'The Turtles' Guitarist Mark Volman Dead at 78

Mark Volman, a founding member of the 1960s rock band The Turtles, is dead ... TMZ has learned.

Mark died Friday morning, according to People. Mark's cause of death is unclear.

Mark was diagnosed two years ago with Lewy body dementia, the second most common type of dementia. LBD is caused by a buildup of proteins in the brain and it affects memory, movement and thinking.

Along with Howard Kaylan, Mark formed the backbone of The Turtles ... the guitarist and vocalist was known for his distinctive, frizzy hair. Their biggest hit with The Turtles was their January 1967 single, "Happy Together," which rose to the top spot on the Billboard Hot 100 back in the day.

When The Turtles folded in 1970, Mark and Howard formed the comedy rock duo Flo & Eddie ... with Mark as Flo, a pseudonym short for The Phlorescent Leech.

Mark and Howard were also recruited into Frank Zappa's band, Mothers of Invention.

In recent times, Mark had been touring with several bands on the "Happy Together" tour, which brought together Jay and the Americans, Little Anthony, Ron Dante of the Archies -- who acted as a representative of The Turtles -- and more.

Sandy Yaguda of Jay and the Americans tells TMZ ... he had enjoyed reconnecting with Mark after years apart thanks to the "Happy Together" tour. They were on the road together for several months in 2024, but Sandy tells us Mark's health forced him to take a break for this year's trek.

He adds ... "He fought long and hard, and part of the joy of his life was performing. He was funny, he was enjoyable. He made everybody smile off stage and on-stage. He made everyone's life better."

Meanwhile, Jay Reincke of Jay and The Americans through his agency BiCoastal Productions tells TMZ Mark was a "wonderful guy" and "certainly the spirit of The Turtles."

Mark was 78.