A rare set of tyrannosaur footprints is giving researchers insight into the walking speed of the prehistoric beasts, and it’s possible that humans might have been able to outrun them. According to the new estimate, Tyrannosaurus rex may have ambled as quickly as 8 kilometers per hour (5 miles per hour), slower than a plodding amateur marathon runner or even a middle-aged power walker.
Fossilized tyrannosaur tracks are rare, even in areas where their skeletal fossils are abundant, says Scott Persons, a vertebrate paleontologist at the University of Alberta, Edmonton in Canada, and lead author of the new study. Well-preserved individual tracks can be used to help identify the size and type of dinosaur that created the imprint. Even rarer sets of footprints, or trackways, can reveal more, says Persons, as the spacing and arrangement of individual footprints can provide insights into dinosaur gaits and walking speeds.
Containing three footprints, the new trackway was found in 66-million-year-old rocks that formed along an ancient shoreline in what is now Wyoming. The first footprint is well preserved, with three toes facing forward and one short, thumblike toe facing rearward. This arrangement marks its maker as a meat-eating theropod dinosaur, Persons says. The only theropods known to have lived in the region at the time—and large enough to have created the 47-centimeter-wide track—were tyrannosaurs. If the trackmaker were the mighty T. rex, it probably would have been an adolescent. The other possibility, says Persons, is a smaller theropod called Nanotyrannus lancensis, which some paleontologists suggest is merely an immature T. rex, as opposed to a separate species.
Whatever species made the track, the calculations reveal that the creature had a “brisk walking speed,” Persons says. To figure out just how fast it was moving, Persons and his team first estimated how high the dinosaur’s hips must have been above the ground, based on the length of the footprint. Using two common formulas, they determined the creature’s hips were likely somewhere between 1.56 and 2.07 meters off the ground. Then, they measured the distance between the footprints and used an equation based on observations of living, walking bipeds to estimate the dinosaur’s walking speed, yielding a result between 4.5 and 8 kilometers per hour (2.8 to 5 miles per hour), they report online this month and in a forthcoming print issue of Cretaceous Research.
The team’s findings “are on par for what little data we have for tyrannosaurs,” says Richard McCrea, a paleontologist at the Peace Region Paleontology Research Centre in Tumbler Ridge, Canada.
The analysis doesn't prove that T. Rex couldn't have gone faster, however. Because trackways are records of single events—one walk along a lakeshore, for example—the odds are that any particular set of footprints doesn’t capture a dinosaur’s peak performance, says Thomas Holtz Jr., a vertebrate paleontologist at the University of Maryland, College Park. Moreover, he notes, the types of sediment that are good for preserving footprints are typically wet and sloppy, not the best surface on which a dinosaur could run full speed. McCrea agrees: “There are as yet no known trackways of running tyrannosaurs, so we don’t know for sure just what their upper speed limit was.”
One previous study of a single footprint of a large tyrannosaur suggests that the beast could have been traveling as fast as 11 kilometers per hour (6.8 miles per hour), says Eric Snively, a vertebrate paleontologist at the University of Wisconsin, La Crosse. That's still a speed that a halfway decent amateur runner could beat. “If you were out walking a juvenile T. rex, you’d be comfortable at a brisk walk,” he says. “If you were walking an adult, you’d be jogging.”