VALLEY OF FIRE STATE PARK -- When Steven Theodore and his son, Evan, set out on a hike on a cool, September morning last year, they didn't know they had crossed paths with a bunch of squirrellike animals that had scampered up a sand dune 175 million years ago.
Now, a local paleontologist with cooperation from state parks officials is studying the fossilized tracks found by the Theodores. He says they were left by warm-blooded animals that roamed what is now Nevada during the middle Jurassic period.
"These were the guys running between the legs of the big Jurassic dinosaurs" so to speak, said Steve Rowland, a geosciences professor at the University of Nevada, Las Vegas, whose specialty is paleontology.
"It's certainly an important discovery that helps us learn more about the history of Nevada, so it's significant," Rowland said last week during a visit to the discovery site in a remote part of the 36,000-acre Valley of Fire's State Park, 45 miles northeast of Las Vegas.
On Sept. 29, 2002, Theodore and his teenage son set out for an after-breakfast jaunt through the park's majestic sandstone formations.
"As we were leaning on this one rock, I said, `Evan, is this what it looks like to me.' And he said, `These are footprints,'" said Theodore, 51, a freelance television producer who has lived in the Las Vegas area since 1978.
Evan Theodore, 16, a high school junior, said the find was exciting. "We found out that no other ones had been reported. That was cool."
What they had discovered as the morning sunlight cast shadows across the slanted, Aztec sandstone rock face was a remarkable set of trace fossils, known as "ichnogenus Brasilichnium," Rowland said.
In all, there are nine parallel sets of tracks containing 90 individual fossilized paw prints on two rock slabs. The tracks appear to be heading uphill, based on the depressions. In some cases there are imprints from three-clawed toes, while other traces depict four toes. Some are about the size of a quarter and others about the size of a 50-cent piece.
Their age, Rowland said, is based on the estimated time that sand dunes dominated the area before they hardened into sandstone. The tracks also have the same characteristics of size and shape of those left by creatures from about the same geologic period of confirmed Brasilichnium traces found elsewhere.
It is rare to find so many sets of trace fossil tracks of this type in one location, Rowland said.
The squirrellike creatures that left them probably were desert animals, he said, tramping across what was a sand dune sea that stretched for thousands of square miles.
The tracks are much smaller and from different creatures than the dinosaur tracks found about three years ago at the Johnson farm near St. George, Utah.
The Valley of Fire tracks were left by fur-bearing animals that were common about the time the so-called supercontinent, known as Pangaea, was drifting apart.
As the name "Brasilichnium" implies, they are trace fossils of the same kind as those found in Brazil in 1911 by a mining engineer. Some of those trace fossils are contained in flagstone used for sidewalks in a town 175 miles northwest of the coastal city Sao Paulo.
Similar trace fossils from early mammals have been recorded in only a few other places on the planet, including southern Africa and in Navajo sandstone in southern Utah.
At the time the Valley of Fire footprints were left, North America and South America still were connected and early mammals were evolving as were a now-extinct group of animals called therapsids, sometimes called mammallike reptiles. There is no conclusive way to distinguish mammal tracks from those of therapsids, Rowland said.
Mammals, he noted, first appear in the fossil record about 220 million years ago, or about 45 million years before the Valley of Fire tracks were made.
"These were desert-dwelling creatures, which makes the preservation of their tracks all the more miraculous. There probably had to have been some moisture and then these guys scrambled up the dune.
"Then, I don't know, a dust storm came along. Then more sand dunes on top of that. Then it had to erode away," he said.
The Theodores' discovery adds a new dimension to the Valley of Fire's link to ancient history. The park is home to petrified wood believed to have been carried from higher elevations by floods that deposited them from 250 million to 200 million years ago, according to Rowland and Jim Hammons, park supervisor.
The park also is widely known for rock art, or petroglyphs, etched in sandstone walls centuries or perhaps thousands of years ago by ancestors of American Indians who used spear-throwing devices, known as atlatls, to hunt bighorn sheep.
While the park's fossilized footprints in Aztec sandstone are evidence of early mammal life about 175 million years ago, they are predated by fossilized remains of marine reptiles, boxcar-size fish-lizards, called ichthyosaurs, that swam off the shore of what is now central Nevada more than 200 million years ago.
Fossils from these needle-nosed beasts are preserved in a state park in Nye County east of Gabbs and some 300 miles north of Las Vegas, where Stanford University professor Slemon Muller found them in 1929 in the Shoshone Mountains near the mining ghost town of Berlin.
Hammons said the State Parks Division is developing a policy and plan for managing trace fossils and he hopes molds of the tracks that the Theodores found can be another interpretative display for park visitors although the location of the actual sandstone slab will remain off-limits.
On the Net: Nevada Division of State Parks: http://parks.nv.gov/vf.htm