During the Late Cretaceous period -- 100 to 66 million ago years ago, just before the dinosaurs went extinct -- all kinds of dinosaurs and mammals inhabited the area.
Researchers continue to be amazed by the diversity and abundance of dinosaur bones found here, along with how well they have been preserved.
A few scattered vertebrae are not enough to identify a previously unknown species. To do that, scientists need many parts of a skeleton and, ideally, specimens from several of the creatures.
"So many of our duck-billed dinosaurs, like this one, are still covered with the impressions of their skin; you can see their scales," said Joe Sertich, dinosaurs curator at the Denver Museum of Nature and Science.
"The mudstone and sandstone of Grand Staircase preserve some of the best quality dinosaur bones you'll see anywhere in the world."
But the land is also rich in minerals such as coal and it is of interest to the tourism industry and ranchers.
Sertich believes the competing interests could co-exist, but taking away protected status opens the door to theft, vandalism, and destruction.
"When you operate a coal mine... many of these fossils are lost forever," Sertich told AFP during a tour of the museum.
The last dinosaur
And while some may think that digging up dinosaur bones is not a priority, scientists say studying how they lived and died provides a better understanding of climate change threatening life today.
"By going back into dinosaur ecosystems, we learn a lot about the world around us right now," Sertich said.
"This is the only way we can learn how evolution works on millions-of-year time scales."
He has been combing the Grand Staircase for fossils for 17 years.
"Being able to find to find new dinosaurs every time you spend one or two weeks out in the field is unlike anything you can do anywhere," the museum curator said.
Sertich pointed to bones from a new species of domed dinosaur that were unearthed just five meters from Trump's shrunken boundary line.
"By preserving a place like this, we have this repository where dinosaurs can always be discovered," Sertich said.