Sunset at Bears Ears National Monument.

President Donald Trump signed an executive order yesterday calling on the Department of the Interior (DOI) to review “all Presidential designations or expansions of designations under the Antiquities Act made since January 1, 1996.” Why would a new president with so much on his plate care about 24 parcels of land and sea that his three immediate predecessors decided to protect permanently?

The answer, not surprisingly, is politics. Opponents of such designations see them as unwanted federal interventions. And that’s why Trump has asked Secretary of the Interior Ryan Zinke to review those decisions, starting with an expanse of land in southeastern Utah surrounding a twin pair of mesas known as Bears Ears. Its designation was one of former President Barack Obama’s last acts in office.

“In December of last year alone, the federal government asserted this power over 1.35 million acres of land in Utah, known as Bears Ears—I’ve heard a lot about Bears Ears, and I hear it’s beautiful—over the profound objections of the citizens of Utah,” Trump said during a signing ceremony at DOI. “The Antiquities Act does not give the federal government unlimited power to lock up millions of acres of land and water, and it’s time we ended this abusive practice,” he added.

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Why do scientists think Bears Ears should be a national monument? And why are some Utahns so angry? Let’s dig in.

What sort of antiquities might it hold?

Bears Ears preserves one of the best records in the United States of the middle to late Triassic, the era of the rise of the dinosaurs. It contains rocks dating to between 240 million and 200 million years old, according to paleontologist Robert Gay, now at the Colorado Canyons Association in Grand Junction, who had spearheaded the push for the designation and talked to ScienceInsider when the monument was created in December 2016. The Triassic was a strange time, Gay notes, with “little tiny, puny things running around getting eaten by everything else out there, giant toilet-headed reptiles, strange plant-eating crocodiles with giant pig snouts. Dinosaurs were a rare and minor component of this ecosystem.”

Right on top of those rocks in Bears Ears, Gay says, are rock layers from the very early Jurassic, with “dinosaurs everywhere. It’s one of the few places in the U.S. where we can directly document that huge faunal turnover.” Archaeologists have also long pushed for the Bears Ears designation, noting that it contains more than 100,000 archaeological sites, including cliff dwellings, rock art, and other structures belonging to Ancestral Pueblo people.