Complex sentence: They were used primarily in the 1960s and 1970s and were replaced by the "Sturgeon" and "Los Angeles" classes, and were the result of a study commissioned in 1956 by the Chief of Naval Operations (CNO), Admiral Arleigh Burke.
Split into simpler sentences: They were used primarily in the 1960s and 1970s, until replaced by the and classes. The "Thresher/Permit" class were the result of a study commissioned in 1956 by the Chief of Naval Operations (CNO), Admiral Arleigh Burke.