The I.M. Chait gallery had previously purchased and sold an illegally smuggled duck-billed dinosaur skeleton from convicted palaeontologist Eric Prokopi, whom US Attorney Preet Bharara called a "one-man black market in prehistoric fossils".
The gallery has not been accused of wrong-doing. A representative did not return a request for comment.
It was unclear whether the Cage skull was connected to Prokopi, who pleaded guilty in December 2012 to smuggling a nearly complete Tyrannosaurus bataar skeleton out of Mongolia's Gobi desert and was later sentenced to three months in prison.
As part of his guilty plea, Prokopi helped prosecutors recover at least 17 other dinosaur fossils.
The Tyrannosaurus bataar, like its more famous relative Tyrannosaurus rex, was a carnivore that lived about 70 million years ago.
Its remains have been discovered only in Mongolia, which criminalised the export of dinosaur fossils in 1924.
Since 2012, Mr Bharara's office has recovered more than a dozen Mongolian fossils, including three full Tyrannosaurus bataar skeletons.
"Each of these fossils represents a culturally and scientifically important artefact looted from its rightful owner," Mr Bharara said last week.
