Walter Stein was exploring a ranch in Perkins County, South Dakota, in 2014 when he stumbled across a root-covered set of bones that had tumbled out of an eroding hillside. Stein realized he was looking at the horns of a Triceratops, and despite the horns’ weathered condition, he could tell that they belonged to a big one.

The founder of a South Dakotan firm called PaleoAdventures, which digs up fossils for commercial sale, Stein nicknamed the fossil “Big John” after the owner of the ranch where he found it. For six years, he held on to the Triceratops in hopes that a U.S. museum would purchase it—but none came forward. Then, in 2020, he sold the fossil to an