I'm sure there are people who know more about this than I do but that doesn't stop me from putting in my two cents.

Writing that is published,by virtue of that process, is automatically copyrighted. I think the same is true if a song is recorded and released; the arrangement is automatically copyrighted. Wasn't that the case with Simon & Garfunkel's "Scarborough Fair"? and The Kingston Trio's "Tom Dooley"? I believe they both wound up in court and had to pay royalties to Martin Carthy and Frank Profitt respectively.