Par Ridder, son of former Knight Ridder chairman Tony Ridder, has abruptly resigned as publisher of the St. Paul Pioneer Press and has taken over as publisher of the rival Minneapolis Star Tribune. The Pioneer Press had been in Ridder’s family for decades until it was sold last year to McClatchy, which then sold it to Hearst and now is being managed for Hearst by Dean Singleton’s MediaNews Group. According to the Pioneer Press, Singleton personally went to St. Paul yesterday to talk to employees about Ridder’s departure, admitting he was “surprised and disappointed.” Ridder, 38, had been publisher of the Pioneer Press for three years after serving as advertising manager of the Knight Ridder-owned San Luis Obispo Tribune.