There was one brief and somewhat tarnished moment during my adolescence — somewhere around 1966-1967 — in which I couldn’t distinguish between the inherent value of the Velvet Underground versus the Monkees or Frank Zappa & the Mothers of Invention and Paul Revere & the Raiders. That confession is not alarming in view of my age (13 going on 14), but consider the circumstances and suspend revisionism. The late Sixties were the last gasp of true Top 40 radio: At one point in the summer of 1966, for instance, Lee Dorsey’s soulful “Working in a Coalmine” was wedged in on the charts with the Sandpipers’ sappy “Guantanamera” and ? & the Mysterians’ still-vibrant “96 Tears.” A similar week in 1967 saw the innocence of the Turtles’ “Happy Together” and the sheer exuberance of Mitch Ryder & the Detroit Wheels shadowed by the call to arms of Buffalo Springfield’s “For What It’s Worth.” Those were the days when 16 magazine was selling over one million copies a month to teenage girls just like me.
– Margaret Moser, The Singer Not the Song, 1999
RockCritics.com 
I like the quote, but I just don’t think “the late Sixties were the last gasp of true Top 40 radio” is true. I can’t think of anything offhand that made it onto Top 40 in 1969 that wouldn’t also have made it in 1972—and you could probably stretch that forward a few years. I think it was really when punk started that there was one big dividing line, as opposed to isolated blips (the Fugs in 1966, the Velvet Underground in 1968, the New York Dolls in 1973, etc.).
I’m not in disagreement with you, Phil, and might take your argument even further. To my mind, Top 40 existed until well into the ’80s or early ’90s — to the time when talk took over AM and “hits” formats became determined by more rigid genre distinctions. On the other hand, depending on what she means by the word “true,” you could actually make an argument that an actual Top 40 ended *earlier* than she suggests, when the charts were much more a combination of youth music and adult music. Elvis and Chuck Berry mixed with Patti Page and Perry Como. (That’s a fair bit different, I think, than the “easy listening” that took up space on Top 40 when we grew up, much of which was still primarily aimed at younger listeners.)
Phil, I don’t expect you to elaborate, but I’ll just toss this in the air: I actually don’t see punk as a dividing line, insofar as pop radio is concerned, but I guess I don’t know what you mean exactly. I just think top 40 and punk existed as mutually exclusive entities, though of course, some punk-influenced pop eventually made its way into the Top 40. I guess what I mean is, I don’t see any startling differences between the pop charts in 1976 and the pop charts in 1978. I see punk as a dividing line, obviously, in a bunch of other ways… but the charts always seemed resistant. Am I completely off-track here?
I guess I mean that there was this huge story out of England in 1976 and 1977, and there were hundreds and hundreds of bands attached to that story that just weren’t going to get played on Top 40 radio. The new wave stuff that followed, yes—and you and I have often joked about how amorphous that line is. (Or put another way: the Clash in 1977, no Top 40 airplay, the Clash in 1982, pretty big hit.) In England, I think it was different—I’d have to check, but I believe some of the ground-zero punk also charted. I don’t know that there was a whole “movement” (that sounds awful, but I can’t think of a better word right now) that was denied airplay on North American Top 40 radio before punk. There were heavy metal hits, art rock hits, glam hits (though not as many as in England), etc.
I’d also have to check this, but wasn’t Top 40 starting to get very segregated for a couple of years during the post-disco backlash/pre-Thriller moment?