There's a hilarious-in-retrospect interview where McCartney praises the unusual placement of notes in a particular bassline on a particular song and attributes it to Brian Wilson's genius. Later it was revealed to be entirely due to Carol Kaye, the bass player on that session.

(Not to take anything away from Brian's actual genius!)

Carol has said though that, for the most part, Brian did write the bass parts that she played.

It also illustrates the difference from the Beatles, who actually did compose most of their cool stuff (although Martin did help).

Just a note on how this came to be - Phil Spector's "Be My Baby", released in '63 became an obsession for Wilson upon first listen. This obsession led to him sitting in on studio sessions w/ Spector, and of course, that was his introduction to the Wrecking Crew and that style of working.

On that note, the Brian Wilson song "Guess I'm Dumb" which was rejected by the other Beach Boys and instead released with vocals by Wrecking Crew member and touring Beach Boy Glen Campbell is an obvious foreshadowing of the Pet Sounds mood and sound, as well as a beautiful song.