A young tyrannosaur that strode across the sand of what’s now Wyoming some 66 million years ago has left us a distinctive legacy — the clearest proof yet of how fast dinosaurs like T. rex really moved.
The footprints are the first known multiple tracks that can be attributed to Tyrannosaurus rex, researchers say, and judging by its stride, the young tyrannosaur probably didn’t walk much faster than we do.
The tracks consist of three distinct prints found in sandstone just north of the town of Glenrock in eastern Wyoming, on the fossil-rich grounds of the Paleon Glenrock Museum.
The fossil trackway was discovered several years ago, but Scott Persons, a graduate student in paleontology at the University of Alberta, worked with researchers at the museum and elsewhere to document and study them for the first time. “At first, it looked like a prehistoric pothole,” Persons said, in a press statement, of the first tyrannosaur print that he saw at Glenrock.
“But soon, I could see the imprints of three big toes, each with sharp claw tips.
“It was so cool, my jaw dropped.”
The prints are about 47 centimeters [18.5 inches] from heel to toe, and at least one left a deep impression, as much as 19 centimeters [7.5 inches] into the ancient sediment.
The marks were left by a large theropod, a two-legged carnivore, the researchers say.
And given that the rocks in which the prints were found date back 66 million years, they must have been made by some type of tyrannosaur, because tyrannosaurids were the only large theropods that lived in that place and time.
Tyrannosaurs and other theropods walked on their toes, leaving tracks smaller than their feet. But even so, the Glenrock prints are on the small side, Persons said.
“The tracks are just a bit too small to belong to a full-grown Tyrannosaurus rex,” he said.
“But they could very well be the tracks of an adolescent T. rex, or they could belong to the closely related, smaller tyrannosaur, Nanotyrannus.”
“We really can’t say which.”
Indeed, many scientists contend that Nanotyrannus — known from only two fossils found in Montana — may actually be the same as Tyrannosaurus, and that its fossils were actually juvenile specimens that were mistaken for a separate genus. [Learn more about the T. rex family tree: “New Tyrannosaur Species, ‘King of Gore,’ Reveals Origins of T. Rex“]
In either case, the fossil tracks provide important new data for experts about the anatomy and physiology of the giant dinos — specifically, the exact measurements of a tyrannosaur’s stride.
“Having a trail of tracks is important,” Persons said.
“With it, you can calculate an estimate of how fast the tyrannosaur was walking.”
Just how quickly — or slowly — tyrannosaurs moved has been a topic of much debate among paleontologists, Persons and his colleagues explained.
The close arrangement of those prints led some scientists to conclude that tyrannosaurs had a short stride and, therefore, were plodding, awkward walkers, moving perhaps as slowly as 1 meter a second [or about 2.25 miles per hour].
But the newly reported tracks at Glenrock tell a different story, the team said.
Based on T. rex (and Nanotyrannus) fossils, an animal with feet the size of the Glenrock specimen probably stood about 2.12 meters [7 feet] at the hip. [See a photo gallery of “magnificent” T. rex fossil recently found in Montana.]
That data, plus the fact that the prints are about 3.33 meters [11 feet] apart, leads the researchers to conclude that the tyrannosaur at Glenrock must have been moving at least 1.24 to 2.23 meters per second [or about 2.8 to 5 miles per hour].
The average human walking speed, by comparison, falls within that range, at about 1.4 meters per second, or 3.1 miles an hour.
So you and I might not have been able to outrun a T. rex going at full tilt, the researchers said.
But the West’s most famous tyrannosaur likely had no problem catching up to its less nimble, more ponderous prey. [Did T. rex eat its own kind?: “T. Rex Fossil Found in Wyoming Reveals Cannibalism Among Tyrannosaurs“]
“The tyrannosaurid track record shows that tyrannosaurid stride lengths and walking speeds fell well within the range of other similarly sized theropods,” the team writes in the paper reporting their findings, “and [it] affirms previous track-based ecological inferences that tyrannosaur stride lengths and walking speeds exceeded those of their presumed hadrosaurid prey.”
Persons and his colleagues report their findings in the journal Cretaceous Research.
Smith, S., Persons, W., & Xing, L. (2016). A tyrannosaur trackway at Glenrock, Lance Formation (Maastrichtian), Wyoming Cretaceous Research, 61, 1-4 DOI: 10.1016/j.cretres.2015.12.020