Nearly 100 years ago, Mongolia made it illegal to own or export objects of cultural importance, such as dinosaur fossils. These bones were among a hoard of illegally obtained dinosaur skeletons seized by U.S. authorities.

More than 18 dinosaur skeletons illegally taken from Mongolia were formally returned to their homeland last week, U.S. authorities announced.

The fossilized bones were handed over to Mongolian officials in a repatriation ceremony held July 10 in New York. "Today, we return a veritable nest of dinosaurs," Manhattan U.S. Attorney Preet Bharara said in a statement after the ceremony.

"This is a historic event for the U.S. Attorney's Office, in addition to being a prehistoric event, and we are proud to participate in the return of these dinosaur skeletons to their rightful home," Bharara said. [Album: A Tarbosaurus Travels from Auction to Courtroom]

The road to repatriation began two years ago, in 2012, when an auction house in New York was offering a skeleton of a Tarbosaurus bataar — an Asian cousin of Tyrannosaurus rex. Mongolian authorities voiced concern that the specimen had likely been smuggled into the United States. The 70-million-year-old dinosaur species was native to the Gobi desert in Asia, and to date, has only been found in modern-day Mongolia.

The Tarbosaurus sale attracted a bid of more than $1 million, but the suspicions of the Mongolian authorities sparked a long legal battle and federal investigation. U.S. authorities froze the sale, and after a lengthy custody battle, the specimen was returned to Mongolia in May 2013.

Eric Prokopi, a self-described commercial paleontologist who imported the dinosaur, pleaded guilty to criminal charges that he smuggled the skeleton and other fossils into the United States. In June, Prokopi was sentenced to three months in federal prison.

The other Mongolian fossils forfeited by Prokopi during the case were returned in the July 10 ceremony, including a second Tarbosaurus, oviraptors and skeletons of the duckbilled, plant-eating Saurolophus angustirostris.

Federal authorities also returned fossils that had been forfeited by Christopher Moore, a onetime business partner of Prokopi in the United Kingdom, including a third Tarbosaurus, skeletons of a roosterlike Gallimimus dinosaur and a nest of fossilized eggs, all looted from Mongolia.

"The fossils returned today do not belong in the hands of any private collection or one owner," James T. Hayes Jr., special agent in charge of Homeland Security Investigations (HSI) in New York, said in a statement. "They belong to the people of Mongolia where they will be displayed in their national museum alongside the Bataar ICE repatriated last year. HSI will not allow the illicit greed of some to trump the cultural history of an entire nation."

The Mongolian government made it illegal in 1924 to own or export items of cultural significance, including dinosaur fossils.

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