A man found a stegosaurus skeleton during a stroll on his Colorado property. He could make $6 million from it.

• Jason Cooper discovered a stegosaurus fossil on his Colorado property in May 2022.
• The fossil, named Apex, is about 70% complete and could fetch up to $6 million at auction.

On his 45th birthday in May 2022, Jason Cooper was strolling through his Colorado property when he came across part of a stegosaurus bone sticking out of a rock wall.

He began excavating it, cleaning and assembling the skeleton, and named it Apex.

As a professional commercial fossil hunter, Cooper knew that Apex was an impressive find. The fossil stands about 11 feet tall, spans over 20 feet long, and is about 70% complete, he told BBC News.

But he might have been surprised to learn that Sotheby's believes it could fetch up to $6 million at auction.

Apex is "virtually complete" and well preserved, with 274 possible bone elements mounted on a steel armature, Sotheby's said in a press statement.

According to the auction house's global head of science and popular culture, Cassandra Hatton, it is "one of the best fossils of its kind ever unearthed."

Apex will be auctioned in July during Sotheby's annual "Geek Week" sale. It's estimated to be worth $4-6 million, with the auction house describing it as one of the "most valuable dinosaur fossils ever offered."

The stegosaurus was discovered in the Morrison Formation in Colorado, a stretch of sedimentary rock dating back to the Jurassic period.

The area is a rich source of dinosaur fossils, according to the National Park Service.

BBC News reported that Cooper and his family live on a property with just under 100 acres of land.

According to the BBC, Cooper recorded data about where the fossil was found, covered up the bones, and then began cleaning and reassembling the skeleton using a pneumatic chisel, microscopes, and sand-blasting jets.

Sotheby's said there were no signs of combat or predation-related injuries on the fossil, but there was evidence of arthritis, which suggests the dinosaur lived to an old age.

The upcoming sale is attracting some controversy.

While the skeleton will be on exhibition at Sotheby's galleries in New York before being auctioned, some paleontologists argue it shouldn't be privately sold.

Steve Brusatte, a professor of paleontology at the University of Edinburgh, told CNN that Apex belongs in a museum.

"It is a great shame when a fossil like this, which could educate and rouse the curiosity of so many people, just disappears into the mansion of an oligarch," he told the news outlet.

BBC News reported in 2019 that celebrities like Leonardo DiCaprio, Russell Crowe, and Nicolas Cage are among the famous people who have bought prehistoric remains.