Mary Boyle Seitz, the scenarist for John Wayne’s first starring role, “The Big Trail,” in 1929, died May 12 in Los Angeles following a short illness. She was 93.A native of Ontario, Canada, she joined MGM Studios on its opening day in 1924, and throughout the following decade worked for Irving Thalberg, David O. Selznick and Raoul Walsh. She was married for 45 years to inventor and cinematographer John F. Seitz (“The Four Horsemen of the Apocalypse,” “Sunset Boulevard,” “The Lost Weekend”), who died in 1979. She is survived by a son and a sister. Donations may be sent to the Motion Picture and Television Home in Woodland Hills.