Kent Hups set out one morning on a research expedition into the Hell Creek Formation, a vast sequence of fossil rich sedimentary rocks across parts of Montana, Wyoming, North Dakota and South Dakota. The formation is packed with Late Cretaceous dinosaur bones, but footprints are much harder to find.

Sediments in the area typically eroded before tracks could be preserved. Yet Hups, a high school science teacher from Northglenn, Colorado, encountered two giant impressions in July 2025 that would lead to the first known trackway left by a T. rex.

“My first reaction was total disbelief,” said Hups, a volunteer research associate at the Denver Museum of Nature & Science. “I sat and looked at it for a period of time before I was convinced it was a track at all.”

The footprints displayed distinct impressions with three prongs that closely resembled other fossilized tracks. To identify the Cretaceous animal responsible, Hups measured each trace fossil from end to end.

Footprints that are longer than they are wide suggest that the animal was probably a meat eater, Hups explained. Each impression measured about 3 feet long and between 2 and 2½ feet wide.

Their shape and immense size pointed toward the largest meat eating dinosaur known to have inhabited the area, Tyrannosaurus rex. At the time, the region was a lush, swampy coastal floodplain.

A study describing the extraordinary footprints was published September 8 in the Journal of Vertebrate Paleontology. Researchers said the tracks preserve evidence of movement from 66.5 million years ago and provide clues about the predator’s behavior and walking stride.

Hups found the first two fossilized prints exposed in the open, with one more eroded than the other. Study coauthor Tyler Lyson, senior curator of vertebrate paleontology at the Denver Museum of Nature & Science, wanted stronger evidence of a repeating pattern.

“Rocks can weather into all different shapes and sizes, you know, some that look like Florida, some that look like Texas, some that look like T. rex footprints,” Lyson said. He marked an X in the dirt where additional prints should have appeared.

Excavation revealed a third footprint and then a fourth. All had three slender toes, with middle digits longer than the others, matching anatomy similar to T. rex feet.

Size also helped narrow the possibilities. Lyson said the only dinosaurs from the Hell Creek Formation large enough to leave such tracks were T. rex and Edmontosaurus, a duck billed dinosaur with very different feet.

The study documented four footprints, but the research team later found a fifth. “So, there’s no doubt in my mind that it’s a trackway that belongs to a T. rex,” Lyson said.

The tracks survived because the dinosaur crossed the right ground at the right time, according to Lyson. Soft mud in what is now North Dakota allowed the impressions to be buried, eventually returned to the surface and exposed through erosion.

Before this discovery, researchers had reported only two isolated footprints attributed to the tyrannosaur. One was discovered by a US Geological Survey geologist in New Mexico and described in 1994, while another from Montana’s Hell Creek Formation was reported in 2008.

Scientists have uncovered only a handful of other dinosaur tracks across the wider formation. Dr. Anthony Romilio, a paleontologist and research associate with the University of Queensland’s Dinosaur Lab who was not involved in the study, called even a poorly preserved example meaningful in this setting.

Multiple prints reveal details that isolated impressions cannot. The trackway provides the first direct evidence of a T. rex stride length, measuring about 13 feet across the four footprints and indicating a walking speed of about 3.5 to 4.5 miles per hour, Lyson said.

Thomas Holtz Jr., a vertebrate paleontologist and principal lecturer at the University of Maryland, College Park, cautioned that this was far below the dinosaur’s maximum speed. Wet soil preserves tracks, while animals move fastest on firm ground, he explained.

The Denver Museum of Nature & Science plans to collect three of the North Dakota prints for further analysis of possible tissue and skin impressions. Publication of the trackway also allows other scientists to examine its measurements and reconstruct the dinosaur’s movement.

Hups hopes the discovery encourages more people to inspect unusual rocks closely. “It is the closest thing that we have to a time machine.”