A US palaeontologist who prosecutors called "a one-man black market in prehistoric fossils" has been sentenced to three months in prison after he admitted to having schemed to smuggle dinosaur remains out of Mongolia.

New York prosecutors said Eric Prokopi sold coral, fossils and other natural treasures out of his home and misrepresented the contents of dinosaur fossil shipments to the US from Mongolia between 2010 to 2012.

Prokopi, who pleaded guilty to three felony counts in December 2012, also illegally procured a second nearly complete Tyrannosaurus bataar skeleton, two Saurolophus skeletons and two Oviraptor skeletons, and in 2010 used bogus paperwork to import from China the remains of a small flying dinosaur.

The case stemmed from US efforts to seize and return to Mongolia a Tyrannosaurus bataar skeleton that was auctioned in New York in May 2012 for $US1.05 million.

"What I did was wrong, and I failed to appreciate the gravity of what I have done," Prokopi told US district judge Alvin Hellerstein at a court hearing in Manhattan.

Georges Lederman, Prokopi's lawyer, had sought a non-prison sentence, reflecting the 39-year-old Virginia resident's help in recovering what a prosecutor said was at least 17 other dinosaur fossils.

Judge Hellerstein, however, said a prison term would send a message to others in the commercial palaeontology field.

"He is clearly not a bad person, but he has done a bad thing," the judge said.

The nearly complete 70 million-year-old skeleton was repatriated to Mongolia in May 2013.

Lawyers in Manhattan US attorney Preet Bharara's office said it had been exported from the Gobi Desert in violation of laws declaring dinosaur fossils to be state property.

Mr Lederman said Prokopi will begin serving the three-month sentence in September, followed by another 15 months of supervised release that begins with three months in a community facility.

Prokopi was arrested in October 2012, and pleaded guilty to charges of conspiracy, entry of goods by means of false statements and interstate and foreign transportation of goods converted and taken by fraud.

He could have faced up to three years and one month in prison under federal sentencing guidelines, but US prosecutors sought a shorter term in light of his cooperation in recovering fossils about which the government was unaware.

Martin Bell, an assistant US attorney, said Prokopi shed light on a black market for dinosaur fossils that was "ignored by the government and hiding in plain sight."

Other investigations by the US Department of Justice into possibly illegally imported fossils have since been opened in Wyoming, California and New York, Mr Bell said.

Reuters