As if Tyrannosaurus rex wasn’t terrifying enough already. A skeleton in Canada belonged to a T. rex that was comfortably heavier than any other previously found, making it the largest land predator on record. The discovery means we may have underestimated just how large predatory dinosaurs could grow.
T. rex was one of the last non-bird dinosaurs to evolve, and it has long been considered a contender for the largest ever predatory dinosaur. That case was made even stronger following the discovery of Sue – a 90 per cent complete T. rex skeleton unearthed in 1990.
Sue was described as the largest animal of its kind in the world, but this title now belongs to the Canadian newcomer: Scotty.
“We’re talking about basically a 400-kilogram difference,” says Scott Persons at the University of Alberta, Canada. According to his team’s calculations, Sue weighed an impressive 8460 kilograms, but Scotty tipped the scales at 8870 kilograms. To put it another way, Scotty was a couple of adult male lions heavier than Sue.
Scotty was actually discovered about the same time as Sue, in the early 1990s (it earned its name from a bottle of scotch whisky used by the excavators to toast the find). But because its bones were encased in particularly hard rock, freeing them took decades.
“It wasn’t until now we’ve been able to take a step back and look at the specimen as a whole,” says Persons. “And doing so there’s an oh gosh moment, because the specimen really is enormous.”
Strictly speaking, Scotty is not the longest or tallest T. rex found, but its bones are the heftiest. Persons and his colleagues used a few methods to work out how heavy Scotty would have been, including using the circumference of Scotty’s thigh bones to calculate how much weight the legs were capable of supporting.
The team suspect they know why Scotty was so heavy: its bones suggest it was an exceptionally old individual, probably in its early 30s. T. rex might have reached its maximum body length and height earlier in adulthood, but then bulked up as it became older and mature.
The idea has big implications. Similar meat-eating dinosaurs known as theropods might have followed the same trend, bulking out only after many years.
But given that life for these animals was tough, very few individuals will have reached that full potential, meaning fossils of the oldest and bulkiest theropods will be very rare. In other words, we might be underestimating how big they could grow.
Steve Brusatte at the University of Edinburgh says many palaeontologists have been waiting a long time for a full description of Scotty. “It is probably our best look yet at what one of the largest, oldest, most fully grown adults would have looked like,” he says.
That Scotty seems to be the heaviest T. rex ever found is a reminder that our sample sizes for giant theropods are small, says Brusatte. “New discoveries can reveal even bigger or weirder individuals.”
One mystery is why T. rex could grow to be so large, says Persons. For other gigantic theropods there are mitigating factors that might explain how they survived – for instance, living in water to support their body weight or hunting very slow-moving dinosaurs that weren’t difficult for a heavy predator to catch.
But T. rex lived on dry land and had to hunt relatively fast-moving duck-billed dinosaurs, he says. “So why do tyrannosaurs go down this road of gigantism? We don’t know the answer to that.”
Journal reference: Anatomical Record, DOI: 10.1002/ar.24118
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