Mark Volman, a founding member and lead vocalist of the 1960s pop-rock group the Turtles, died Sept. 5 in Nashville after a brief, unexpected illness, his representative told People. He was 78.

Volman helped propel the band to mainstream success with its 1967 Billboard No. 1 single "Happy Together" and the follow-up hit "Elenore" in 1968. He joined the group in 1963, while a student at Westchester High School in California, and remained a distinctive presence in the band through its early, most commercially successful years.

The group that became the Turtles started out as the Nightriders and later the Crossfires, playing surf music. Volman first performed as a saxophonist before he and fellow founding member Howard Kaylan took on lead vocal duties when the band shifted toward folk rock. The founding lineup included Volman, Kaylan, Don Murray, Al Nichol, Jim Tucker and Chuck Portz.

Over time, the Turtles' roster expanded to include later contributors such as bassists Chip Douglas and Jim Pons and drummers including Joel Larson. The band's combination of tight harmonies and pop hooks earned them broad radio play and enduring recognition for songs that became staples of 1960s American rock.

Volman and Kaylan's voices and onstage chemistry were central to the group's image and sound, anchoring recordings and performances that have been reassessed by critics and fans in the decades since the band's chart successes. "Happy Together" in particular has had continued cultural resonance, appearing in films, television and advertising and remaining a touchstone of the era.

Details about memorial services and any survivors were not immediately released. The representative's statement to People described the illness as brief and unexpected but gave no further medical information.

Volman's death marks the passing of one of the prominent voices of a band that bridged the surf, folk-rock and pop sensibilities of the mid-1960s and left a catalog that continues to be heard and covered. The Turtles' work is frequently cited in discussions of 1960s pop craftsmanship and the commercial apex of American harmony-driven rock.