Tracks from half a dozen species of dinosaurs turned up in an area of southern Utah that is popular with recreational riders of all-terrain vehicles (ATVs).
A section the size of a football field has been closed off to protect thousands of three-toed and other tracks, according to the Bureau of Land Management (BLM).
"Some people knew the tracks were out there, but we didn't," BLM spokesperson Larry Crutchfield told the Salt Lake Tribune for a story posted online Thursday. "But most people didn't even know they were riding over dinosaur tracks."
The footprints were laid across dozens of layers of rock and include tracks of a sharp-toothed and clawed carnivore, a three-toed crocodile, and a large plant-eating species.
Southern Utah is known for dinosaur fossils and tracks from the Jurassic period. These animals lived about 190 million years ago in a harsh desert that got intermittent deluges.
"You rarely find herbivores in a desert," said Martin Lockley, who heads the Dinosaur Tracks Museum at the University of Colorado at Denver.
The new sandstone prints are at least a hundred million years older than the fossils being extracted in nearby Grand Staircase-Escalante National Monument, said paleontologist Andrew Milner.