How fast could a T. rex run?

According to the GaitSym simulations done by the UK’s NW-Grid, T-rex was not as fast as everyone believes it to be. Image courtesy Wikimedia Commons.

New grid computing simulations have shown that the smallest dinosaurs, not the biggest, were the fastest. This work is helping palaeontologists understand how dinosaurs moved around and what roles they played in the prehistoric world.

With its sharp teeth and massive jaws, the T. rex(officially known as Tyrannosaurus rex)is the stuff of nightmares. It’s not surprising that scientists are convinced the T. rexwas a carnivorous predator, because of its menacing jaws, but huge teeth don’t tell the whole story. Was it like the modern cheetah, catching its prey in short burst-like sprints? Or was the T. rexa sneaky stalk-and-ambush hunter like the jaguar?

Since we can’t see a real T. rex in action (it disappeared along with the other dinosaurs 65 million years ago), palaeontologists need to look elsewhere to understand its role as a predator and its speed. If zebras were to become extinct, the palaeontologists of the future could probably use living horses or donkeys as comparisons. People looking at dinosaur behavior don’t have that luxury because there is nothing alive today quite like a T. rex. The solution is to create a detailed computer simulation of the animal’s skeleton and muscles.

ISGTW reported last year about computer simulation research done by the Animal Simulation Laboratory at the University of Manchester, UK, that showed carnivores similar to T. rex such as Acrocanthosaurus couldn’t even outrun a bicycle. The computer models stated that this therapod (beast feet) dinosaur had an average running speed of 15 miles per hour (24.5 kilometers per hour) and walked at about 5.5 mph (9 kph). Now, researchers have simulated the running speeds of a number of dinosaurs compared with living animals today. The research suggests the smaller the dinosaur, the faster it was.

Teaching a T. rex how to run

William Sellers and Phillip Manning, two palaeontologists from the University of Manchester, used a programme called GaitSym – a simulation environment that respects the real laws of physics (e.g. gravity, inertia) –  to model the top running speeds of five types of bipedal dinosaur –Compsognathus,Velociraptor, Dilophosaurus, Allosaurus and T. rex. They also modelled three living animals – the ostrich, the emu and humans – with well-known top speeds to use as comparison.

First, they used the information available from known fossils to reconstruct the animal’s locomotive anatomy and to build a 2D musculoskeletal model. The model specifies, for example, where the joints are, where the muscles are, the weight or mass of the thighs, feet and other parts of the animal, alongside the size and properties of its muscles.

Then, they ‘released’ this virtual robot in GaitSym and told it to run as fast as possible. The key to the model is that the palaeontologists didn’t specify which muscle activation sequence the dinosaurs should use. GaitSym looked for the muscle activation pattern that allowed the animal to cover the most ground in a given amount of time. The program experiments with different combinations of muscle activation patterns and searches for an optimum solution.

Patterns that caused the animal to stagger, stumble or fall were abandoned while promising patterns were selected for further investigation. Each individual computation is not complex, but the problem is that GaitSym needs to go through thousands of muscle activation patterns. This makes the work computationally demanding and impractical to complete using a single computer.  Instead, Sellers and Manning accessed the grid computing services provided by the UK’s NW-Grid and used about 170,000 hours of computing time to complete the project in a few months.

And the dinosaur speeding record goes to...

Sellers and Manning reported in their Proceedings of the Royal Society Bpaper that all simulations generated high-quality running gaits (the way locomotion is achieved using limbs) for the seven tested animals. The computer model also assigned top speeds to living animals that are reasonably close (although slightly slower) than what is measured in real life.

They found that the smallest dinosaur was also the fastest: little Compsognathuswas roughly the size of a turkey, but with a top speed of 64 km per hour it could keep up with a racing greyhound. The Velociraptorand the Dilophosaurus(both species responsible for more than a few fatalities in Jurassic Park)had top speeds of about 38 km per hour, just below what a modern elephant can manage.

The T. rexwas the slowest animal in the contest and according to the model could only get up to 29 km per hour. This means that Usain Bolt – the world’s fastest man – could probably outrun it with his 9.58 second 100 metre record at 37.5 km per hour, albeit for a short while at least.

Sense and sensitivity analysis

The GaitSym models show that studying dinosaur locomotion is no longer science fiction. Recent advances in software, processing speeds, the availability of high performance computing clusters and grid computing made it possible to create very detailed simulations.

There is a lot to learn from these models, but palaeontologists are aware that they are only best-estimate representations. Simulations cannot provide definitive answers because there is a fair amount of guesswork involved in deciding which values about muscle size for example, to input into the model. This is a difficult problem to solve. We know roughly what dinosaurs looked like, thanks to skeletons, sometimes exquisitely preserved. But muscles rot away and very rarely make it into the fossil record.

Instead of adding precise input values to the model, palaeontologists performed sensitivity analysis tests, where inputs are tested in a given range to see how they affect the model’s behavior. Sensitivity analysis doesn’t give a definite answer of A or B, but it gives a good idea about which factors influenced the animals' speed the most – which is equally important if the ultimate goal is to understand how dinosaurs lived in their world.

Karl Bates, also from the University of Manchester, used sensitivity analysis tests to take a closer look at the Sellers & Manning results. For this work, which was part of his PhD, he focused on the Allosaurus– a huge carnivore, which lived in the Jurassic period, 100 million years before the T. rex– and accessed the grid computing resources provided by the UK’s National Grid Service.

Bates repeated the model runs described above, but instead of inputting precise values, he analyzed the ranges of five input parameters: muscle contraction velocity; force per unit area (a proxy for muscle mass); muscle fiber length; body weight and centre of mass.

Two specimens of Compsognathus compared in size with each other and to a human. The new research shows that the tiny Compsognathus could keep up with a modern day Greyhound and maybe could even surpass it in some cases. Image courtesy Wikimedia Commons.

The first finding was that body-weight related parameters don’t have much influence on top speed. Changing the total body mass of the Allosaurusto values within 1,100 to 2,300 kilograms has minimal impact on top speed, which ranges between 32.4 and 32.7 kilometers per hour. The position of the center of mass also didn’t seem to have much effect.

What made a real difference were the muscle parameters. Playing with the input values of muscle mass and contraction velocity caused top speed to vary by up to 66% and 42%, respectively.

Sellers and Manning considered that the maximum contraction velocity of the Allosaurus'muscles was 8 per second. But, since he couldn’t know this for sure, Bates tested the model within the 4 to 12 per second range and analysed how this change influenced top speed.

Thinking of top speed in possible ranges, instead of precise values, is very informative and allowed the palaeontologists to interpret their data with a greater degree of confidence.

Even considering the maximum values for all muscle parameters, the Allosaurusmodel was not able to run very fast. In fact, any speed in excess of about 43 km per hour would require extreme adaptations for high speed, including improbable proportions of muscle-to-body weight. There are still many unknowns, but models such as this let us know that the Allosaurusand its cousin T-rex were certainly not the cheetahs of their time, but perhaps more like a jaguar.

This is an edited version of a story that first appeared on egi.eu