The rat-like creatures ran over prehistoric sand dunes about 190 million years ago. Now their ancient tracks have been discovered in Dinosaur National Monument in eastern Utah.
"Its an amazing find," said Dan Chure,a paleontologist with the National Park Service who discovered the site with George Engelmannof the University of Nebraska, Omaha.
Chure said in a telephone interview Thursday that the pair were exploring an area of the monument for the fourth consecutive summer when they found the tracks on July 8. He said he is hoping to find tiny skeletons, too, on further investigation.
The pair also found the fossilized trail of a scorpion tail near the tracks.
The tracks, which number in the hundreds, were fossilized in the Glen Canyon Formation, sand dunes which covered much of Utah and Wyoming as well as parts of Colorado, northern Arizona, and New Mexico during the Early Jurassic period.
Chure said the find is a testament to the diversity of life in the arid region when the giant dunes, which reached heights of up to several hundred feet, were interrupted by an occasional oasis.
The tracks, discovered in the Utah section of the monument in Uintah County, indicate the little mammals were walking uphill because the heel imprints are more distinct than the toes.
He said mammal tracks are rare in the area but not unknown. Other previously discovered tracks, made by creatures known as Brasilichnium, measure about the size of a quarter. The latest tracks are smaller -- about the size of a dime -- and may have been made by different primitive creature.
"It was almost like a bunch of juveniles running around," Chure said.
One of the challenges is accurately mapping the tracks, some of which are so faint they can only be seen when the light is at a certain angle.
A news release about the find said the paleontologists were excited to discover something besides dinosaurs, and surprised to find so many tracks on the surface.
"A few are so well preserved that individual toe impressions can be seen," Engelmann states in the release.
Mixed among the mammal footprints are other tracks made by larger animals, possibly small dinosaurs.
"This was a time when the ancestors of modern mammals were losing dominance on land to the dinosaurs," Engelmann said. "It's near the beginning of a long time when dinosaurs ruled and our ancestors tried to stay out of their way."