-a-half times the weight of a white rhino – the largest rhinoceros.)
But again, by factoring in a degree of scientific leeway, Scotty may not have outweighed Sue by a significant amount. Longer dinosaurs weren't always heavier than shorter ones.
"There are many different approaches to estimating dinosaur size," Dr Persons explains. "You could try making a scale model of what you think the dinosaur looked like in the flesh, directly calculate the mass of your model, and then scale your calculation up. You could create a 3D scan of the dinosaur's entire skeleton and model the flesh over it.
"But that technique only works if you have a very complete skeleton (better for Sue than Scotty). Both of these approaches involve many assumptions about what the missing flesh would have been like."
An alternative approach, and one adapted by Dr Persons and his team, is to estimate the animal's size based on the leg bones. "The legs of T. rex were the pillars that held the mighty dinosaur up. It makes sense that the girth of those pillars would correlate with the amount of weight they were adapted to support.
"Based on the strength of Scotty's leg bones, we have calculated the dinosaur's weight at roughly 8,800 kg [19,400 lb]. But take that mass lightly, because such leg-based estimations are not an exact science. Perhaps Tyrannosaurus rex put extra pressure on its legs, because it frequently chased after fast prey. If so, its leg bones may be evolutionarily overengineered. So, our number could be off by a few tons!"
Until palaeontologists reach a definitive and universally accepted decision, then, Guinness World Records (GWR) believes that Scotty and Sue should jointly share the title of largest T. rex skeleton.
Dr Persons was delighted when we contacted him about the new joint record, adding: "I hope the honour will draw attention to the very cool work being done in the fossil-rich badlands of Saskatchewan. Excavating, cleaning and studying Scotty's enormous skeleton has been a correspondingly tremendous undertaking.
"I am delighted to have been part of a huge team of researchers, volunteers and expert diggers that has dedicated years towards exhuming the dinosaur."
Scotty and Sue may have male and female nicknames, but in reality scientists struggle to accurately determine dinosaur gender. "Determining the sex of prehistoric mammals is usually much easier," Dr Persons tells us.
"Because most mammals give live birth, females tend to have diagnostically wider hips. But dinosaurs seem to have all been egg layers. Big dinosaurs, like T. rex, laid relatively small eggs, which required hip bones of no greater width or unique form."
Recently, palaeontologists have experimented with a different sexing technique, based on mother birds. When they're preparing themselves to produce eggs – the shells of which will need calcium – they produce a calcium-rich layer of bone ("medullary bone") inside thicker parts of their skeleton, such as the legs. None was found in Scotty.
"Now, this does not prove that Scotty was a 'him'," acknowledges Dr Persons. "Unfortunately, medullary bone doesn't stick around for very long. It is only present at and near the time of egg laying. So, while the presence of medullary bone would prove that a dinosaur is female (because only females ever produce eggs), its absence only proves that the dinosaur wasn't pregnant when it died.
"Scotty's gender identity remains ambiguous. And I'm cool with that."
Find out about more record-breaking animals in our records showcase
The name Tyrannosaurus rex literally means "tyrant lizard king", a reference to this apex predator's fearsome reputation. One of the largest prehistoric carnivores, it lived during the Late Cretaceous period, 67–65 million years ago.
T. rex mainly populated forests and river plains in prehistoric North America, although in 2012 fossils of one of its ancestors, the feathered Yutyrannus huali, were discovered in north-eastern China.
Recent research into the size and weight of T. rex suggests that it moved less speedily than previously thought, walking rather than running at around 19 km/h (12 mph) – although its prey was usually even slower. So that classic scene from Jurassic Park (USA, 1993) when the jeep is being chased probably had longer to get away from the pursuing dinosaur than we once thought!
By contrast, the fastest dinosaur was the ostrich-like Gallimimus, which scientists believe could maintain speeds of 60 km/h (35 mph). It would comfortably outpace Jamaica's Usain Bolt, who reached a top speed of 44.16 km/h (27.44 mph) during his record-breaking fastest 100 m run in 2009.
T. rex had up to 60 conical, serrated teeth, each about the size of a banana, and could bite with a force of up to 57,000 N (12,814 lb/f) – the strongest land-animal bite ever. To put that in context, it's five times greater than the canine bite of a saltwater crocodile (the strongest caniniform bite force for a crocodile today).
The "tyrant lizard king" mostly hunted herbivorous dinosaurs such as Triceratops and Edmontosaurus, although one study from 2010 also suggested that it willingly fed on its own kind as well. Given the need to eat whenever possible, it probably hunted live prey but also scavenged too. But there’s much that is still unclear about its dining habits – for example, whether it was a lone hunter or attacked in packs.
While it may better known than most of its prehistoric peers, T. rex wasn't the largest carnivorous dinosaur. That title goes to Spinosaurus. Analysis of skull fragments suggests that it grew to 17 m (56 ft) in length and weighed up to 9 tonnes (19,850 lb). Indeed, Spinosaurus may well have been the largest terrestrial predator ever known.
It would never have had a prehistoric face-off with the tyrant lizard king, though: by the time T. rex was stomping the Earth, Spinosaurus had been extinct for 10 million years.
In evolutionary terms, T. rex didn't have long left either, though. Around 65.5 million years ago, a massive extinction event abruptly wiped out all the dinosaurs (except for the birds) along with about half of all animal species.
Scotty and Sue must have been remarkably strong, and resilient, to live as long as they did. Both specimens were of a similar age when they died, although Dr Persons believes that Scotty edges it as the oldest known T. rex, perhaps having reached 30-plus years old. That makes them old for their species.
Dinosaurs were constantly engaged in an often-violent struggle for survival and many didn't get past their first year. The bones of Scotty and Sue bear enough teeth marks to suggest that they'd each weathered plenty of attacks during their long lives. Scotty had poorly healed ribs, an infected jaw and possibly a tail bite from another T. rex.
It's unlikely that those injuries killed him, though. "They are old scars and all from battles the T. rex survived," Dr Persons says. "I cannot say what killed Scotty, that remains a mystery. Although I can tell you that Scotty's skeleton records none of the bite marks that would have been left by other carnivorous dinosaurs munching and gnawing on its bones. In the end, Scotty was not another dinosaur's meal."
To date, some 50 partial T. rex skeletons have been discovered. But until a truly unprecedented specimen is discovered, these two tyrannosaurs remain GWR title holders as the largest of their kind.
So how likely is it that we'll find an even larger, or more complete skeleton?
"Very likely," Dr Persons affirms. "As a species, Tyrannosaurus rex roamed across the whole of western North America, for over a million years. I find it impossible to think that we have been so lucky as to discover the two largest individuals that ever lived. There must be even bigger (though probably just slightly) T. rex skeletons waiting to be found.
"As Scotty illustrates, world records are made to be broken."