About 190 million years ago, a desert larger than the Sahara covered much of the area that is now the southwestern U.S. with sands much like the ones shown in this image taken in Capitol Reef National Park. In the image background can be seen the Waterpocket Fold—a 100-mile-long warp in the earth's crust where erosion over millions of years has exposed layers of rocks and fossils. Laurie J. Schmidt
About 190 million years ago, during the Early Jurassic Period, a vast desert larger than the Sahara covered much of what is now Utah, Wyoming, Colorado, New Mexico, Arizona, and Nevada. Given that Jurassic time was the “Age of Dinosaurs,” it’s not surprising that fossil evidence of the great reptiles would show up there now and then. But recently, geologists from the University of Utah uncovered an exceptional find — a large concentration of dinosaur tracks and rare tail-drag marks.
Located in the Coyote Buttes North area of the Vermilion Cliffs National Monument, which straddles the Arizona-Utah border, the three-quarter-acre site has more than 1,000, and maybe even thousands, of dinosaur tracks. The tracks were previously believed to be potholes — circular depressions in sandstone caused by erosion — but because of their physical features and the fact that they were concentrated along only one surface, the scientists realized they were actually dinosaur footprints.
The results of the study were published in the October issue of the international paleontology journal Palaios and authored by Marjorie Chan, professor at the University of Utah, and her graduate student, Winston Seiler.
According to Seiler, the variety of track shapes and sizes indicates that at least four dinosaur species were gathered at what he believes was a watering hole in a desert oasis. “The different-size tracks (one to 20 inches long) may tell us that we are seeing mothers walking around with babies,” he said. The 2.4-inch-wide tail-drag marks, some of which are as long as 24 feet, are particularly noteworthy, as there are fewer than a dozen such sites known worldwide, according to Seiler.
The tracks and tail marks were preserved when shifting sand dunes covered the area where the dinosaurs left their prints. Over millions of years, the dunes became the Navajo Sandstone, which is visible today throughout much of the Colorado Plateau region. More than 60 track sites have been found in Navajo Sandstone, but the density of the newly discovered “trample surface” makes it unique.