The ammonite, formed by millions of years of compression and mineralisation, has an opalescent quality, iridescent as prisms of light diffract off its coiled outer shell. More than 70 million years ago, that shell housed a now-extinct creature, a cephalopod that was a member of the squid family. Found in the Canadian province of Alberta, the whorled fossil, more than 14 inches tall and 16 inches wide, will be for sale at this year’s Masterpiece Online fair by the gallery David Aaron. 

It is not the only prehistoric natural object that will be offered at the sale. ArtAncient, a gallery that deals in antiquities and, increasingly, elements of natural history, is offering a “gogotte,” a sandstone concretion formed roughly 30 million years ago near Fontainebleau, France, when superheated water flowed through tiny grains of sand, fusing them into an intricate formation resembling a sculpture. 

The market for natural history, and especially fossils, has exploded in recent years, even among collectors who usually buy fine art and antiquities. “This is a growth area, because I think people are increasingly interested in diversifying their collections and surrounding themselves with beautiful and historically important objects that are imbued with meaning,” said Salomon Aaron, director of David Aaron. “Natural history is really a very fascinating area of the art market now.” 

Dinosaur skeletons are the hottest commodity in the growing market. The most recent iteration of fossil craze began, perhaps, with Sue, a complete Tyrannosaurus rex skeleton sold in 1997 for $8.36 million. Then there was Stan, another complete T-rex, that sold at auction at Christie’s last year for a record-shattering $31.8 million. James Hyslop, head of science and natural history at Christie’s, said that he had seen the market grow every year since 2007. But it is not entirely new. Hyslop said this kind of periodic enthusiasm had roots in the 19th century. “Throughout the Victorian period, the best fossils were on par with the most expensive paintings in the world.” 

“The market for American fossils is going through the roof, especially when it comes to dinosauria,” said Tom Lindgren, a natural history specialist for Bonhams, which last month held a natural history auction that included a tyrannosaurus tooth that sold for $11,475 and a double butterfly in amber that went for nearly $38,000. “When Christie’s sold the T-rex, it got people champing at the bit, wondering, when is the next Stan going to appear?” he said. “It created a real dinosaur rush, like the gold rush of the late 1800s in California and the West. Now we’re seeing a dinosaur rush in that part of the U.S.” 

The American West is rich in fossils, especially in Wyoming, Montana, Utah and Colorado. And in the US it is legal to sell and export fossils found on private land. Many other countries heavily regulate fossil sales, or forbid exports altogether, something dealers have to be cautious about when establishing provenance. (Fossils have been illegally exported from Mongolia and China and stolen from public lands in the US). But the appeal of owning a fossil is distinct from that of an art object, in part because of the miracle of its age. “It really puts things into perspective, the history of our planet, the nature of evolution, when you’re dealing with things which are 50 million years old,” Aaron said. “It makes you think about things when you celebrate your birthday.”