Looking for an efficient approach to isolating a petrosal

I am trying to isolate the petrosal bone from a CT scan of a fossil skull. I’ve managed to segment it out the hard way before, using threshold and paint.
But this takes a considerable amount of time, and I always end up with jagged surfaces where the bone closely contacts another or small spikes on what should be a smooth surface. I’ve tried the smoothing tool but it does not seem to change anything, regardless of smoothing method.
Are there any good techniques or extensions for dealing with such a complex bone?