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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. Getty

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

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