Sexy, ballad-belting chanteuses don’t often share the stage with decapitated heads and mentally challenged former Good Humor men, but all piece together compellingly in the Signature Theater Company’s revival of John (Six Degrees of Separation) Guare’s odd patchwork of a play/musical/family drama/whodunit. Set in 1977, the story centers on Betty Yearn (Lili Taylor), a single mom from Maine sent by her mother to retrieve her sister Rosalie (the brassy and highly entertaining Sherie Rene Scott) from the Big Bad Apple. When Rosalie is killed in a freak cycling accident, Betty easily slips — perhaps too easily for her liking — into her sister’s sordid life as a travel agent and part-time porn actress. Given her lifestyle, Betty also becomes the prime suspect in her own son’s death when his body turns up sans head in the West Village — a gruesome mystery that is narrated in flashback by a back-from-the-grave Rosalie. Surreal, jarring and oftentimes just plain weird, Guare’s Landscape is a work of art worth examining up close and sticking your nose into.