Skull and mandible of Delphinapterus leucas from the MNHN, borrowed from Wikipedia.
Occasionally I'll see a photograph that speaks to me and looks like it would be fun (or challenging) to illustrate. A few weeks ago I found a neat looking photo of a beluga (Delphinapterus leucas) skull and mandible from the MNHN in Paris (shown above). It's a rather old mount as can be gathered from the nails and wires securing the teeth. This one was of sufficiently high resolution to elaborate minute details of the bone texture, and was an excellent candidate for an experiment in substantially more detailed illustration than what I've previously attempted.
The completed illustration, measuring about ~14" wide or so.
Close up of the temporal region. I went a bit crazy here.