The song is a ripoff
The opening chords of both songs are built off a bass line that descends by half steps: A-G#-G-F#-F. They share the key of A minor. A similar guitar fingering in steady, stately eighth notes outlines the chord progression. Led Zeppelin’s Jimmy Page heard Spirit perform “Taurus” when the bands toured together in 1968 and 1969.
The song isn’t a ripoff
The chord progression is a standard progression used in other songs such as “Michelle” by The Beatles and “Time in a Bottle” by Jim Croce. The musically similar section is only one part of each song, and the phrases end on different chords. “Stairway to Heaven” is more musically developed, including lyrics, and apart from the opening guitar progression is orchestrated differently; in fact, the guitar pattern and recorders are used to evoke a Renaissance feel.
You can’t copyright a standard chord progression. But recent court cases have ruled that the “feel” of a song, including its rhythm, can be copyrighted (see my post “Copyright: what the ?*$!“). I believe that Jimmy Page was influenced by “Taurus” when he came up with the opening of “Stairway to Heaven.” And since the opening is such a signature part of the song, it’s probably fair that he and Robert Plant should share copyright credit with Spirit. What irks me, though, is that it took Spirit 40 some years to file a copyright infringement lawsuit. And the amount sought for damages—$60 million—seems way out of line, as if they deliberately waited that long to maximize their profit. Something doesn’t smell right there.