An international legal dispute over a dinosaur skeleton sprouted last month from an innocent stack of mail at the American Museum of Natural History.

Mark A. Norell, who heads the museum’s paleontology division, was flipping through letters and packages in his office atop one of the museum’s turrets when he noticed an alarming curiosity in a catalog of scheduled auctions in Manhattan: a perfectly assembled Tyrannosaurus bataar skeleton.

Mr. Norell believed there was almost no chance that the 24-foot-long, 8-foot-tall fossil had been legally removed from Mongolia, the only country where that type has been found. On May 17, three days before the auction, he wrote an open letter that was posted to listservs frequented by paleontologists.

“As someone who is intimately familiar with these faunas, these specimens were undoubtedly looted from Mongolia,” the letter said.