Crafted into an open-mouth pose, the T. rex skeleton measuring 11.6 meters long (38 feet long) and 3.9 meters high (12.8 feet) high came in under the anticipated range of 5 million to 8 million francs when it went under the hammer at the Koller auction house in Zurich.

Koller had said Tuesday's sale would be the first time such a T. rex skeleton would go up for auction in Europe. The composite skeleton was a showpiece of an auction that featured some 70 lots, and the skull was set up next to the auctioneer's podium throughout.

"It could be that it was a composite—that could be why the purists didn't go for it," Karl Green, the auction house's marketing director, said by phone. "It's a fair price for the dino. I hope it's going to be shown somewhere in public."

Green did not immediately identify the buyer. Including the "buyer's premium" and fees, the sale came to 5.5 million Swiss francs (about $6.1 million), Koller said.

Promoters said the composite T. rex, dubbed "Trinity," was built from specimens retrieved from three sites in the Hell Creek and Lance Creek formations of Montana and Wyoming between 2008 and 2013.