Complex sentence: In 1966, Gene Clark, who then was the band's primary songwriter, left the group because of stress and this placed all the group's songwriting responsibilities in the hands of McGuinn, Crosby, and Hillman.
Split into simpler sentences: In 1966, Gene Clark, who then was the band's primary songwriter, left the group because of stress. This placed all the group's songwriting responsibilities in the hands of McGuinn, Crosby, and Hillman.