The world's most famous dinosaur, Tyrannosaurus rex, is back in the news again! This time, new research is claiming the big predator wasn't so fast on its feet, which has come as a surprise – and even an outrage! – to those who like to imagine the huge dinosaur racing across the prehistoric landscape.

It's no big mystery why Tyrannosaurus gets so much time in the spotlight. For scientists, it's a uniquely fascinating animal, and for the general public, it's mostly just awesome.

"It was a carnivorous biped the size of an elephant: basically, a reptilian killer whale on legs," said tyrannosaur expert Thomas Holtz Jr. of the University of Maryland in an email. "That kind of animal gets some attention."

T rex skeleton_2017_07_26.jpg
Tyrannosaurus rex skeleton at the Naturalis Biodiversity Center in Leiden, Netherlands. There's no doubt these huge dinosaurs were formidable predators, but just how fast were they? Image: -JvL-/Flickr

It's no wonder palaeontologists have spent decades trying to unravel the question of how this creature got around. Early on, some suggested it might have hit high speeds of over 65kph (40mph), based on the fact that it seemed built to move: the structure of its foot bones makes them particularly efficient for walking or running, and the rear of its skeleton shows the tell-tale scars of the massive muscles that powered its hefty long legs.

But newer research has challenged those big numbers. Using biomechanics – a marriage of biology and physics – scientists have plugged the animal into computer simulations to try to figure out exactly how a tyrannosaur's skeleton, muscles and huge mass factored into its movement. From such models, we can see that it's just plain difficult to move a colossal animal. Biomechanical studies (including one just last week) tend to predict top speeds for T. rex in the range of about 25-40kph (15-25mph).

And then there's the newest study on the block, also published last week, by researchers in the UK and the US – the one that says T. rex couldn't run at all.

This study uses an advanced technique to explore not just the strength of the tyrant king's muscles, but also the strength of its bones. Moving such a big body puts a lot of strain on the legs, and these new calculations suggest the big dinosaur's limbs would not have been able to handle moving at more than a fast walk, which for T. rex clocks in at around 18kph (12mph).

An animation showing a biomechanical model of T. rex moving at a fast walk, the highest speed it could move according to these calculations. Credit: Sellers, W et al. (2017)

Is this the final answer to the question of the big dinosaur's speed? Certainly not. For one thing, this new approach will require more testing and refining, although it is a big step (so to speak) in the right direction.

"[This study] points the way we are going to need to go for more accurate estimations of speeds in fossil animals," said Holtz, who was not involved in this new research. He pointed out – as did the authors of the new study – that there's a lot more to be considered in future work. "Locomotion doesn't come from just one anatomical system," he added. "It is produced by the interaction of the skeleton, muscles, tendons, joints, respiration, circulatory system, and so on."

Some parts of Tyrannosaurus, like the skeleton, are known very well. But the softer parts don't typically fossilise, so scientists need to get creative when trying to estimate the animal's weight or the size of its muscles.

"These things are really quite hard to work out," said David Hone, another tyrannosaur expert at the Queen Mary University of London, who also wasn't involved in this new research, "[but] we can actually do some fairly good ballpark estimates."

And those estimates seem to be getting better over time. We may not have a definitive T. rex speed limit (if there is such a thing), but Hone described over the phone that "we're actually repeatedly getting more or less the same number from multiple different sources … they're all kind of converging on about the same value, which really rather suggests that's probably about right."

So, more and more studies hint that Tyrannosaurus was slower than the fastest humans. Does this mean it was a pathetic predator? According to the experts, not at all!

"Everyone's hung up on speed [but] you don't need to be fast to be a predator," Hone emphasised. "Do you know how many species of predatory snail there are? Or predatory starfish? There are loads!"

Indeed, some of the world's largest predators today, including crocodiles and polar bears, aren't in the habit of running for their food.

"There's way more to it than speed," Hone added. "There's endurance and also there's acceleration, [and] it depends what their behaviour is like."

Holtz also noted that any speed estimate for T. rex "isn't really meaningful without the same data for the other dinosaurs" that shared its environment. He wants to see more calculations for the creatures Tyrannosaurus might have called food, such as Edmontosaurus, Triceratops, and so on. If T. rex was hunting big prey, they may have been moving at a similar pace – and there's no need for speed if your food isn't fast!

It's also pretty likely that the hunting habits of history's most famous carnivore changed as it grew. Maybe a young, medium-sized T. rex  (only the size of a rhino) would have made the most use of those running adaptations, while big bad grown-ups took a more patient approach.

These were fascinating animals, with diverse habits, long lives, ancient histories and many more secrets waiting to be uncovered.



Top header image: Garrett Miller/Flickr