At 11 feet tall and more than 20 feet long, the fossil stands out in the scientific world for its massive size and its nearly complete skeleton, which was discovered by a commercial paleontologist in a sprawling rock formation on his private land near the town of Dinosaur, Colo. — yes, Dinosaur — in May 2022.

Named for its prominent size among stegosaurus fossils, Apex is estimated to be up to 161 million years old, according to Sotheby’s auction listing, and is about 30 percent larger than another notable stegosaurus specimen — Sophie — on display at the Museum of Natural History in London.

Sotheby’s said Apex appeared to live a peaceful life; it has “no signs of combat or predation-related injuries,” the listing stated, adding that evidence of arthritis “indicates that it lived to an advanced age.”

Julien Louys, a paleontologist and professor at Griffith University in Queensland, Australia, said the fossil was a “phenomenal discovery” for the scientific community.

“In terms of vertebrates, it’s incredibly rare to get most of the skeleton preserved and probably even more so for big animals like dinosaurs. Elements tend to get lost through the fossilization process and the recovery process,” he said. “But with such a spectacular specimen, it’d be sad to see it not available to both the general population as well as for scientists to look at.”

Past sales of rare fossils to private bidders have drawn significant backlash, particularly when corporations or anonymous collectors pay what some have called “extortion-level prices” for rare specimens. Sotheby’s previously sold a 200-pound Tyrannosaurus rex head to an anonymous buyer for $6 million, though that was well below its estimated value of up to $20 million. In 2018, members of the Society for Vertebrate Paleontology wrote an open letter calling for a Parisian auction house to halt plans to sell a dinosaur fossil advertised to be from a new species. It later sold to an anonymous buyer for more than 2 million euros.

Sotheby’s did not immediately respond to a request for comment.

Diego C. García-Bellido, a senior researcher of paleontology at the South Australian Museum and a professor at Adelaide University in Australia, noted that Sotheby’s listed Apex as an undetermined species of stegosaurus, which he said could mean its species is being kept secret or that scientists haven’t had a chance to properly study it yet. Excavation of the stegosaurus was completed in October.

“We have no chance,” he said. “Science cannot compete with private companies [or wealthy buyers] in terms of getting their hands on a specimen.”

“I know how strapped for cash, in general, museums are. … Five or 6 million dollars is the sort of money that almost no museum in the world can afford to pay,” he said.

However, many institutions would not have the infrastructure to display such a massive skeleton, García-Bellido added, and commercial paleontologists often spend a considerable amount of their own money excavating and preparing a specimen.

“I’m not in the position to criticize a land owner who prepares and puts a lot of work into getting one of these fossils out,” he said, adding: “If it wasn’t for him, you know what? We wouldn’t have known it existed.”