Just north of the town of Glenrock, Wyoming, a stretch of ancient yellow sandstone is studded with a killer’s footprints. About 66 million years ago, a Tyrannosaurus rex strode through the wet sand of what was then an ocean shore. The three massive footprints it left behind are just the second tyrannosaur trackway ever discovered. But there’s far more to these tracks than their rarity, according to a forthcoming study in the June issue of Cretaceous Research.
Paleontologists have long debated how fast and athletic tyrannosaurs could have been. Most known tyrannosaur tracks are single, isolated prints—a snapshot that just tells paleontologists which dinosaur left the track. By contrast, a series of tracks lets paleontologists watch the tyrannosaur stride along, and compare its speed with the estimates for its favorite prey.
But the first time study author Scott Persons saw the tracks, calculating a tyrannosaur’s speed was the furthest thing from his mind. At the time, Persons was just a 13-year-old visiting the Glenrock Paleon Museum, when its curator happened to invite him to the nearby fossil bed to see the 19-inch-long footprints. “Before Glenrock,” Persons tells University of Alberta news staff, “paleontology was [to me] dinosaurs in books and their skeletons in display halls.” Getting to see real tracks in the field made such an impression that, when Persons became a graduate student in paleontology at the University of Alberta, he wanted to go back and formally study the footprints.
Working with museum’s curator, Sean Smith, and Chinese scientist Lida Xing, Persons carefully examined the tracks. Based on their size, the toe pattern, and the gouges left by sharp claws, the paleontologists knew that the tracks could only come from a large theropod like T. rex. The pattern of the prints also showed that the dinosaur had been walking along the muddy shoreline, instead of running.
To calculate how fast the tyrannosaur had been walking, the scientists used two different approaches based off of its stride length and estimated hip height. The first approach used an equation derived from observations of modern animals. The second approach used data from humans, who walk upright on two legs, like tyrannosaurs. Using these approaches, the paleontologists estimated that the tyrannosaurus was walking at 1.24 to 2.23 meters per second—roughly 3 to 5 miles per hour, a slow jog for a human. At this walking speed, the tyrannosaur moved faster and covered more ground with each step than the duck-billed dinosaurs, one of its prey species.
Of course, tyrannosaurs probably didn’t power-walk after their prey. Just how fast could tyrannosaurs run?
So far, scientists don’t know. “There are as yet no known trackways of running tyrannosaurs,” says paleontologist Richard McCrea, who wasn’t involved in the study. Given the rarity of tyrannosaur trackways, it may be some time before paleontologists find the fossilized data they need to make the calculations. In the meantime, the 66-million-year-old tracks are still out in the yellow sandstone of Wyoming, waiting to inspire future paleontologists and dinosaur fans alike. “If you go to Glenrock, visit the Paleon Museum and are up for a little hike,” Persons says, then “you can see the prints just like I did.”
Written by Megan Cartwright
Megan Cartwright is a freelance science/medical writer near Seattle. Before becoming a full-time writer, she worked as a scientist studying infectious diseases and vaccines, and earned her Ph.D. in Toxicology from the University of Washington. Megan has written for Slate and Bitesize Bio, and helps write blog posts and learning modules on chemistry for Visionlearning.