Yes, I agree with squirrelborg, you could buffer in-between the teeth with darker shades, instead of black, to trick the eye into believing that there are more teeth there.
The contrast between light and dark will make the teeth more defined and stylized, while lower contrast will make the brain believe that there are a lot of teeth there, since there is color there, and the brain knows that there are teeth present in those areas of a skull, and fills us in with that information.
That's one of the things one has to master with them pixels, to make details out of almost nothing, and make stuff happen in the nothing-space between pixels. It's especially important in animation. Heh.