Tritylodonts are herbivores related to the medium-sized mammals that roamed the deserts of the National Park Service at Lake Powell 200 million years ago. Scientists have made one of this year's most important vertebrate discoveries in the US. They've discovered what they call extremely rare fossils.

These findings include the skulls and teeth of dozens of medium-sized, mammalian herbivores called tritylodonts that once roamed the desert. These findings could be a start. Because the decreasing water level of the reservoir reveals new rock layers that may hold clues to understanding the history and evolution of mammals.

Tritylodonts, social animals that resembled rodents, lived together in burrows and fed on stems, leaves and roots, lived at the same time as dinosaurs. Their remains have been discovered in various parts of the world and they survived from the Jurassic to the Cretaceous period.

"The high geological formations surrounding Lake Powell are home to numerous undiscovered fossil remains, making the Glen Canyon national recreation area one of the National Park Service's most important sites for paleontological research," the researchers said.

Eroded millions of years ago by streams, deserts and ancient rivers, this region harbours the remains of one of the largest mass extinctions in Earth's history. After identifying the fossils, scientists had to scramble to recover them in just 120 days before the snowmelt flooded the area again.

Scientists will spend years combing through hundreds of fossil-bearing rocks collected from the site, examining fragments that may show how some animals survived in conditions that caused others to disappear.

These fossils will also be kept at the Prehistoric Museum in Price, Utah.