A Tyrannosaurus rex fossil billed as one of the world’s largest and most complete specimens has sold for a record $50.1 million at an auction.

Private buyers are increasingly outbidding museums for fossils. That’s making it difficult—or even impossible—for researchers to improve our understanding of the past.

A Tyrannosaurus rex skeleton from South Dakota, nicknamed Gus, has become the world’s most expensive fossil sold at auction.