Unfortunately after attacking this piece from every angle with some plastic opening tools, I'm quite certain there is no non-destructive way to open this piece.
I tried slipping a spudger in every seam where the plastic joins and was unable to find anywhere I could get any leverage, or even push it in more than a few millimeters.
Even the jaw piece is held on by a cylinder of plastic running from cheek to cheek. There isn't a good way to photograph it, but here's an illustration to show you what I mean.
Based on this experience, I believe it is held together by single-use plastic clips that snap into place on the inside without any way to detach them, short of breaking them. I do not think it is glued together, because the two halves do shift a tiny amount under pressure, and some points were much more resistant to inserting a spudger than others (perhaps because a clip is in that area).