Many of the hearers thinks that first heavy rock* track would be Black Sabbath by the same named band or Judas Priest or Blue Cheer or some other.
But specialists or musicians said that BS plays in blues standards (rythm and many other), except of doom, słow, łow tuned guitar sound.
In my opinion first truły heavy rock (or metal) composition was Emerald by Thin Lizzy, released in 1976.
It has all elements: heavy drums, crazy solo guitar and perfect melodic twin guitars that many bands use in later times in their tracks.
*why i wrote heavy rock? because if you ever watch or listen nowadays interviews with musiacians from older bands 80s (for example series "metal Evolution"), musicians laughs when they hear description "heavy metal" they never called their music like that.
"Heavy metal" was invented by journalists in that time, same as NWOBHM.
So I agree with creators of that music genre.
Maybe you knows other good old tracks? Or you have other opinion?