In Geology, a fossil is usually considered to be the remains of a living organism which has been preserved in the sedementary record by being covered up with sediments - thus having survived, at least partially, the usual fate of nearly all flora and fauna of being "recycled" by Mother Nature into nutrients for other oganisms to live on.  Accordingly, until a shell is buried under enough sand, silt, turbidite, etc., to begin preserving it from being eaten, eroded, dissolved, etc., it is still  a shell, and not yet a fossil.  A "sub-fossil" is an organism or part of one, which is in the early stages of this process.
Cheers,
Ross M.
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