Geologist Winston Seiler with some of the dinosaur tracks he identified for his thesis as a University of Utah master's degree student. The impressions once were thought to be potholes eroded by water. But Seiler and Marjorie Chan, chair of geology and geophysics at the University of Utah, published a scientific paper in the October 2008 issue of the journal Palaios identifying the abundant impressions as comprising a large dinosaur "trample surface" in northern Arizona. There are so many tracks they wryly refer to the site as "a dinosaur dance floor."

University of Utah geologist Winston Seiler walks among hundreds of dinosaur footprints in a "trample surface" that likely was a watering hole amid desert sand dunes during the Jurassic Period 190 million years ago. The track site, which also includes some dinosaur tail-drag marks, is located in Coyote Buttes North area along the Arizona-Utah border.

This Eubrontes dinosaur footprint, including three toes and a heel, measures roughly 16 inches long. Dinosaur footprints are named by their shape because the species and genus of animal that made them isn't known, although Eubrontes tracks are believed to have been made by upright-walking, meat-eaters smaller than Tyrannosaurus rex. Eubrontes is one of four types of dinosaur footprints identified by University of Utah geologists at a Jurassic Period dinosaur "trample surface" in northern Arizona. The footprints previously had been thought to be modern potholes eroded by water. The inset outlines the footprint shape.

Photo on left shows eroded dinosaur footprints, and tail-drag marks highlighted in the diagram at right, at a northern Arizona site that University of Utah geologists are calling "a dinosaur dance floor."

This 4-inch long Grallator dinosaur track is among four types of dinosaur footprints identified by University of Utah geologists at a large dinosaur "trample surface" in the Paria Canyon-Vermilion Cliffs Wilderness near the Arizona-Utah border. They were left by a small dinosaur, perhaps only 3 feet tall, some 190 million years ago.

This photo shows a trackway, or set of prints made by the same dinosaur, as it walked through a wet, sandy oasis some 190 million years ago in what is now the Coyote Buttes North area straddling the Utah-Arizona border. University of Utah geologists published a new study showing that numerous impressions at the site are dinosaur tracks, not erosion-caused potholes as was believed previously.
This 14-inch-long Sauropodomorph dinosaur track actually is two footprints in one and was left by a creature that walked on four legs. The imprint includes the deeper central circular portion, which was left when a dinosaur's "pes" or rear foot, stepped into the larger, shallower print left by a "manus" or front foot. The toe prints, top and upper right, were left by the front foot, obscuring prints from the rear toes. The print is one of many identified by University of Utah scientists at a large dinosaur "trample surface" in the Vermilion Cliffs National Monument in northern Arizona.

University of Utah geologist Winston Seiler walks in the path of dinosaurs. The dinosaur tracks were preserved in a "trample surface" where the reptiles likely gathered to drink water at an oasis among arid sand dunes some 190 million years ago. The site is in the Paria Canyon-Vermilion Cliffs Wilderness along the Arizona-Utah border.

The "dinosaur dance floor," formally known as a dinosaur "trample surface," is outlined by white dashes in this photo taken from a hill above the three-quarter-acre site. The site's numerous holes in Jurassic sandstone were identified as dinosaur tracks by University of Utah geologists Marjorie Chan and Winston Seiler.

A dinosaur trample surface has been identified on the Arizona side of that state's border with Utah. Geologists from the University of Utah determined the numerous impressions at the site are dinosaur tracks, not erosion features.