Large cassid gastropods found in close association with Oligocene mysticete whale skeletons in a deep water setting indicate a novel trophic relationship. Bones of early mysticetes are preserved in close association with a recurring assemblage of bivalve taxa that are common components of fossil and recent hydrothermal vent and cold methane seep localities. Living members of these bivalve groups harbor sulfide-oxidizing autotrophic bacteria and comparisons with recent whale-falls indicate that they utilized bone-oil seepage. But unlike recent whale-falls, this fossil invertebrate assemblage is volumetrically dominated by large individuals of the cassid gastropod Liracassis apta that are preserved with their apertures adhering to the bone surfaces. Comparisons with recent cassoidean taxa and their ecological settings suggests that Liracassis apta belongs to the subfamily Oocorythinae, not Cassininae, and was an opportunistic scavenger in deep water, low oxygen environments. Oocorythine gastropods radiated rapidly on the northeastern Pacific margin during the Late Eocene and Oligocene to become the most commonly recorded gastropod in shelf and slope settings.