Fifteen sheep from a flock of around 70 were found dead last week at a private ranch in Nicasio, prompting an investigation by the Marin County Sheriff’s Office. A sheepherder and the owner, whose property borders Skywalker Ranch, initially reported that the animals appeared to have gunshot wounds, but they may have fallen prey to a puma. Dr. Quinton Martins, director of True Wild Conservation, a nonprofit that studies mountain lions in the North Bay,…