Dr. Donald Ledbetter is Ross' boss at the Museum of Prehistoric History.
It is revealed in "The One With Ross' Sandwich" that Dr. Ledbetter ate Ross' leftover Thanksgiving sandwich, for which Ross gets really mad at him. He then lets Ross take a sabbatical.
In "The One Where Everybody Finds Out", while Dr. Ledbetter visits Ross to ask him to come back to work at the museum, Ross discovers that Monica and Chandler are in a relationship and gets mad again.
After that episode, Ross never returns working the Museum, although it is not revealed if he resigned or got fired.