"I was perplexed by what I was seeing," Seiler recalled. "There were these mounds that were pushed up around each of the holes. The mounds would have to form when the sand was still soft. I saw patterns that were repeating themselves. There was this lemon-drop impression that didn't fit in with potholes."
His intuition suddenly told him the depressions were not erosion-caused potholes, common in sandstone, but rather impressions left by hordes of dinosaurs. Two years of further study corroborated his hunch, documenting what may have been a veritable "dinosaur dance floor" recorded in the three-quarter-acre site, according to findings Seiler published this month in the paleontology journal Palaois.
This dinosaur "trample surface" is different than another recent find near Coral Pink Sand Dunes, although both were found in 190 million-year-old Navajo sandstone. This formation, hundreds of feet deep in places, is comprised of iron-rich windblown sand, deposited when a vast desert covered the region. At the time, this region occupied tropical latitudes, approximately 10 degrees north.
Seiler documented four different types of prints at the site, indicating that both fleet-footed three-toed carnivores and lumbering sauropod plant eaters of varying ages crossed this location.
"It tells us about social behavior, who is dancing together," said co-author Marjorie Chan, the U. geology professor who first saw the holes in 2005. She had directed Seiler to the spot to gather data for her work measuring the redness of some of the most colorful rock in canyon country.
Of further intrigue are sinuous drag marks apparently left by the dinosaurs' tails.
"We think of tails as a counterbalance to the upper part of their body, so they are not usually dragging their tails," said Seiler, now an oil company geologist in California. "It could have something to do with the environment."
The geologists believe the site was part of a soggy network of oases in a sea of sand dunes, indicating a gathering place for dinosaurs in an otherwise uninhabitable landscape. The sand was not only wet enough to record footprints, but was easier to walk on and may indicate the presence of algae, said Jim Kirkland, a paleontologist with the Utah Geological Survey.
"Here is the biggest desert we have in the world's history. We have these stacks of sand and here's the surface that's just trampled by dinosaurs," said Kirkland, whom Seiler consulted for his study. "Because these tracks are preserved it shows there was a high water table."
The dense patchwork of prints averages 12 per square meter. The bottoms of the holes tilt in one direction, suggesting the ancient reptiles were moving in a south to southwest direction, perpendicular to the wind, whose direction is recorded in the formation's striations, according to Chan.
"You're looking for anything that looks different. As sedimentary geologists we are already focusing on details that give clues about the environment," said Chan, who chairs the U.'s department of geology and geophysics.
"It's pieces like these that add to the puzzle, that will help us understand how and why dinosaurs lived in this desert," she added. "That's the serendipity of science. That's what makes it fun."
