You know, it kind of makes sense that it is a mask, or a whole suit. Because well, if it was really bone, and the trunk is attached to the chest, that's what it looks like to me at least, and if that is the case, how would a Facehugger ever get to impregnate it? It was covering the front of Jockey's mouth. And we can clearly see the Jockey was "chest bursted."
So I think it would have to be a mask for this to happen, the Jockey must not have been wearing it at the time that the Facehugger impregnated it.