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Lesh's death was announced on his Instagram account. He was the oldest and one of the longest surviving members of the band that came to define the acid rock sound emanating from San Francisco in the 1960s.

"Phil Lesh, bassist and founding member of The Grateful Dead, passed peacefully this morning. He was surrounded by his family and full of love. Phil brought immense joy to everyone around him and leaves behind a legacy of music and love," reads in part.

The statement did not cite a specific cause of death and attempts to reach representatives for additional details were not immediately successful.

Lesh previously survived bouts of prostate cancer, and a 1998 liver transplant necessitated by a hepatitis C infection and years of heavy drinking.

Lesh's death came two days after MusiCares named the Grateful Dead its Persons of the Year. MusiCares, which helps music professionals needing financial or other kinds of assistance, cited Lesh's Unbroken Chain Foundation among other philanthropic initiatives. The Dead will be honored in January at a benefit gala ahead of the in Los Angeles.

Though he kept a relatively low public profile, fans and fellow band members recognized Lesh as a critical member of the Grateful Dead whose thundering lines on the six-string electric bass provided a brilliant counterpoint to 's soaring solos and anchored the band's famous marathon jams.

"When Phil's happening the band's happening," Garcia once said.

Drummer Mickey Hart called him the group's intellectual who brought a classical composer's mindset and skills to a five-chord rock 'n' roll band.

Lesh credited Garcia with teaching him to play the bass in the unorthodox lead-guitar style that he would become famous for, mixing thundering arpeggios with snippets of spontaneously composed orchestral passages.

Fellow bass player Rob Wasserman once said Lesh's style set him apart from every other bassist he knew of. While most others were content to keep time and take the occasional solo, Wasserman said Lesh was both good enough and confident enough to lead his fellow musicians through a song's melody.

"He happens to play bass but he's more like a horn player, doing all those arpeggios — and he has that counterpoint going all the time," he said.

Lesh began his long musical odyssey as a classically trained violinist, starting with lessons in third grade. He took up the trumpet at 14, eventually earning the second chair in California's Oakland Symphony Orchestra while still in his teens.

He largely put both instruments aside and was driving a mail truck and working as a sound engineer for a small radio station in 1965 when Garcia recruited him to play bass in a fledgling rock band called The Warlocks.

When Lesh told Garcia he didn't play the bass, the musician asked, "Didn't you used to play violin?" When he said yes, Garcia told him, "There you go, man."

With a cheap four-string instrument his girlfriend bought him, Lesh sat down for a seven-hour lesson with Garcia, following the latter's advice that he tune his instrument's strings an octave lower than the four bottom strings on Garcia's guitar.

Then Garcia turned him loose, allowing Lesh to develop the spontaneous style of playing that he would embrace for the rest of his life.

Lesh and Garcia would frequently exchange leads, often spontaneously, while the band as a whole would frequently break into long experimental, jazz-influenced jams during concerts.

The result was that even well-known Grateful Dead songs like "Truckin'" or "Sugar Magnolia" rarely sounded the same two performances in a row, something that would inspire loyal fans to attend show after show.

Phillip Chapman Lesh was born on March 15, 1940, in Berkeley, California, the only child of Frank Lesh, an office equipment repairman, and his wife, Barbara.

He would say in later years that his love of music came from listening to broadcasts of the New York Philharmonic on his grandmother's radio. Musical influences he often cited were not rock musicians but composers like Bach and Edgard Varèse, as well as jazz greats like John Coltrane and Miles Davis.

Soon after he took up the bass, The Warlocks renamed itself the Grateful Dead and Lesh began captivating audiences with his dexterity. Crowds gathered in what came to be known as "The Phil Zone" directly in front of his position onstage.

Lesh also composed music for, and sometimes sang, some of the band's most beloved songs. Among them were the upbeat country rocker "Pride of Cucamonga," the jazz-influenced "Unbroken Chain" and the ethereally beautiful "Box of Rain."

Lesh composed the latter on guitar as a gift for his dying father, and he recalled that Grateful Dead lyricist Robert Hunter, upon hearing the instrumental recording, approached him the next day with a lyric sheet. On that sheet, he said, were "some of the most moving and heartfelt lyrics I've ever had the good fortune to sing."

The group dissolved after Garcia's 1995 death.

Lesh took part in a 2009 Grateful Dead tour and again in 2015 for a handful of "Fare Thee Well" concerts marking both the band's 50th anniversary and what Lesh said would be the last time he would play with the others.

He continued to play with a rotating cast of musicians he called Phil Lesh and Friends. In later years he usually held those performances at Terrapin Crossroads, a restaurant and nightclub he opened near his Northern California home in 2012, which was named after the Grateful Dead song and album "Terrapin Station."

Lesh is survived by his wife, Jill, and sons Brian and Grahame.