It's probably worth mentioning that one of the motivations behind my Phish/Dead comparison post in the first place - besides pure cussedness of course - was to clear a little space for actual musicological work on the evolution of 'improvisatory rock,' and the way Phish and later 'jam bands' (ugh) relate genealogically to musical sources from free jazz to Afrobeat to punk to techno to avant-garde ambient soundscaping. The coherence of Phish's improv, their particular weird one-musical-machine approach, is unique, as is their devotion to a particular kind of listening-exercise. But several other genres/bands serve as 'conditions of possibility' for Phish. Past the limited (but useful because of its stark contrast) comparison of Phish's and the Dead's improv styles, the lineage is way way more interesting.

Shit, as far as I'm concerned all 'jam band' music was born the moment Tony Williams switched to the ride cymbal at the climax of Miles's solo on 'It's About That Time'...