A paleontologist has discovered a giant footprint most likely left by a towering tyrannosaur as it pounded the Earth 65 million years ago.
The footprint, which measures about 2.5 feet (74 centimeters) in length, was found in rocks in Montana's Hell Creek Formation, a well-known site for Tyrannosaurus rex fossils.
"We are relatively confident that it's been made by a theropod, or predatory dinosaur," said paleontologist Phillip Manning of the University of Manchester in England, who was part of the team that found the print.
Based on the footprint's slender toes, toe positions and overall size, Manning and his colleagues have narrowed down the dinosaur's species name to either T. rex or Nanotyrannus, a tyrannosaur whose name means "tiny tyrant."
"Predatory dinosaurs have much more gracile toes than their dumpy hadrosaur [duck-billed dinosaur] friends that are lolloping around at the end of the Cretaceous," Manning told LiveScience.
T. rex is thought to have grown to about 40 feet (12 meters) in length, while Nanotyrannus was likely just about 17 feet (5 meters) long. Both dinosaurs lived 67 million to 65 million years ago at the end of the Cretaceous Period.
Of course there's always the possibility that a species of dinosaur that is new to science left the print, Manning said.
A future discovery of a more pristine and unweathered footprint at the site, made by the same species, would also help in identifying the dinosaur that made the latest print.
"The only way you'd know whether an animal had left its footprint 65 million years ago in a specific package of rock would be to find the animal dead in its tracks," Manning said.
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Jeanna served as editor-in-chief of Live Science. Previously, she was an assistant editor at Scholastic's Science World magazine. Jeanna has an English degree from Salisbury University, a master's degree in biogeochemistry and environmental sciences from the University of Maryland, and a graduate science journalism degree from New York University. She has worked as a biologist in Florida, where she monitored wetlands and did field surveys for endangered species. She also received an ocean sciences journalism fellowship from Woods Hole Oceanographic Institution.