Complex sentence: He remained at South Dakota until 1971 when he was selected to succeed Jack Friel as commissioner of the Big Sky Conference, and served until June 1977, when he was replaced by Steve Belko.
Split into simpler sentences: He remained at South Dakota through 1971 when he was selected to serve as commissioner of the Big Sky Conference. He served as commissioner through June 1977 when he was replaced by Steve Belko.