Plenty of people have wondered just how fast a Tyrannosaurus rex could run. But that might be the wrong question. People should have been asking if this fearsome dinosaur was able to run at all. In fact, a new analysis suggests, the dino’s leg bones would not have stood up to the stress of a full-on run. Yet this creature could obviously outrace its prey. And scientists now say that if it were alive today, T. rex probably could catch most people, too.
William Sellers is a vertebrate paleontologist at the University of Manchester in England. His team described its new findings online July 17 in PeerJ.
Past studies have used several methods to try to estimate the speed of a T. rex, Sellers notes. Some accounted for the creature’s size, weight and muscle bulk. (Researchers have to guess at the size of a dinosaur’s muscles, since they don’t survive in fossils.) Others have looked at fossil footprints, taking detailed measurements of their size and spacing. Some researchers, including those on Sellers’ team, have even used computers to model, or simulate, the dino’s gait.
Such studies gave mixed results. Their estimates for a T. rex’s top speed ranged from 5 to 20 meters per second. That’s about 18 to 72 kilometers per hour (or 11 to 45 miles per hour). So T. rex topped out somewhere between a car inching its way through a school zone and one barreling down a highway.
Now Sellers and his team have added a new dimension to their computer model. Such models almost always include the dino’s overall size, how its bones were arranged and how strong its muscles likely were. But they have to simplify the dino in digital form. In the model used by Seller’s team, for instance, the T. rex was made of 15 pieces. It had a body, two legs with four parts each and two forelimbs, each with three parts. “Our T. rex model is the most complicated we’ve built, but it’s still pretty simple,” Sellers admits.
His team also included one more aspect that previous studies did not. They added data about the presumed strength of the dino’s leg bones.
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Teaching a digital dino to run
The team ran its simulations on hundreds of computers at once. Altogether, the programs ran for more than 5,000 hours. (That’s equivalent to one computer running nonstop for nearly seven months.) Over that time, the simulations “taught” the virtual dinosaur how to walk and run.
The scientists define walking as a gait in which at least one foot is always on the ground. In a run, both feet are sometimes in the air at once.
Earlier studies had suggested T. rex’s long legs helped give this creature great speed. The longer an animal’s legs, the greater the distance they can cover with each stride. But Sellers’ team also turned up a possible disadvantage to T. rex’s long legs. Leg bones experience certain stresses with the push-off and landing of each stride. And these stresses increase as bones lengthen.
If the new simulations ever suggested those bones were being stressed to the point of damage, it would flag that running speed as too high, Sellers explains. And at every speed, the stresses in a running T. rex’s leg bones were so high that they likely would have been damaged or even broken, the team found. This was especially true for the lower leg bones.
For a walking T. rex, the model found that a safe speed maxed out at just below 28 kilometers per hour (17 miles per hour). That’s a little faster than the top speed that elite marathoners reach.
“I’m glad the team introduced bone strength as a constraint” on T. rex’s top speed, says Eric Snively. He’s a vertebrate paleontologist at the University of Wisconsin-La Crosse. But T. rex’s top speed actually could have been a little bit higher than the new estimates, he adds. That’s because the dino might have used a bouncing style of walking, called “grounded running.”
Many modern-day birds, such as sandpipers and quail, use this style of walking. (Researchers don’t know why, though, because grounded running hasn’t been studied in detail). That gait is difficult to simulate in a computer. There are signs, however, that it may be slightly faster and more efficient than a normal walk.
“This is an interesting and extremely ambitious study,” says John Hutchinson. He’s an evolutionary biologist at the Royal Veterinary College in Hatfield, England. “But it’s a bit soon to accept this conclusion,” he adds. That’s because Sellers’ team has not done similar simulations for living creatures to see if the results match reality, he says.
Such tests, plus studies of walking or jogging creatures (to verify those computer models), might reveal whether the bouncy style of walking actually reduces stresses in leg bones, says Sellers. “It would be neat to think that our modeling of dinosaurs could lead to new insights about living creatures,” he says.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
digital (in computer science and engineering) An adjective indicating that something has been developed numerically on a computer or on some other electronic device, based on a binary system (where all numbers are displayed using a series of only zeros and ones).
dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. For many decades, they have been distinguished by their hips. But a new 2017 analysis now calls into question that characterization of relatedness based on that hip shape.
evolutionary biologist Someone who studies the adaptive processes that have led to the diversity of life on Earth. These scientists can study many different subjects, including the microbiology and genetics of living organisms, how species change to adapt, and the fossil record (to assess how various ancient species are related to each other and to modern-day relatives).
forelimb The arms, wings, fins or legs in what might be thought of as the top half of the body. It’s the opposite of a hindlimb.
gait The pattern of leg motions by which an animal walks from place to place.
insight The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
muscle A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.
prey (n.) Animal species eaten by others. (v.) To attack and eat another species.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
stress (in biology) A factor — such as unusual temperatures, moisture, movements or pollution — that affects the health of a species or ecosystem. (in physics) Pressure or tension exerted on a material object.
Tyrannosaurus rex A top-predator dinosaur that roamed Earth during the late Cretaceous period. Adults could be 12 meters (40 feet) long.
verify (n. verification) To demonstrate or confirm in some way that a particular claim or suspicion is true.
vertebrate paleontology The study of animals in the fossil record that had backbones (or a notochord). Someone who works in this field is a vertebrate paleontologist.
veterinary Having to do with animal medicine or health care.
virtual Being almost like something. An object or concept that is virtually real would be almost true or real — but not quite. The term often is used to refer to something that has been modeled — by or accomplished by — a computer using numbers, not by using real-world parts. So a virtual motor would be one that could be seen on a computer screen and tested by computer programming (but it wouldn’t be a three-dimensional device made from metal).
Journal: W.I. Sellers et al. Investigating the running abilities of Tyrannosaurus rex using stress-constrained multibody dynamic analysis. PeerJ. Published online July 18, 2017. doi:10.7717/peerj.3420.