A set of four three-toed footprints preserved in southwestern North Dakota is the first known series of tracks left by an adult Tyrannosaurus rex, scientists say. The 23-foot trackway was found in the Hell Creek Formation, a fossil-rich region known for deposits from the final part of the Cretaceous Period. Researchers from the Denver Museum of Nature and Science and Liverpool John Moores University identified the trail last year and have now documented their findings in a scientific journal. The footprints—two from the dinosaur’s left foot and two from its right—are each about 3 feet long. The distance between successive steps is approximately 13 feet, forming a record of the animal’s movement across the ancient landscape about 66.5 million years ago. Using high-resolution 3D scans, the team recorded the shape and details of each impression and produced physical reproductions. Their analysis indicates that the T. rex was walking at an estimated 3.5 to 4.5 miles per hour, comparable to a brisk human pace. “Bones tell us an enormous amount about these animals, but footprints capture a moment of behavior,” said Tyler Lyson, senior curator of vertebrate paleontology at the Denver museum. “With a trackway, we can actually follow an animal as it moved across the landscape. That is incredibly rare for T. rex.” Individual dinosaur footprints and tracks made by juvenile T. rex or closely related species have been reported previously. But researchers say this is the first identified sequence attributable to an adult T. rex, offering evidence about how one of the most famous predators moved rather than only what its skeleton looked like. T. rex lived approximately 66 million to 68 million years ago and stood about 12 to 13 feet tall. It was among the last non-avian dinosaurs before the mass extinction that ended the Cretaceous Period. Three of the footprints are scheduled to be recovered by helicopter later this year and transported to the Denver museum for preservation and further study. The discovery was made by Kent Hups, a Colorado schoolteacher, longtime museum volunteer and co-author of the research paper. “As a teacher, I’m always telling my students that science starts with curiosity, passion and careful observation,” Hups said. “I never imagined that could lead me to a discovery like this.”