A 67-million-year-old Tyrannosaurus rex skeleton, nicknamed Gus, sold for $50.1 million at Sotheby's in New York, becoming the most expensive fossil ever auctioned.

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.