It could be a case of convergent evolution...fusion really started in 1968-69 with Miles Davis "In a silent way" (or some might say with "filles de kilimangaro")...and that was about the same time period, if not slightly before. It could be what one poster above had mentioned, that some of these burgeoning prog rock bands were definitely taking notice of the new sound of fusion taking place primarily in the US with bands like Mahavishu, head hunters, early weather report. Who knows if they heard "Bitches brew" and went "wow"! There were artists like Sun Ra, that were doing "progressive" jazz with sci-fi song titles even as early as the late 50's, and one can't but wonder how many English prog rockers heard Sun Ra's records.
But I would think that by the mid 70's, most had abandoned the sound of saxes and whatnot, and really moved farther towards the classical side of the thing. Fusion, for the most part, has a much larger degree of improv going on then say a band like YES, who although don't read music still have their songs completed, arranged and orchestrated ahead of time (they have really great memories, obviously!)