Lips, close enough that they stop being lips and turn into terrain.
The method was backwards from how it looks. I laid down a flat mid-grey over the whole shape with a stump, then cut every crease out of it with a sharpened eraser, then went back in with pencil to deepen the darks. All those pale cracks are erased, not drawn.
Once you are working at this scale you are not drawing a mouth any more. You are drawing dry skin, and light sitting in the bottom of a groove, and the small wet line where the two halves meet.