The fossil came from a close relative of the Tyrannosaurus rex and is worth between $250,000 and $400,000.
The Tyrannosaurus bataar, also known as the Tarbosaurus, lived about 70 million years ago.
The Jackson Hole News and Guide reports that federal officials have declined to provide any details about the case, saying it remains under investigation.
Federal officials recently seized a nearly complete Tarbosaurus skeleton that was sold at auction in New York and arrested a Florida man for illegally importing dinosaur fossils.
It’s unclear whether there’s any connection between the Jackson skull and the New York case.